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500 kSPS, 2-Channel, Software-Selectable, True Bipolar Input, 12-Bit Plus Sign ADC AD7321
FEATURES
12-bit plus sign SAR ADC True bipolar input ranges Software-selectable input ranges 10 V, 5 V, 2.5 V, 0 V to +10 V 500 kSPS throughput rate Two analog input channels with channel sequencer Single-ended, true differential, and pseudo differential analog input capability High analog input impedance Low power: 17 mW Full power signal bandwidth: 22 MHz Internal 2.5 V reference High speed serial interface Power-down modes 14-lead TSSOP package iCMOSTM process technology
FUNCTIONAL BLOCK DIAGRAM
VDD REFIN/OUT VCC
AD7321
2.5V VREF I/P MUX T/H
VIN0 VIN1
13-BIT SUCCESSIVE APPROXIMATION ADC
DOUT CHANNEL SEQUENCER CONTROL LOGIC AND REGISTERS SCLK CS DIN VDRIVE AGND VSS DGND
05399-001
Figure 1.
GENERAL DESCRIPTION
The AD7321 1 is a 2-channel, 12-bit plus sign successive approximation ADC designed on the iCMOS (industrial CMOS) process. iCMOS is a process combining high voltage silicon with submicron CMOS and complementary bipolar technologies. It enables the development of a wide range of high performance analog ICs capable of 33 V operation in a footprint that no previous generation of high voltage parts could achieve. Unlike analog ICs using conventional CMOS processes, iCMOS components can accept bipolar input signals while providing increased performance, dramatically reduced power consumption, and reduced package size. The AD7321 can accept true bipolar analog input signals. The AD7321 has four software-selectable input ranges, 10 V, 5 V, 2.5 V, and 0 V to +10 V. Each analog input channel can be independently programmed to one of the four input ranges. The analog input channels on the AD7321 can be programmed to be single-ended, true differential, or pseudo differential. The ADC contains a 2.5 V internal reference. The AD7321 also allows for external reference operation. If a 3 V reference is applied to the REFIN/OUT pin, the AD7321 can accept a true bipolar 12 V analog input. Minimum 12 V VDD and VSS supplies are required for the 12 V input range. The ADC has a high speed serial interface that can operate at throughput rates up to 500 kSPS.
PRODUCT HIGHLIGHTS
1. 2. The AD7321 can accept true bipolar analog input signals, 10 V, 5 V, 2.5 V, and 0 V to +10 V unipolar signals. The two analog inputs can be configured as two singleended inputs, one true differential input pair, or one pseudo differential input. 500 kSPS serial interface. SPI(R)-/QSPITM-/DSP-/MICROWIRETMcompatible interface. Low power, 17 mW, at a maximum throughput rate of 500 kSPS. Channel sequencer.
Throughput Rate 1000 kSPS 1000 kSPS 500 kSPS 1000 kSPS 500 kSPS 1000 kSPS Number of bits 12-bit plus sign 12-bit plus sign 12-bit plus sign 12-bit plus sign 12-bit plus sign 12-bit plus sign Number of Channels 8 8 8 4 4 2
3. 4. 5.
Table 1. Similar Devices
Device Number AD7329 AD7328 AD7327 AD7324 AD7323 AD7322
1
Protected by U.S. Patent No. 6,731,232.
Rev. 0
Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. Specifications subject to change without notice. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. Trademarks and registered trademarks are the property of their respective owners.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781.329.4700 www.analog.com Fax: 781.461.3113 (c)2006 Analog Devices, Inc. All rights reserved.
AD7321 TABLE OF CONTENTS
Features .............................................................................................. 1 Functional Block Diagram .............................................................. 1 General Description ......................................................................... 1 Product Highlights ........................................................................... 1 Revision History ............................................................................... 2 Specifications..................................................................................... 3 Timing Specifications .................................................................. 7 Absolute Maximum Ratings............................................................ 8 ESD Caution.................................................................................. 8 Pin Configuration and Function Descriptions............................. 9 Typical Performance Characteristics ........................................... 10 Terminology .................................................................................... 14 Theory of Operation ...................................................................... 16 Circuit Information.................................................................... 16 Converter Operation.................................................................. 16 Analog Input Structure.............................................................. 17 Typical Connection Diagram ................................................... 19 Analog Input ............................................................................... 19 Driver Amplifier Choice............................................................ 21 Registers ........................................................................................... 22 Addressing Registers.................................................................. 22 Control Register ......................................................................... 23 Range Register ............................................................................ 25 Sequencer Operation ..................................................................... 26 Reference ..................................................................................... 27 VDRIVE............................................................................................ 27 Modes of Operation ....................................................................... 28 Normal Mode.............................................................................. 28 Full Shutdown Mode.................................................................. 28 Auto Shutdown Mode................................................................ 29 Autostandby Mode..................................................................... 29 Power vs. Throughput Rate....................................................... 30 Serial Interface ................................................................................ 31 Microprocessor Interfacing........................................................... 32 AD7321 to ADSP-21xx.............................................................. 32 AD7321 to ADSP-BF53x ........................................................... 32 Application Hints ........................................................................... 33 Layout and Grounding .............................................................. 33 Outline Dimensions ....................................................................... 34 Ordering Guide .......................................................................... 34
REVISION HISTORY
1/06--Revision 0: Initial Version
Rev. 0 | Page 2 of 36
AD7321 SPECIFICATIONS
VDD = 12 V to 16.5 V, VSS = -12 V to -16.5 V, VCC = 2.7 V to 5.25 V, VDRIVE = 2.7 V to 5.25 V, VREF = 2.5 V to 3.0 V internal/external, fSCLK = 10 MHz, fS = 500 kSPS, TA = TMAX to TMIN, unless otherwise noted. Table 2.
Parameter 1 DYNAMIC PERFORMANCE Signal-to-Noise Ratio (SNR) 2 Min 76 75.5 72.5 72 Signal-to-Noise + Distortion (SINAD)2 75 74 76 72 72.5 Total Harmonic Distortion (THD)2 -82 -77 -79 -80 Peak Harmonic or Spurious Noise (SFDR)2 -82 -78 -80 -79 Intermodulation Distortion (IMD)2 Second-Order Terms Third-Order Terms Aperture Delay 3 Aperture Jitter3 Common-Mode Rejection (CMRR)2 Channel-to-Channel Isolation2 Full Power Bandwidth -81 -80 -80 -79 dB dB dB dB dB dB dB dB dB dB dB dB dB dB B Version Typ Max Unit dB dB dB dB dB Test Conditions/Comments FIN = 50 kHz sine wave Differential mode, VCC = 4.75 V to 5.25 V Differential mode, VCC < 4.75 V Single-ended/pseudo differential mode; 10 V, 2.5 V and 5 V ranges, VCC = 4.75 V to 5.25 V Single-ended/pseudo differential mode ; 0 V to 10 V VCC = 4.75 V to 5.25 V and all ranges at VCC < 4.75 V Differential mode; 2.5 V and 5 V ranges Differential mode; 0 V to 10 V Differential mode; 10 V range Single-ended/pseudo differential mode; 2.5 V and 5 V ranges Single-ended/pseudo differential mode; 0 V to +10 V and 10 V ranges Differential mode; 2.5 V and 5 V ranges Differential mode; 0 V to 10 V ranges Differential mode; 10 V range Single-ended/pseudo differential mode; 5 V range Single-ended/pseudo differential mode; 2.5 V range Single-ended/pseudo differential mode; 0 V to +10 V and 10 V ranges Differential mode; 2.5 V and 5 V ranges Differential mode; 0 V to 10 V ranges Differential mode; 10 V ranges Single-ended/pseudo differential mode; 5 V range Single-ended/pseudo differential mode; 2.5 V range Single-ended/pseudo differential mode; 0 V to +10 V and 10 V ranges fa = 50 kHz, fb = 30 kHz
-88 -90 7 50 -79 -72 22 5
dB dB ns ps dB dB MHz MHz
Up to 100 kHz ripple frequency; see Figure 17 FIN on unselected channels up to 100 kHz; see Figure 14 At 3 dB At 0.1 dB
Rev. 0 | Page 3 of 36
AD7321
Parameter 1 DC ACCURACY 4 Resolution No Missing Codes Min 13 12-bit plus sign 11-bit plus sign 1.1 1 -0.7/+1.2 Differential Nonlinearity2 -0.9/+1.2 0.9 -0.7/+1 Offset Error2, 5 Offset Error Match2, 5 Gain Error2, 5 Gain Error Match2, 5 Positive Full-Scale Error2, 6 Positive Full-Scale Error Match2, 6 Bipolar Zero Error2, 6 Bipolar Zero Error Match2, 6 Negative Full-Scale Error2, 6 Negative Full-Scale Error Match2, 6 -4/+9 -7/+10 0.6 0.5 8 14 0.5 0.5 4 7 0.5 0.5 8.5 7.5 0.5 0.5 4 6 0.5 0.5 B Version Typ Max Unit Bits Bits Bits LSB LSB LSB LSB LSB LSB LSB LSB LSB LSB LSB LSB LSB LSB LSB LSB LSB LSB LSB LSB LSB LSB LSB LSB LSB LSB Test Conditions/Comments
Differential mode Single-ended/pseudo differential mode Differential mode; VCC = 3 V to 5.25 V, typ for VCC = 2.7 V Single-ended/pseudo differential mode, VCC = 3 V to 5.25 V, typ for VCC = 2.7 V Single-ended/pseudo differential mode (LSB = FSR/8192) Differential mode; guaranteed no missing codes to 13 bits Single-ended mode; guaranteed no missing codes to 12 bits Single-ended/psuedo differential mode (LSB = FSR/8192) Single-ended/pseudo differential mode Differential mode Single-ended/pseudo differential mode Differential mode Single-ended/pseudo differential mode Differential mode Single-ended/pseudo differential mode Differential mode Single-ended/pseudo differential mode Differential mode Single-ended/pseudo differential mode Differential mode Single-ended/pseudo differential mode Differential mode Single-ended/pseudo differential mode Differential mode Single-ended/pseudo differential mode Differential mode Single-ended/pseudo differential mode Differential mode
Integral Nonlinearity2
Rev. 0 | Page 4 of 36
AD7321
Parameter 1 ANALOG INPUT Input Voltage Ranges (Programmed via Range Register) Min B Version Typ Max Unit Test Conditions/Comments Reference = 2.5 V; see Table 6 VDD = 10 V min, VSS = -10 V min, VCC = 2.7 V to 5.25 V VDD = 5 V min, VSS = -5 V min, VCC = 2.7 V to 5.25 V VDD = 5 V min, VSS = -5 V min, VCC = 2.7 V to 5.25 V VDD = 10 V min, VSS = AGND min, VCC = 2.7 V to 5.25 V VDD = 16.5 V, VSS = -16.5 V, VCC = 5 V; see Figure 40 and Figure 41 Reference = 2.5 V; range = 10 V Reference = 2.5 V; range = 5 V Reference = 2.5 V; range = 2.5 V Reference = 2.5 V; range = 0 V to +10 V VIN = VDD or VSS Per input channel, VIN = VDD or VSS When in track, 10 V range When in track, 5 V and 0 V to +10 V ranges When in track, 2.5 V range When in hold, all ranges
10 5 2.5 0 to 10
V V V V
Pseudo Differential VIN(-) Input Range 3.5 6 5 +3/-5 DC Leakage Current Input Capacitance3 3 13.5 16.5 21.5 3 2.5 10 2.5 5 10 25 3 7 2.4 0.8 0.4 1 10 VDRIVE - 0.2 V 0.4 1 5 Straight natural binary Twos complement 1.6 305 500 s ns kSPS 3 1 80 V V V V nA nA pF pF pF pF V A pF V mV mV ppm/C ppm/C V V V A pF V V A pF
REFERENCE INPUT/OUTPUT Input Voltage Range Input DC Leakage Current Input Capacitance Reference Output Voltage Reference Output Voltage Error @ 25C Reference Output Voltage TMIN to TMAX Reference Temperature Coefficient Reference Output Impedance LOGIC INPUTS Input High Voltage, VINH Input Low Voltage, VINL Input Current, IIN Input Capacitance, CIN3 LOGIC OUTPUTS Output High Voltage, VOH Output Low Voltage, VOL Floating-State Leakage Current Floating-State Output Capacitance3 Output Coding CONVERSION RATE Conversion Time Track-and-Hold Acquisition Time2, 3 Throughput Rate
VCC = 4.75 V to 5.25 V VCC = 2.7 to 3.6 V VIN = 0 V or VDRIVE
ISOURCE = 200 A ISINK = 200 A
Coding bit set to 1 in control register Coding bit set to 0 in control register 16 SCLK cycles with SCLK = 10 MHz Full-scale step input; see the Terminology section See the Serial Interface section
Rev. 0 | Page 5 of 36
AD7321
Parameter 1 POWER REQUIREMENTS VDD VSS VCC VDRIVE Normal Mode (Static) Normal Mode (Operational) IDD ISS ICC and IDRIVE Autostandby Mode (Dynamic) IDD ISS ICC and IDRIVE Autoshutdown Mode (Static) IDD ISS ICC and IDRIVE Full Shutdown Mode IDD ISS ICC and IDRIVE POWER DISSIPATION Normal Mode (Operational) Full Shutdown Mode
1 2 3
Min 12 -12 2.7 2.7
B Version Typ
Max 16.5 -16.5 5.25 5.25
Unit V V V V mA A A mA A A mA A A A A A A mW W
Test Conditions/Comments Digital inputs = 0 V or VDRIVE See Table 6 See Table 6 See Table 6 VDD/VSS = 16.5 V, VCC/VDRIVE = 5.25 V fSAMPLE = 500 kSPS VDD = 16.5 V VSS = -16.5 V VCC/VDRIVE = 5.25 V fSAMPLE = 250 kSPS VDD = 16.5 V VSS = -16.5 V VCC/VDRIVE = 5.25 V SCLK on or off VDD = 16.5 V VSS = -16.5 V VCC/VDRIVE = 5.25 V SCLK on or off VDD = 16.5 V VSS = -16.5 V VCC/VDRIVE = 5.25 V VDD = 16.5 V, VSS = -16.5 V, VCC = 5.25 V VDD = 16.5 V, VSS = -16.5 V, VCC = 5.25 V
0.9 180 205 2 100 110 0.75 1 1 1 1 1 1 17 38.25
Temperature range is -40C to +85C. See the Terminology section. Sample tested during initial release to ensure compliance. 4 For dc accuracy specifications, the LSB size for differential mode is FSR/8192. For single-ended mode/pseudo differential mode, the LSB size is FSR/4096, unless otherwise noted. 5 Unipolar 0 V to 10 V range with straight binary output coding. 6 Bipolar range with twos complement output coding.
Rev. 0 | Page 6 of 36
AD7321
TIMING SPECIFICATIONS
VDD = 12 V to 16.5 V, VSS = -12 V to -16.5 V, VCC = 2.7 V to 5.25 V, VDRIVE = 2.7 V to 5.25 V, VREF = 2.5 V to 3.0 V internal/external, TA = TMAX to TMIN. Timing specifications apply with a 32 pF load, unless otherwise noted. 1 Table 3.
Parameter fSCLK tCONVERT tQUIET t1 t2 2 t3 t4 t5 t6 t7 t8 t9 t10 tPOWER-UP Limit at TMIN, TMAX VCC < 4.75 V VCC = 4.75 V to 5.25 V 50 50 10 10 16 x tSCLK 16 x tSCLK 75 60 12 5 25 20 45 35 26 14 57 43 0.4 x tSCLK 0.4 x tSCLK 0.4 x tSCLK 0.4 x tSCLK 13 8 40 22 10 9 4 4 2 2 750 750 500 500 25
1 2
Unit kHz min MHz max ns max ns min ns min ns min ns min ns max ns max ns min ns min ns min ns max ns min ns min ns min ns max s max s typ
Description VDRIVE VCC
25
tSCLK = 1/fSCLK Minimum time between end of serial read and next falling edge of CS Minimum CS pulse width CS to SCLK set-up time; bipolar input ranges (10 V, 5 V, 2.5 V) Unipolar input range (0 V to 10 V) Delay from CS until DOUT three-state disabled Data access time after SCLK falling edge SCLK low pulse width SCLK high pulse width SCLK to data valid hold time SCLK falling edge to DOUT high impedance SCLK falling edge to DOUT high impedance DIN set-up time prior to SCLK falling edge DIN hold time after SCLK falling edge Power-up from autostandby Power-up from full shutdown/autoshutdown mode, internal reference Power-up from full shutdown/autoshutdown mode, external reference
Sample tested during initial release to ensure compliance. All input signals are specified with tr = tf = 5 ns (10% to 90% of VDRIVE) and timed from a voltage level of 1.6 V. When using the 0 V to 10 V unipolar range, running at 500 kSPS throughput rate with t2 at 20 ns, the mark space ratio needs to be limited to 50:50.
t1
CS
t2
SCLK 1 2 3 4 IDENTIFICATION BIT ADD0 SIGN
tCONVERT t6
5 13 14 15 16
t3
t4
DB11
t7
DB10 DB2
t5
DB1 DB0
t8
ZERO DOUT THREE- ZERO t9 STATE DIN WRITE ZERO
tQUIET
t10
REG SEL MSB LSB DON'T CARE
THREE-STATE
05399-002
Figure 2. Serial Interface Timing Diagram
Rev. 0 | Page 7 of 36
AD7321 ABSOLUTE MAXIMUM RATINGS
TA = 25C, unless otherwise noted Table 4.
Parameter VDD to AGND, DGND VSS to AGND, DGND VDD to VCC VCC to AGND, DGND VDRIVE to AGND, DGND AGND to DGND Analog Input Voltage to AGND 1 Digital Input Voltage to DGND Digital Output Voltage to GND REFIN to AGND Input Current to Any Pin Except Supplies 2 Operating Temperature Range Storage Temperature Range Junction Temperature TSSOP Package JA Thermal Impedance JC Thermal Impedance Pb-Free Temperature, Soldering Reflow ESD
1
Rating -0.3 V to +16.5 V +0.3 V to -16.5 V VCC - 0.3 V to 16.5 V -0.3 V to +7 V -0.3 V to +7 V -0.3 V to +0.3 V VSS - 0.3 V to VDD + 0.3 V -0.3 V to +7 V -0.3 V to VDRIVE + 0.3 V -0.3 V to VCC + 0.3 V 10 mA -40C to +85C -65C to +150C 150C 113.5C/W 30C/W 260(0)C 2.5 kV
Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device. This is a stress rating only; functional operation of the device at these or any other conditions above those indicated in the operational section of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
If the analog inputs are driven from alternative VDD and VSS supply circuitry, Schottky diodes should be placed in series with the AD7321's VDD and VSS supplies. 2 Transient currents of up to 100 mA do not cause SCR latch-up.
ESD CAUTION
ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily accumulate on the human body and test equipment and can discharge without detection. Although this product features proprietary ESD protection circuitry, permanent damage may occur on devices subjected to high energy electrostatic discharges. Therefore, proper ESD precautions are recommended to avoid performance degradation or loss of functionality.
Rev. 0 | Page 8 of 36
AD7321 PIN CONFIGURATION AND FUNCTION DESCRIPTIONS
CS 1 DIN 2 DGND 3 AGND 4 REFIN/OUT 5 VSS 6 VIN0 7
14 13
SCLK DGND DOUT VDRIVE VCC
05399-003
AD7321
TOP VIEW (Not to Scale)
12 11 10 9 8
VDD VIN1
Figure 3. TSSOP Pin Configuration
Table 5. AD7321 Pin Function Descriptions
Pin No. 1 2 3, 13 Mnemonic CS DIN DGND Description Chip Select. Active low logic input. This input provides the dual function of initiating conversions on the AD7321 and frames the serial data transfer. Data In. Data to be written to the on-chip registers is provided on this input and is clocked into the register on the falling edge of SCLK (see the Registers section). Digital Ground. Ground reference point for all digital circuitry on the AD7321. The DGND and AGND voltages ideally should be at the same potential and must not be more than 0.3 V apart, even on a transient basis. Analog Ground. Ground reference point for all analog circuitry on the AD7321. All analog input signals and any external reference signal should be referred to this AGND voltage. The AGND and DGND voltages ideally should be at the same potential and must not be more than 0.3 V apart, even on a transient basis. Reference Input/Reference Output. The on-chip reference is available on this pin for use external to the AD7321. The nominal internal reference voltage is 2.5 V, which appears at this pin. A 680 nF capacitor should be placed on the reference pin (see the Reference section). Alternatively, the internal reference can be disabled and an external reference applied to this input. On power-up, the external reference mode is the default condition. Negative Power Supply Voltage. This is the negative supply voltage for the analog input section. Analog Input 0 to Analog Input 1. The analog inputs are multiplexed into the on-chip track-and-hold. The analog input channel for conversion is selected by programming the Channel Address Bit ADD0 in the control register. The inputs can be configured as two single-ended inputs, one true differential input pair, or one pseudo differential input. The configuration of the analog inputs is selected by programming the mode bits, Bit Mode 1 and Bit Mode 0, in the control register. The input range on each input channel is controlled by programming the range register. Input ranges of 10 V, 5 V, 2.5 V, and 0 V to +10 V can be selected on each analog input channel when a +2.5 V reference voltage is used (see the Registers section). Positive Power Supply Voltage. This is the positive supply voltage for the analog input section. Analog Supply Voltage, 2.7 V to 5.25 V. This is the supply voltage for the ADC core on the AD7321. This supply should be decoupled to AGND. Logic Power Supply Input. The voltage supplied at this pin determines at what voltage the interface operates. This pin should be decoupled to DGND. The voltage at this pin may be different to that at VCC, but it should not exceed VCC by more than 0.3 V. Serial Data Output. The conversion output data is supplied to this pin as a serial data stream. The bits are clocked out on the falling edge of the SCLK input, and 16 SCLKs are required to access the data. The data stream consists of two ZERO bits, a channel identification bit, the sign bit, and 12 bits of conversion data. The data is provided MSB first (see the Serial Interface section). Serial Clock, Logic Input. A serial clock input provides the SCLK used for accessing the data from the AD7321. This clock is also used as the clock source for the conversion process.
4
AGND
5
REFIN/OUT
6 7, 8
VSS VIN0 to VIN1
9 10 11
VDD VCC VDRIVE
12
DOUT
14
SCLK
Rev. 0 | Page 9 of 36
AD7321 TYPICAL PERFORMANCE CHARACTERISTICS
0 -20 -40 4096 POINT FFT VCC = VDRIVE = 5V VDD, VSS = 15V TA = 25C INT/EXT 2.5V REFERENCE 10V RANGE FIN = 50kHz SNR = 77.30dB SINAD = 76.85dB THD = -86.96dB SFDR = -88.22dB
1.0 0.8 0.6 0.4
INL ERROR (LSB)
VCC = VDRIVE = 5V INT/EXT 2.5V REFERENCE TA = 25C 10V RANGE VDD, VSS = 15V +INL = +0.55LSB -INL = -0.68LSB
0.2 0 -0.2 -0.4 -0.6
SNR (dB)
-60 -80 -100 -120 -140
-0.8 -1.0
0 50 100 150 200 250 FREQUENCY (kHz)
05399-004
05399-007 05399-044 05399-043
0
8192 1024 2048 3072 4096 5120 6144 7168 512 1536 2560 3584 4608 5632 6656 7680 CODE
Figure 4. FFT True Differential Mode
1.0
0 -20 -40 4096 POINT FFT VCC = VDRIVE = 5V VDD, VSS = 15V TA = 25C INT/EXT 2.5V REFERENCE 10V RANGE FIN = 50kHz SNR = 74.67dB SINAD = 74.03dB THD = -82.68dB SFDR = -85.40dB
Figure 7. Typical INL True Differential Mode
0.8 0.6
DNL ERROR (LSB)
0.4 0.2 0 -0.2 -0.4 -0.6 -0.8 -1.0 0 VCC = VDRIVE = 5V 10V RANGE TA = 25C +DNL = +0.79LSB -DNL = -0.38LSB VDD, VSS = 15V INT/EXT 2.5V REFERENCE 8192 1024 2048 3072 4096 5120 6144 7168 512 1536 2560 3584 4608 5632 6656 7680 CODE
SNR (dB)
-60 -80 -100 -120 -140
0
50
100
150
200
250
FREQUENCY (kHz)
05399-005
Figure 5. FFT Single-Ended Mode
1.0 0.8 0.6 0.4
INL ERROR (LSB)
Figure 8. Typical DNL Single-Ended Mode
1.0 0.8 0.6
DNL ERROR (LSB)
0.4 0.2 0 -0.2 -0.4 -0.6 -0.8 -1.0 0 VCC = VDRIVE = 5V TA = 25C VDD, VSS = 15V INT/EXT 2.5V REFERENCE 10V RANGE +DNL = +0.72LSB -DNL = -0.22LSB
05399-006
0.2 0 -0.2 VCC = VDRIVE = 5V TA = 25C VDD, VSS = 15V -0.6 INT/EXT 2.5V REFERENCE 10V RANGE -0.8 +INL = +0.87LSB -INL = -0.49LSB -1.0 0 8192 1024 2048 3072 4096 5120 6144 7168 512 1536 2560 3584 4608 5632 6656 7680 CODE -0.4
8192 1024 2048 3072 4096 5120 6144 7168 512 1536 2560 3584 4608 5632 6656 7680 CODE
Figure 6. Typical DNL True Differential Mode
Figure 9. Typical INL Single-Ended Mode
Rev. 0 | Page 10 of 36
AD7321
-50 -55 -60 -65 VCC = VDRIVE = 3V VDD/VSS = 12V TA = 25C fS = 500kSPS INTERNAL REFERENCE
80 5V DIFF 2.5V DIFF 75
0V TO +10V SE 10V SE 10V DIFF 0V TO +10V DIFF 5V SE 5V DIFF 2.5V DIFF 2.5V SE
55 70
5V SE 2.5V SE 10V DIFF 10V SE 0V TO +10V DIFF 0V TO +10V SE
SINAD (dB)
THD (dB)
-70 -75 -80 -85 -90 -95 -100 10 100
65
60 VCC = VDRIVE = 5V VDD/VSS = 12V TA = 25C fS = 500kSPS INTERNAL REFERENCE 100 ANALOG INPUT FREQUENCY (kHz) 1000
05399-063
1000
ANALOG INPUT FREQUENCY (kHz)
Figure 10. THD vs. Analog Input Frequency for Single-Ended (SE) and True Differential Mode (Diff) at 3 V VCC
-50 VCC = VDRIVE = 5V VDD/VSS = 12V TA = 25C fS = 500kSPS INTERNAL REFERENCE
05399-060
50 10
Figure 13. SINAD vs. Analog Input Frequency for Single-Ended (SE) and True Differential Mode (Diff) at 5 V VCC
-50
CHANNEL-TO-CHANNEL ISOLATION (dB)
-55 -60 -65
-55 -60 -65 -70 -75 -80 -85 -90 -95 VDD/VSS = 12V SINGLE-ENDED MODE fS = 500kSPS TA = 25C 50kHz ON SELECTED CHANNEL 0 100 200 300 400 500 600
05399-012
0V TO +10V SE 10V SE 10V DIFF 0V TO +10V DIFF 5V SE 5V DIFF
VCC = 3V VCC = 5V
THD (dB)
-70 -75 -80 -85 -90 -95 -100 10 100
2.5V SE 2.5V DIFF 1000
05399-061
ANALOG INPUT FREQUENCY (kHz)
FREQUENCY OF INPUT NOISE (kHz)
Figure 11. THD vs. Analog Input Frequency for Single-Ended (SE) and True Differential Mode (Diff) at 5 V VCC
80 5V DIFF 2.5V DIFF 5V SE 2.5V SE 0V TO +10V DIFF 10V DIFF 10V SE 65 0V TO +10V SE 60 VCC = VDRIVE = 3V VDD/VSS = 12V TA = 25C fS = 500kSPS INTERNAL REFERENCE 100 ANALOG INPUT FREQUENCY (kHz) 1000
05399-062
Figure 14. Channel-to-Channel Isolation
10k 9k
9469
75
NUMBER OF OCCURRENCES
8k 7k 6k 5k 4k 3k 2k 1k 0 0 -2 228 -1 0 CODE
VCC = 5V VDD/VSS = 12V RANGE = 10V 10k SAMPLES TA = 25C
70
SINAD (dB)
55
303 1
2
Figure 12. SINAD vs. Analog Input Frequency for Single-Ended (SE) and True Differential Mode (Diff) at 3 V VCC
Figure 15. Histogram of Codes, True Differential Mode
Rev. 0 | Page 11 of 36
05399-013
50 10
0
AD7321
8k 7k 7600
2.0
VCC = 5V VDD/VSS = 12V RANGE = 10V 10k SAMPLES TA = 25C INL ERROR (LSB)
1.5 1.0 0.5 0 -0.5 -1.0 INL = 500kSPS
NUMBER OF OCCURENCES
6k 5k 4k 3k 2k 1201 1k 0 0 -3 23 -2 -1 0 CODE 1 1165
-1.5
11 2 0 3
05399-014
-2.0
5V RANGE VCC = VDRIVE = 5V INTERNAL REFERENCE SINGLE-ENDED MODE
05399-050 05399-064 05399-054
5
7
9
11
13
15
17
19
VDD/VSS SUPPLY VOLTAGE (V)
Figure 16. Histogram of Codes, Single-Ended Mode
Figure 19. INL Error vs. Supply Voltage at 500 kSPS
-50 -55 -60 -65
-50 -55 -60 -65 100mV p-p SINE WAVE ON EACH SUPPLY NO DECOUPLING SINGLE-ENDED MODE fS = 500kSPS VCC = 5V VCC = 3V
CMRR (dB)
PSRR (dB)
-70 -75 -80 -85 -90 -95 -100 0
VCC = 5V
-70 -75 -80 -85 -90 -95
VDD = 12V
VCC = 3V DIFFERENTIAL MODE FIN = 50kHz VDD/VSS = 12V fS = 500kSPS TA = 25C 200 400 600 800 1000 1200
05399-055
VSS = -12V
-100
0
200
400
600
800
1000
1200
RIPPLE FREQUENCY (kHz)
SUPPLY RIPPLE FREQUENCY (kHz)
Figure 17. CMRR vs. Common-Mode Ripple Frequency
Figure 20. PSRR vs. Supply Ripple Frequency Without Supply Decoupling
2.0 1.5 1.0
DNL ERROR (LSB)
-50 -55 -60 -65
0.5 DNL = 500kSPS 0 -0.5 -1.0 -1.5 -2.0 5V RANGE VCC = VDRIVE = 5V INTERNAL REFERENCE SINGLE-ENDED MODE
05399-049
VCC = VDRIVE = 5V VDD/VSS = 12V TA = 25C INTERNAL REF RANGE = 10V AND 2.5V fS = 500kSPS DIFFERENTIAL MODE
THD (dB)
-70 -75 -80 -85 -90 -95
10V RANGE RIN = 4000 RIN = 3000 RIN = 2000 RIN = 1000 RIN = 100 RIN = 12 2.5V RANGE RIN = 9000 RIN = 5500 RIN = 2000 RIN = 100 RIN = 12
5
7
9
11
13
15
17
19
-100 10
100 INPUT FREQUENCY (kHz)
1000
VDD/VSS SUPPLY VOLTAGE (V)
Figure 18. DNL Error vs. Supply Voltage at 500 kSPS
Figure 21. THD vs. Analog Input Frequency for Various Source Impedances, True Differential Mode
Rev. 0 | Page 12 of 36
AD7321
-50 VCC = VDRIVE = 5V -55 VDD/VSS = 12V TA = 25C -60 INTERNAL REF RANGE = 10V AND 2.5V -65 fS = 500kSPS SINGLE-ENDED MODE
THD (dB)
-70 -75 -80 -85 -90 -95 100 INPUT FREQUENCY (kHz)
10V RANGE RIN = 4000 RIN = 2000 RIN = 1000 RIN = 100 RIN = 50 2.5V RANGE RIN = 4700 RIN = 3000 RIN = 1000 RIN = 100 RIN = 50
1000
Figure 22. THD vs. Analog Input Frequency for Various Source Impedances, Single-Ended Mode
05399-065
-100 10
Rev. 0 | Page 13 of 36
AD7321 TERMINOLOGY
Differential Nonlinearity This is the difference between the measured and the ideal 1 LSB change between any two adjacent codes in the ADC. Integral Nonlinearity This is the maximum deviation from a straight line passing through the endpoints of the ADC transfer function. The endpoints of the transfer function are zero scale (a point 1 LSB below the first code transition) and full scale (a point 1 LSB above the last code transition). Offset Code Error This applies to straight binary output coding. It is the deviation of the first code transition (00 ... 000) to (00 ... 001) from the ideal, that is, AGND + 1 LSB. Offset Error Match This is the difference in offset error between any two input channels. Gain Error This applies to straight binary output coding. It is the deviation of the last code transition (111 ... 110) to (111 ... 111) from the ideal (that is, 4 x VREF - 1 LSB, 2 x VREF - 1 LSB, VREF -1 LSB) after adjusting for the offset error. Gain Error Match This is the difference in gain error between any two input channels. Bipolar Zero Code Error This applies when using twos complement output coding and a bipolar analog input. It is the deviation of the midscale transition (all 1s to all 0s) from the ideal input voltage, that is, AGND - 1 LSB. Bipolar Zero Code Error Match This refers to the difference in bipolar zero code error between any two input channels. Positive Full-Scale Error This applies when using twos complement output coding and any of the bipolar analog input ranges. It is the deviation of the last code transition (011 ... 110) to (011 ... 111) from the ideal (4 x VREF - 1 LSB, 2 x VREF - 1 LSB, VREF - 1 LSB) after adjusting for the bipolar zero code error. Positive Full-Scale Error Match This is the difference in positive full-scale error between any two input channels. Negative Full-Scale Error This applies when using twos complement output coding and any of the bipolar analog input ranges. This is the deviation of the first code transition (10 ... 000) to (10 ... 001) from the ideal (that is, -4 x VREF + 1 LSB, -2 x VREF + 1 LSB, -VREF + 1 LSB) after adjusting for the bipolar zero code error. Negative Full-Scale Error Match This is the difference in negative full-scale error between any two input channels. Track-and-Hold Acquisition Time The track-and-hold amplifier returns into track mode after the 14th SCLK rising edge. Track-and-hold acquisition time is the time required for the output of the track-and-hold amplifier to reach its final value, within 1/2 LSB, after the end of a conversion. For the 2.5 V range, the specified acquisition time is the time required for the track-and-hold amplifier to settle to within 1 LSB. Signal to (Noise + Distortion) Ratio This is the measured ratio of signal to (noise + distortion) at the output of the A/D converter. The signal is the rms amplitude of the fundamental. Noise is the sum of all non-fundamental signals up to half the sampling frequency (fS/2), excluding dc. The ratio is dependent on the number of quantization levels in the digitization process. The more levels, the smaller the quantization noise. Theoretically, the signal to (noise + distortion) ratio for an ideal N-bit converter with a sine wave input is given by Signal to (Noise + Distortion) = (6.02 N + 1.76) dB For a 13-bit converter, this is 80.02 dB. Total Harmonic Distortion Total harmonic distortion (THD) is the ratio of the rms sum of harmonics to the fundamental. For the AD7321 it is defined as
THD(dB) = 20 log V2 2 + V3 2 + V 4 2 + V5 2 + V6 2 V1
where V1 is the rms amplitude of the fundamental, and V2, V3, V4, V5, and V6 are the rms amplitudes of the second through the sixth harmonics. Peak Harmonic or Spurious Noise Peak harmonic or spurious noise is defined as the ratio of the rms value of the next largest component in the ADC output spectrum (up to fS/2, excluding dc) to the rms value of the fundamental. Normally, the value of this specification is determined by the largest harmonic in the spectrum, but for ADCs where the harmonics are buried in the noise floor, the largest harmonic could be a noise peak.
Rev. 0 | Page 14 of 36
AD7321
Channel-to-Channel Isolation Channel-to-channel isolation is a measure of the level of crosstalk between any two channels. It is measured by applying a full-scale, 100 kHz sine wave signal to all unselected input channels and determining the degree to which the signal attenuates in the selected channel with a 50 kHz signal. Figure 14 shows the worstcase across all eight channels for the AD7321. The analog input range is programmed to be the same on all channels. Intermodulation Distortion With inputs consisting of sine waves at two frequencies, fa and fb, any active device with nonlinearities creates distortion products at sum and difference frequencies of mfa nfb, where m, n = 0, 1, 2, 3, and so on. Intermodulation distortion terms are those for which neither m nor n are equal to 0. For example, the second-order terms include (fa + fb) and (fa - fb), whereas the third-order terms include (2fa + fb), (2fa - fb), (fa + 2fb), and (fa - 2fb). The AD7321 is tested using the CCIF standard where two input frequencies near the top end of the input bandwidth are used. In this case, the second-order terms are usually distanced in frequency from the original sine waves, whereas the third-order terms are usually at a frequency close to the input frequencies. As a result, the second- and third-order terms are specified separately. The calculation of the intermodulation distortion is per the THD specification, where it is the ratio of the rms sum of the individual distortion products to the rms amplitude of the sum of the fundamentals expressed in decibels. PSR (Power Supply Rejection) Variations in power supply affect the full-scale transition but not the linearity of the converter. Power supply rejection is the maximum change in the full-scale transition point due to a change in power supply voltage from the nominal value (see the Typical Performance Characteristics section). CMRR (Common-Mode Rejection Ratio) CMRR is defined as the ratio of the power in the ADC output at full-scale frequency, f, to the power of a 100 mV sine wave applied to the common-mode voltage of the VIN+ and VIN- frequency, fS, as CMRR (dB) = 10 log (Pf/PfS) where Pf is the power at frequency f in the ADC output, and PfS is the power at frequency fS in the ADC output (see Figure 17).
Rev. 0 | Page 15 of 36
AD7321 THEORY OF OPERATION
CIRCUIT INFORMATION
The AD7321 is a fast, 2-channel, 12-bit plus sign, bipolar input, serial A/D converter. The AD7321 can accept bipolar input ranges that include 10 V, 5 V, and 2.5 V; it can also accept a 0 V to +10 V unipolar input range. A different analog input range can be programmed on each analog input channel via the on-chip registers. The AD7321 has a high speed serial interface that can operate at throughput rates up to 500 kSPS. The AD7321 requires VDD and VSS dual supplies for the high voltage analog input structures. These supplies must be equal to or greater than the largest analog input range selected. See Table 6 for the requirements of these supplies for each analog input range. The AD7321 requires a low voltage 2.7 V to 5.25 V VCC supply to power the ADC core. Table 6. Reference and Supply Requirements for Each Analog Input Range
Selected Analog Input Range (V) 10 5 2.5 0 to +10 FullScale Input Range (V) 10 12 5 6 2.5 3 0 to +10 0 to +12
The analog inputs can be configured as two single-ended inputs, one true differential input pair, or one pseudo differential input. Selection can be made by programming the mode bits, Mode 0 and Mode 1, in the control register. The serial clock input accesses data from the part and provides the clock source for the successive approximation ADC. The AD7321 has an on-chip 2.5 V reference. However, the AD7321 can also work with an external reference. On power-up, the external reference operation is the default option. If the internal reference is the preferred option, the user must write to the reference bit in the control register to select the internal reference operation. The AD7321 also features power-down options to allow power savings between conversions. The power-down modes are selected by programming the on-chip control register, as described in the Modes of Operation section.
CONVERTER OPERATION
AVCC (V) 3/5 3/5 3/5 3/5 3/5 3/5 3/5 3/5 Minimum VDD/VSS (V) 10 12 5 6 5 5 +10/AGND +12/AGND
Reference Voltage (V) 2.5 3.0 2.5 3.0 2.5 3.0 2.5 3.0
The AD7321 is a successive approximation analog-to-digital converter built around two capacitive DACs. Figure 23 and Figure 24 show simplified schematics of the ADC in singleended mode during the acquisition and conversion phases, respectively. Figure 25 and Figure 26 show simplified schematics of the ADC in differential mode during acquisition and conversion phases, respectively. The ADC is composed of control logic, a SAR, and capacitive DACs. In Figure 23 (the acquisition phase), SW2 is closed and SW1 is in Position A, the comparator is held in a balanced condition, and the sampling capacitor array acquires the signal on the input.
CAPACITIVE DAC B CS SW2 COMPARATOR CONTROL LOGIC
It may be necessary to decrease the throughput rate when the AD7321 is configured with the minimum VDD and VSS supplies in order to meet the performance specifications (see the Typical Performance Characteristics section). Figure 31 shows the change in THD as the VDD and VSS supplies are reduced. For ac performance at the maximum throughput rate, the THD degrades slightly as VDD and VSS are reduced. It might therefore be necessary to reduce the throughput rate when using minimum VDD and VSS supplies so that there is less degradation of THD and the specified performance can be maintained. The degradation is due to an increase in the on resistance of the input multiplexer when the VDD and VSS supplies are reduced. Figure 18 and Figure 19 show the change in INL and DNL as the VDD and VSS voltages are varied. For dc performance when operating at the maximum throughput rate, as the VDD and VSS supply voltages are reduced, the typical INL and DNL error remains constant.
VIN0
A SW1 AGND
Figure 23. ADC Acquisition Phase (Single-Ended)
When the ADC starts a conversion (Figure 24), SW2 opens and SW1 moves to Position B, causing the comparator to become unbalanced. The control logic and the charge redistribution DAC are used to add and subtract fixed amounts of charge from the capacitive DAC to bring the comparator back into a balanced condition. When the comparator is rebalanced, the conversion is complete. The control logic generates the ADC output code.
Rev. 0 | Page 16 of 36
05399-017
AD7321
CAPACITIVE DAC B CS SW2 COMPARATOR CONTROL LOGIC
VIN0
A SW1 AGND
The ideal transfer characteristic for the AD7321 when twos complement coding is selected is shown in Figure 27. The ideal transfer characteristic for the AD7321 when straight binary coding is selected is shown in Figure 28.
05399-018
Figure 24. ADC Conversion Phase (Single-Ended)
011...111 011...110
ADC CODE
Figure 25 shows the differential configuration during the acquisition phase. For the conversion phase, SW3 opens and SW1 and SW2 move to Position B (see Figure 26). The output impedances of the source driving the VIN+ and VIN- pins must match; otherwise, the two inputs have different settling times, resulting in errors.
CAPACITIVE DAC B CS SW3 COMPARATOR CONTROL LOGIC
000...001 000...000 111...111
100...010 100...001 100...000
05399-021 05399-022
-FSR/2 + 1LSB AGND + 1LSB
AGND - 1LSB
+FSR/2 - 1LSB BIPOLAR RANGES +FSR - 1LSB UNIPOLAR RANGE ANALOG INPUT
Figure 27. Twos Complement Transfer Characteristic
VIN+ VIN-
A SW1 A SW2 B VREF CS
111...111 111...110
ADC CODE
05399-019
CAPACITIVE DAC
111...000 011...111
Figure 25. ADC Differential Configuration During Acquisition Phase
CAPACITIVE DAC B CS SW3 COMPARATOR CONTROL LOGIC
000...010 000...001 000...000 -FSR/2 + 1LSB +FSR/2 - 1LSB BIPOLAR RANGES AGND + 1LSB +FSR - 1LSB UNIPOLAR RANGE ANALOG INPUT
VIN+ VIN-
A SW1 A SW2 B VREF CS
Figure 28. Straight Binary Transfer Characteristic
05399-020
CAPACITIVE DAC
ANALOG INPUT STRUCTURE
The analog inputs of the AD7321 can be configured as singleended, true differential, or pseudo differential via the control register mode bits (see Table 9). The AD7321 can accept true bipolar input signals. On power-up, the analog inputs operate as two single-ended analog input channels. If true differential or pseudo differential is required, a write to the control register is necessary after power-up to change this configuration. Figure 29 shows the equivalent analog input circuit of the AD7321 in single-ended mode. Figure 30 shows the equivalent analog input structure in differential mode. The two diodes provide ESD protection for the analog inputs.
VDD D VIN0 C1 D VSS R1 C2
Figure 26. ADC Differential Configuration During Conversion Phase
Output Coding
The AD7321 default output coding is set to twos complement. The output coding is controlled by the coding bit in the control register. To change the output coding to straight binary coding, the coding bit in the control register must be set. When operating in sequence mode, the output coding for each channel in the sequence is the value written to the coding bit during the last write to the control register.
Transfer Functions
The designed code transitions occur at successive integer LSB values (that is, 1 LSB, 2 LSB, and so on). The LSB size is dependent on the analog input range selected. Table 7. LSB Sizes for Each Analog Input Range
Input Range 10 V 5 V 2.5 V 0 V to +10 V Full-Scale Range/8192 Codes 20 V 10 V 5V 10 V LSB Size 2.441 mV 1.22 mV 0.61 mV 1.22 mV
Figure 29. Equivalent Analog Input Circuit (Single-Ended)
Rev. 0 | Page 17 of 36
05399-023
AD7321
VDD D VIN+ C1 D VSS VDD D VIN- C1 D VSS R1 C2 R1 C2
The AD7321 enters track mode on the 14th SCLK rising edge. When running the AD7321 at a throughput rate of 500 kSPS with a 10 MHz SCLK signal, the ADC has approximately 1.5 SCLK + t8 + tQUIET to acquire the analog input signal. The ADC goes back into hold mode on the CS falling edge. As the VDD/VSS supply voltage is reduced, the on resistance of the input multiplexer increases. Therefore, based on the equation for tACQ, it is necessary to increase the amount of acquisition time provided to the AD7321, and hence decrease the overall throughput rate. Figure 31 shows that as the VDD and VSS supplies are reduced, the specified THD performance degrades slightly. If the throughput rate is reduced when operating with the minimum VDD and VSS supplies, the specified THD performance is maintained.
-75 VCC = VDRIVE = 5V INTERNAL REFERENCE TA = 25C FIN = 10kHz 5V RANGE SE MODE
Figure 30. Equivalent Analog Input Circuit (Differential)
Care should be taken to ensure that the analog input does not exceed the VDD and VSS supply rails by more than 300 mV. Exceeding this value causes the diodes to become forward biased and to start conducting into either the VDD supply rail or VSS supply rail. These diodes can conduct up to 10 mA without causing irreversible damage to the part. In Figure 29 and Figure 30, Capacitor C1 is typically 4 pF and can primarily be attributed to pin capacitance. Resistor R1 is a lumped component made up of the on resistance of the input multiplexer and the track-and-hold switch. Capacitor C2 is the sampling capacitor; its capacitance varies depending on the analog input range selected (see the Specifications section).
05399-024
-80
THD (dB)
-85
Track-and-Hold Section
The track-and-hold on the analog input of the AD7321 allows the ADC to accurately convert an input sine wave of full-scale amplitude to 13-bit accuracy. The input bandwidth of the trackand-hold is greater than the Nyquist rate of the ADC. The AD7321 can handle frequencies up to 22 MHz. The track-and-hold enters its tracking mode on the 14th SCLK rising edge after the CS falling edge. The time required to acquire an input signal depends on how quickly the sampling capacitor is charged. With 0 source impedance, 305 ns is sufficient to acquire the signal to the 13-bit level. The acquisition time required is calculated using the following formula: tACQ = 10 x ((RSOURCE + R) C) where C is the sampling capacitance and R is the resistance seen by the track-and-hold amplifier looking back on the input. For the AD7321, the value of R includes the on resistance of the input multiplexer and is typically 300 . RSOURCE should include any extra source impedance on the analog input.
-90 500kSPS
-95
VDD/VSS SUPPLIES (V)
Figure 31. THD vs. VDD/VSS Supply Voltage at 500 kSPS
Unlike other bipolar ADCs, the AD7321 does not have a resistive analog input structure. On the AD7321, the bipolar analog signal is sampled directly onto the sampling capacitor. This gives the AD7321 high analog input impedance. An approximation for the analog input impedance can be calculated from the following formula: Z = 1/(fS x CS) where fS is the sampling frequency, and CS is the sampling capacitor value. CS depends on the analog input range chosen (see the Specifications section). When operating at 500 kSPS, the analog input impedance is typically 145 k for the 10 V range. As the sampling frequency is reduced, the analog input impedance further increases. As the analog input impedance increases, the current required to drive the analog input therefore decreases.
Rev. 0 | Page 18 of 36
05399-051
5
7
9
11
13
15
17
19
AD7321
TYPICAL CONNECTION DIAGRAM
Figure 32 shows a typical connection diagram for the AD7321. In this configuration, the AGND pin is connected to the analog ground plane of the system, and the DGND pin is connected to the digital ground plane of the system. The analog inputs on the AD7321 can be configured to operate in single-ended, true differential, or pseudo differential mode. The AD7321 can operate with either an internal or external reference. In Figure 32, the AD7321 is configured to operate with the internal 2.5 V reference. A 680 nF decoupling capacitor is required when operating with the internal reference. The VCC pin can be connected to either a 3 V supply voltage or a 5 V supply voltage. The VDD and VSS are the dual supplies for the high voltage analog input structures. The voltage on these pins must be equal to or greater than the highest analog input range selected on the analog input channels (see Table 6). The VDRIVE pin is connected to the supply voltage of the microprocessor. The voltage applied to the VDRIVE input controls the voltage of the serial interface. VDRIVE can be set to 3 V or 5 V.
+15V 0.1F + 10F 10F + 0.1F VCC + 2.7V TO 5.25V
V+
5V
AGND
VIN+
VDD VCC
AD73211
VSS
1ADDITIONAL PINS OMITTED FOR CLARITY.
Figure 33. Single-Ended Mode Typical Connection Diagram
True Differential Mode
The AD7321 can have one true differential analog input pair. Differential signals have some benefits over single-ended signals, including better noise immunity based on the device's common-mode rejection and improvements in distortion performance. Figure 34 defines the configuration of the true differential analog inputs of the AD7321.
VIN+
AD73211
VIN-
05399-027
VDD1
VCC VDRIVE 10F + 0.1F
+3V SUPPLY
1ADDITIONAL
AD7321
CS DOUT ANALOG INPUTS 10V, 5V, 2.5V 0V TO +10V VIN0 VIN1 SCLK DIN C/P
PINS OMITTED FOR CLARITY.
Figure 34. True Differential Inputs
DGND 680nF REFIN/OUT VSS1 AGND
SERIAL INTERFACE
-15V
05399-025
ANALOG INPUT
Single-Ended Inputs
The AD7321 has a total of two analog inputs when operating the AD7321 in single-ended mode. Each analog input can be independently programmed to one of the four analog input ranges. In applications where the signal source is high impedance, it is recommended to buffer the signal before applying it to the ADC analog inputs. Figure 33 shows the configuration of the AD7321 in single-ended mode.
+
0.1F
10F
1MINIMUM
VDD AND VSS SUPPLY VOLTAGES DEPEND ON THE HIGHEST ANALOG INPUT RANGE SELECTED.
The amplitude of the differential signal is the difference between the signals applied to the VIN+ and VIN- pins in each differential pair (VIN+ - VIN-). VIN+ and VIN- should be simultaneously driven by two signals each of amplitude 4 x VREF (depending on the input range selected) that are 180 out of phase. Assuming the 4 x VREF mode, the amplitude of the differential signal is -20 V to +20 V p-p (2 x 4 x VREF), regardless of the common mode. The common mode is the average of the two signals (VIN+ + VIN-)/2 and is therefore the voltage on which the two input signals are centered. This voltage is set up externally, and its range varies with reference voltage. As the reference voltage increases, the common-mode range decreases. When driving the differential inputs with an amplifier, the actual common-mode range is determined by the amplifier's output swing. If the differential inputs are not driven from an amplifier, the common-mode range is determined by the supply voltage on the VDD supply pin and the VSS supply pin. When a conversion takes place, the common mode is rejected, resulting in a noise-free signal of amplitude -2 x (4 x VREF) to +2 x (4 x VREF) corresponding to Digital Codes -4096 to +4095.
Figure 32. Typical Connection Diagram
Rev. 0 | Page 19 of 36
05399-026
V-
AD7321
5 4 3 2 5V RANGE 2.5V RANGE 5V RANGE 6 4 10V RANGE 2.5V RANGE 8 5V RANGE
VCOM RANGE (V)
1 0 -1 -2 -3 -4 -5 -6 16.5V VDD/VSS 12V VDD/VSS VCC = 3V VREF = 3V
05399-045
VCOM RANGE (V)
2 0 -2 -4 -6 -8
10V RANGE
10V RANGE 10V RANGE
2.5V RANGE
5V RANGE VCC = 5V VREF = 2.5V 16.5V VDD/VSS 12V VDD/VSS 2.5V RANGE
Figure 35. Common-Mode Range for VCC = 3 V and REFIN/OUT = 3 V
8 5V RANGE 2.5V RANGE 5V RANGE 2.5V RANGE
Figure 38. Common-Mode Range for VCC = 5 V and REFIN/OUT = 2.5 V
Pseudo Differential Inputs
The AD7321 can have one pseudo differential pair. The VIN+ inputs are coupled to the signal source and must have an amplitude within the selected range for that channel as programmed in the range register. A dc input is applied to the VIN- pin. The voltage applied to this input provides an offset for the VIN+ input from ground or a pseudo ground. Pseudo differential inputs separate the analog input signal ground from the ADC ground, allowing cancellation of dc common-mode voltages. When a conversion takes place, the pseudo ground corresponds to Code -4096 and the maximum amplitude corresponds to Code +4095.
V+ 5V
6
VCOM RANGE (V)
4
10V RANGE
2 10V RANGE 0
-2 VCC = 5V VREF = 3V -4 16.5V VDD/VSS 12V VDD/VSS
05399-046
Figure 36. Common-Mode Range for VCC = 5 V and REFIN/OUT = 3 V
6 4 2
VIN+
5V RANGE 5V RANGE
VDD VCC
AD73211
VIN- VSS
VCOM RANGE (V)
0 -2 -4 -6 -8 16.5V VDD/VSS 12V VDD/VSS 10V RANGE
2.5V RANGE
1ADDITIONAL PINS OMITTED FOR CLARITY.
Figure 39. Pseudo Differential Inputs
VCC = 3V VREF = 2.5V
Figure 37. Common-Mode Range for VCC = 3 V and REFIN/OUT = 2.5 V
Figure 40 and Figure 41 show the typical voltage range on the VIN- pin for the different analog input ranges when configured in the pseudo differential mode. For example, when the AD7321 is configured to operate in pseudo differential mode and the 5 V range is selected with 16.5 V VDD/VSS supplies and 5 V VCC, the voltage on the VIN- pin can vary from -6.5 V to +6.5 V.
05399-047
Rev. 0 | Page 20 of 36
05399-028
10V 2.5V RANGE RANGE
V-
05399-048
AD7321
8 6 4 2 0 -2 -4 -6 -8 5V RANGE 2.5V RANGE 10V RANGE 5V RANGE 2.5V RANGE
10V RANGE
The driver amplifier must be able to settle for a full-scale step to a 13-bit level, 0.0122%, in less than the specified acquisition time of the AD7321. An op amp such as the AD8021 meets this requirement when operating in single-ended mode. The AD8021 needs an external compensating NPO type of capacitor. The AD8022 can also be used in high frequency applications where a dual version is required. For lower frequency applications, op amps such as the AD797, AD845, and AD8610 can be used with the AD7321 in single-ended mode configuration. Differential operation requires that VIN+ and VIN- be simultaneously driven with two signals of equal amplitude that are 180 out of phase. The common mode must be set up externally to the AD7321. The common-mode range is determined by the REFIN/OUT voltage, the VCC supply voltage, and the particular amplifier used to drive the analog inputs. Differential mode with either an ac input or a dc input provides the best THD performance over a wide frequency range. Because not all applications have a signal preconditioned for differential operation, there is often a need to perform the single-ended-todifferential conversion. This single-ended-to-differential conversion can be performed using an op amp pair. Typical connection diagrams for an op amp pair are shown in Figure 42 and Figure 43. In Figure 42, the common-mode signal is applied to the noninverting input of the second amplifier.
05399-040
16.5V VDD/VSS
12V VDD/VSS
Figure 40. Pseudo Input Range with VCC = 5 V
4 5V RANGE 2 5V RANGE 2.5V RANGE
0
-2 2.5V RANGE
10V RANGE 10V RANGE
-4
-6 VCC = 3V VREF = 2.5V -8
0V TO +10V RANGE
0V TO +10V RANGE
05399-039
VCC = 5V VREF = 2.5V
0V TO +10V RANGE
0V TO +10V RANGE
16.5V VDD/VSS
12V VDD/VSS
1.5k 3k
Figure 41. Pseudo Input Range with VCC = 3 V
VIN
AD845
V+
DRIVER AMPLIFIER CHOICE
In applications where the harmonic distortion and signal-tonoise ratio are critical specifications, the analog input of the AD7321 should be driven from a low impedance source. Large source impedances significantly affect the ac performance of the ADC and can necessitate the use of an input buffer amplifier. When no amplifier is used to drive the analog input, the source impedance should be limited to low values. The maximum source impedance depends on the amount of THD that can be tolerated in the application. The THD increases as the source impedance increases and performance degrades. Figure 21 and Figure 22 show graphs of the THD vs. the analog input frequency for various source impedances. Depending on the input range and analog input configuration selected, the AD7321 can handle source impedances of up to 5.5 k before the THD starts to degrade. Due to the programmable nature of the analog inputs on the AD7321, the choice of op amp used to drive the inputs is a function of the particular application and depends on the input configuration and the analog input voltage ranges selected.
100
1.5k 1.5k 1.5k
V- VCOM 10k
AD845
20k
05399-029
Figure 42. Single-Ended-to-Differential Configuration with the AD845
442 442
VIN
AD8021
V+
442 442 442 442
V-
AD8021
05399-030
Figure 43. Single-Ended-to-Differential Configuration with the AD8021
Rev. 0 | Page 21 of 36
AD7321 REGISTERS
The AD7321 has two programmable registers, the control register and the range register. These registers are write-only registers.
ADDRESSING REGISTERS
A serial transfer on the AD7321 consists of 16 SCLK cycles. The three MSBs on the DIN line during the 16 SCLK transfer are decoded to determine which register is addressed. The three MSBs consist of the write bit, a ZERO bit and the register select bit. The register select bit is used to determine which of the two on-board registers is selected. The write bit determines if the data on the DIN line following the register select bit loads into the addressed register. If the write bit is 1, the bits load into the register addressed by the register select bit. If the write bit is 0, the data on the DIN line does not load into any register. Table 8. Decoding Register Select Bits and Write Bit
Write 0 1 1 ZERO 0 0 0 Register Select 0 0 1 Description Data on the DIN line during this serial transfer is ignored. This combination selects the control register. The subsequent 12 bits are loaded into the control register. This combination selects the range register. The subsequent 6 bits are loaded into the range register.
Rev. 0 | Page 22 of 36
AD7321
CONTROL REGISTER
The control register is used to select the analog input channel, analog input configuration, reference, coding, and power mode. The control register is a write-only, 12-bit register. Data loaded on the DIN line corresponds to the AD7321 configuration for the next conversion. Data should be loaded into the control register after the range register has been initialized. The bit functions of the control register are shown in Table 9 (the power-up status of all bits is 0).
MSB 15 Write
14 ZERO
13 Register Select
12 ZERO
11 ZERO
10 ADD0
9 Mode 1
8 Mode 0
7 PM1
6 PM0
5 Coding
4 Ref
3 Seq1
2 Seq2
1 ZERO
LSB 0 0
Table 9. Control Register Details
Bit 12, 11, 1 10 Mnemonic ZERO ADD0 Description A 0 should be written to these bits at all times. This channel address bit is used to select the analog input channel for the next conversion if the sequencer is not being used. If the sequencer is being used, the channel address bit is used to select the final channel in a consecutive sequence. These two mode bits are used to select the configuration of the two analog input pins, VIN0 to VIN1. These pins are used in conjunction with the channel address bit. On the AD7321, the analog inputs can be configured as two single-ended inputs, one true differential input pair, or one pseudo differential input (see Table 10). The power management bits are used to select different power mode options on the AD7321 (see Table 11). This bit is used to select the type of output coding the AD7321 uses for the next conversion result. If coding = 0, the output coding is twos complement. If coding = 1, the output coding is straight binary. When operating in sequence mode, the output coding for each channel is the value written to the coding bit during the last write to the control register. The reference bit is used to enable or disable the internal reference. If Ref = 0, the external reference is enabled and used for the next conversion, and the internal reference is disabled. If Ref = 1, the internal reference is used for the next conversion. When operating in sequence mode, the reference used for each channel is the value written to the Ref bit during the last write to the control register. The Sequence 1 and Sequence 2 bits are used to control the operation of the sequencer (see Table 12).
9, 8
Mode 1, Mode 0
7, 6 5
PM1, PM0 Coding
4
Ref
3, 2
Seq1/Seq2
The two analog input channels can be configured as one pseudo differential analog input, one true differential input pair, or two singleended analog inputs. Table 10. Analog Input Configuration Selection
Channel Address Bit ADD0 0 1 Mode 1 = 1, Mode 0 = 1 Not Allowed VIN+ VIN- - - - - Mode 1 = 1, Mode 0 = 0 1 Fully Differential I/P VIN+ VIN- VIN0 VIN1 VIN0 VIN1 Mode 1 = 0, Mode 0 =1 1 Pseudo Differential I/P VIN+ VIN- VIN0 VIN1 VIN0 VIN1 Mode 1 = 0, Mode 0 = 0 2 Single-Ended I/Ps VIN+ VIN- VIN0 AGND VIN1 AGND
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AD7321
Table 11. Power Mode Selection
PM1 1 1 0 0 PM0 1 0 1 0 Description Full Shutdown Mode. In this mode, all internal circuitry on the AD7321 is powered down. Information in the control register is retained when the AD7321 is in full shutdown mode. Autoshutdown Mode. The AD7321 enters autoshutdown on the 15th SCLK rising edge when the control register is updated. All internal circuitry is powered down in autoshutdown. Autostandby Mode. In this mode, all internal circuitry is powered down, excluding the internal reference. The AD7321 enters autostandby mode on the 15th SCLK rising edge after the control register is updated. Normal Mode. All internal circuitry is powered up at all times.
Table 12. Sequencer Selection
Seq1 0 1 Seq2 0 0 Description The channel sequencer is not used. The analog channel, selected by programming the ADD0 bit in the control register, selects the next channel for conversion. This configuration is used in conjunction with the channel address bit in the control register. This allows continuous conversions on a consecutive sequence of channels, from Channel 0 through a final channel selected by the channel address bit in the control register. The range for each channel defaults to the range previously written into the range register. The channel sequencer is not used. The analog channel, selected by programming the ADD0 bit in the control register, selects the next channel for conversion.
1
1
Rev. 0 | Page 24 of 36
AD7321
RANGE REGISTER
The range register is used to select one analog input range per analog input channel and to set the ranges for Channel 0 and Channel 1. It is a 6-bit, write-only register with two dedicated range bits for each of the analog input channels, Channel 0 and Channel 1. There are four analog input ranges, 10 V, 5 V, 2.5 V, and 0 V to +10 V. A write to the range register is selected by setting the write bit to 1, the ZERO bit to 0, and the register select bit to 1. After the initial write to the range register occurs, each time an analog input is selected, the AD7321 automatically configures the analog input to the appropriate range, as indicated by the range register. The 10 V input range is selected by default on each analog input channel (see Table 13).
MSB 16 Write
15 ZERO
14 Register Select
13 VIN0A
12 VIN0B
11 0
10 0
9 VIN1A
8 VIN1B
7 0
6 0
5 0
4 0
3 0
2 0
LSB 1 0
Table 13. Range Selection
VINxA 0 0 1 1 VINxB 0 1 0 1 Description This combination selects the 10 V input range on VINx. This combination selects the 5 V input range on VINx. This combination selects the 2.5 V input range on VINx. This combination selects the 0 V to +10 V input range on VINx.
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AD7321 SEQUENCER OPERATION
The AD7321 can be configured to convert a sequence of consecutive channels (see Figure 44). This sequence begins by converting on Channel 0 and ends with a final channel as selected by Bit ADD0 in the control register. In this configuration, there is no need for a write to the sequence register. To operate the AD7321 in this mode, set Seq1 to 1 and Seq2 to 0, and then select the final channel in the sequence by programming Bit ADD0 in the control register. Once the control register is configured to operate the AD7321 in this mode, the DIN line can be held low, or the write bit can be set to 0. To return to traditional multichannel operation, a write to the control register to set Seq1 to 0 and Seq2 to 0 is necessary. When Seq1 and Seq2 are both set to 0, or when both are set to 1, the AD7321 is configured to operate in traditional multichannel mode, where a write to Channel Address Bit ADD0 in the control register selects the next channel for conversion.
POWER ON. CS DIN: WRITE TO RANGE REGISTER TO SELECT THE RANGE FOR THE ANALOG INPUT CHANNELS. DOUT: CONVERSION RESULT FROM CHANNEL 0, 10V RANGE, SINGLE-ENDED MODE. CS DIN: WRITE TO CONTROL REGISTER TO SELECT THE FINAL CHANNEL IN THE CONSECUTIVE SEQUENCE, SET Seq1 = 1 AND Seq2 = 0. SELECT OUTPUT CODING FOR SEQUENCE. DOUT: CONVERSION RESULT FROM CHANNEL 0, RANGE SELECTED IN RANGE REGISTER 1, SINGLE-ENDED MODE. CS DIN: WRITE BIT = 0 OR DIN LINE HELD LOW TO CONTINUE TO CONVERT THROUGH THE SEQUENCE OF CONSECUTIVE CHANNELS. DOUT: CONVERSION RESULT FROM CHANNEL 0, RANGE AS SELECTED IN RANGE REGISTER. CS DIN: WRITE BIT = 0 OR DIN LINE HELD LOW TO CONTINUE THROUGH SEQUENCE OF CONSECUTIVE CHANNELS. DOUT: CONVERSION RESULT FROM CHANNEL1, RANGE AS SELECTED IN RANGE REGISTER. DIN TIED LOW/WRITE BIT = 0.
STOPPING A SEQUENCE. CS DIN: WRITE TO CONTROL REGISTER TO STOP THE SEQUENCE, Seq1 = 0, Seq2 = 0.
CONTINUOUSLY CONVERT ON CONSECUTIVE SEQUENCE OF CHANNELS.
Figure 44. Flowchart for Consecutive Sequence of Channels
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05399-032
DOUT: CONVERSION RESULT FROM CHANNEL IN SEQUENCE.
AD7321
REFERENCE
The AD7321 can operate with either the internal 2.5 V on-chip reference or an externally applied reference. The internal reference is selected by setting the Ref bit in the control register to 1. On power-up, the Ref bit is 0, which selects the external reference for the AD7321 conversion. Suitable reference sources for the AD7321 include AD780, AD1582, ADR431, REF193, and ADR391. The internal reference circuitry consists of a 2.5 V band gap reference and a reference buffer. When operating the AD7321 in internal reference mode, the 2.5 V internal reference is available at the REFIN/OUT pin, which should be decoupled to AGND using a 680 nF capacitor. It is recommended that the internal reference be buffered before applying it elsewhere in the system. The internal reference is capable of sourcing up to 90 A. On power-up, if the internal reference operation is required for the ADC conversion, a write to the control register is necessary to set the Ref bit to 1. During the control register write, the conversion result from the first initial conversion is invalid. The reference buffer requires 500 s to power up and charge the 680 nF decoupling capacitor during the power-up time. The AD7321 is specified for a 2.5 V to 3 V reference range. When a 3 V reference is selected, the ranges are 12 V, 6 V, 3 V, and 0 V to +12 V. For these ranges, the VDD and VSS supply must be equal to or greater than the maximum analog input range selected (see Table 6).
VDRIVE
The AD7321 has a VDRIVE feature to control the voltage at which the serial interface operates. VDRIVE allows the ADC to easily interface to both 3 V and 5 V processors. For example, if the AD7321 is operated with a VCC of 5 V, the VDRIVE pin can be powered from a 3 V supply. This allows the AD7321 to accept large bipolar input signals with low voltage digital processing.
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AD7321 MODES OF OPERATION
The AD7321 has several modes of operation that are designed to provide flexible power management options. These options can be chosen to optimize the power dissipation/throughput rate ratio for different application requirements. The mode of operation of the AD7321 is controlled by the power management bits, Bit PM1 and Bit PM0, in the control register as shown in Table 11. The default mode is normal mode, where all internal circuitry is fully powered up. The AD7321 remains fully powered up at the end of the conversion if both PM1 and PM0 contain 0 in the control register. To complete the conversion and access the conversion result, 16 serial clock cycles are required. At the end of the conversion, CS can idle either high or low until the next conversion. Once the data transfer is complete, another conversion can be initiated after the quiet time, tQUIET, has elapsed.
NORMAL MODE
(PM1 = PM0 = 0) This mode is intended for the fastest throughput rate performance with the AD7321 being fully powered up at all times. Figure 45 shows the general operation of the AD7321 in normal mode. The conversion is initiated on the falling edge of CS, and the track-and-hold section enters hold mode as described in the Serial Interface section. Data on the DIN line during the 16 SCLK transfer is loaded into one of the on-chip registers if the write bit is set. The register is selected by programming the register select bits (see Table 8).
CS 1 SCLK 16
FULL SHUTDOWN MODE
(PM1 = PM0 = 1) In this mode, all internal circuitry on the AD7321 is powered down. The part retains information in the registers during full shutdown. The AD7321 remains in full shutdown mode until the power management bits, Bit PM1 and Bit PM0, in the control register are changed. A write to the control register with PM1 = 1 and PM0 = 1 places the part into full shutdown mode. The AD7321 enters full shutdown mode on the 15th SCLK rising edge once the control register is updated. If a write to the control register occurs while the part is in full shutdown mode with the power management bits, Bit PM1 and Bit PM0, set to 0 (normal mode), the part begins to power up on the 15th SCLK rising edge once the control register is updated. Figure 46 shows how the AD7321 is configured to exit full shutdown mode. To ensure the AD7321 is fully powered up, tPOWER-UP should elapse before the next CS falling edge.
DOUT
2 LEADING ZEROS, CHANNEL I.D. BIT, SIGN BIT + CONVERSION RESULT DATA INTO CONTROL/RANGE REGISTER
05399-035
DIN
Figure 45. Normal Mode
PART IS IN FULL SHUTDOWN
THE PART IS FULLY POWERED UP ONCE tPOWER-UP HAS ELAPSED PART BEGINS TO POWER UP ON THE 15TH SCLK RISING EDGE AS PM1 = PM0 = 0
tPOWER-UP
CS 1 SCLK 16 1 16
SDATA
INVALID DATA
CHANNEL IDENTIFIER BITS + CONVERSION RESULT
DIN
DATA INTO CONTROL REGISTER CONTROL REGISTER IS LOADED ON THE FIRST 15 CLOCKS, PM1 = 0, PM0 = 0
DATA INTO CONTROL REGISTER TO KEEP THE PART IN NORMAL MODE, LOAD PM1 = PM0 = 0 IN CONTROL REGISTER
05399-041
Figure 46. Exiting Full Shutdown Mode
Rev. 0 | Page 28 of 36
AD7321
AUTO SHUTDOWN MODE
(PM1 = 1, PM0 = 0) Once the autoshutdown mode is selected, the AD7321 automatically enters shutdown on the 15th SCLK rising edge. In autoshutdown mode, all internal circuitry is powered down. The AD7321 retains information in the registers during autoshutdown. The track-and-hold is in hold mode during autoshutdown. On the rising CS edge, the track-and-hold, which was in hold during shutdown, returns to track as the AD7321 begins to power up. The power-up from autoshutdown is 500 s. When the control register is programmed to transition to autoshutdown mode, it does so on the 15th SCLK rising edge. Figure 47 shows the part entering autoshutdown mode. The AD7321 automatically begins to power up on the CS rising edge. The tPOWER-UP is required before a valid conversion, initiated by bringing the CS signal low, can take place. Once this valid conversion is complete, the AD7321 powers down again on the 15th SCLK rising edge. The CS signal must remain low again to keep the part in autoshutdown mode. As is the case with autoshutdown mode, the AD7321 enters standby on the 15th SCLK rising edge once the control register is updated (see Figure 47). The part retains information in the registers during standby. The AD7321 remains in standby until it receives a CS rising edge. The ADC begins to power up on the CS rising edge. On the CS rising edge, the track-and-hold, which was in hold mode while the part was in standby, returns to track. The power-up time from standby is 700 ns. The user should ensure that 700 ns have elapsed before bringing CS low to attempt a valid conversion. Once this valid conversion is complete, the AD7321 again returns to standby on the 15th SCLK rising edge. The CS signal must remain low to keep the part in standby mode. Figure 47 shows the part entering autoshutdown mode. The sequence of events is the same when entering autostandby mode. In Figure 47, the power management bits are configured for autoshutdown. For autostandby mode, the power management bits, PM1 and PM0, should be set to 0 and 1, respectively.
AUTOSTANDBY MODE
(PM1 = 0, PM0 =1) In autostandby mode, portions of the AD7321 are powered down, but the on-chip reference remains powered up. The reference bit in the control register should be 1 to ensure that the on-chip reference is enabled. This mode is similar to autoshutdown but allows the AD7321 to power up much faster, which allows faster throughput rates.
PART BEGINS TO POWER UP ON CS RISING EDGE PART ENTERS SHUTDOWN MODE ON THE 15TH RISING SCLK EDGE AS PM1 = 1, PM0 = 0
THE PART IS FULLY POWERED UP ONCE tPOWER-UP HAS ELAPSED
tPOWER-UP
CS 1 SCLK
15 16
1
15 16
SDATA
VALID DATA
VALID DATA
DIN
DATA INTO CONTROL REGISTER CONTROL REGISTER IS LOADED ON THE FIRST 15 CLOCKS, PM1 = 1, PM0 = 0
DATA INTO CONTROL REGISTER
05399-042
Figure 47. Entering Autoshutdown/Autostandby Mode
Rev. 0 | Page 29 of 36
AD7321
POWER VS. THROUGHPUT RATE
The power consumption of the AD7321 varies with throughput rate. The static power consumed by the AD7321 is very low, and significant power savings can be achieved as the throughput rate is reduced. Figure 48 and Figure 49 shows the power vs. throughput rate for the AD7321 at a VCC of 3 V and 5 V, respectively. Both plots clearly show that the average power consumed by the AD7321 is greatly reduced as the sample frequency is reduced. This is true whether a fixed SCLK value is used or if it is scaled with the sampling frequency. Figure 48 and Figure 49 show the power consumption when operating in normal mode for a variable SCLK that scales with the sampling frequency.
12 VCC = 3V VDD/VSS = 12V TA = 25C 10 INTERNAL REFERENCE
20 18 16 VCC = 5V VDD/VSS = 12V TA = 25C INTERNAL REFERENCE
AVERAGE POWER (mW)
14 12 10 8 6 4 2 0 100 200 300 400 500
05399-053
VARIABLE SCLK
0
THROUGHPUT RATE (kSPS)
Figure 49. Power vs. Throughput Rate with 5 V VCC
AVERAGE POWER (mW)
8
6
4
VARIABLE SCLK
2
0
100
200
300
400
500
THROUGHPUT RATE (kSPS)
Figure 48. Power vs. Throughput Rate with 3 V VCC
05399-052
0
Rev. 0 | Page 30 of 36
AD7321 SERIAL INTERFACE
Figure 50 shows the timing diagram for the serial interface of the AD7321. The serial clock applied to the SCLK pin provides the conversion clock and controls the transfer of information to and from the AD7321 during a conversion. The CS signal initiates the data transfer and the conversion process. The falling edge of CS puts the track-and-hold into hold mode and takes the bus out of three-state. Then the analog input signal is sampled. Once the conversion is initiated, it requires 16 SCLK cycles to complete. The track-and-hold goes back into track mode on the 14th SCLK rising edge. On the 16th SCLK falling edge, the DOUT line returns to three-state. If the rising edge of CS occurs before 16 SCLK cycles have elapsed, the conversion is terminated, and the DOUT line returns to three-state. Depending on where the CS signal is brought high, the addressed register may be updated. Data is clocked into the AD7321 on the SCLK falling edge. The three MSBs on the DIN line are decoded to select which register is being addressed. The control register is a 12-bit register. If the control register is addressed by the three MSBs, the data on the DIN line is loaded into the control on the 15th SCLK falling edge. If the range register is addressed, the data on the DIN line is loaded into the addressed register on the 11th SCLK falling edge. Conversion data is clocked out of the AD7321 on each SCLK falling edge. Data on the DOUT line consists of two leading ZEROs, a channel identifier bit, a sign bit, and a 12-bit conversion result. The channel identifier bit is used to indicate which channel corresponds to the conversion result. The first ZERO bit is clocked out on the CS falling edge, and the second bit is clocked out on the first SCLK falling edge.
t1
CS
t2
SCLK 1 2 3 4 IDENTIFICATION BIT ADD0 SIGN
tCONVERT t6
5 13 14 15 16
t3
t4
DB11
t7
DB10 DB2
t5
DB1 DB0
t8
ZERO DOUT THREE- ZERO t9 STATE DIN WRITE ZERO
tQUIET
t10
REG SEL MSB LSB DON'T CARE
THREE-STATE
05399-036
Figure 50. Serial Interface Timing Diagram (Control Register Write)
Rev. 0 | Page 31 of 36
AD7321 MICROPROCESSOR INTERFACING
The serial interface on the AD7321 allows the part to be directly connected to a range of different microprocessors. This section explains how to interface the AD7321 with some common microcontroller and DSP serial interface protocols. The frequency of the serial clock is set in the SCLKDIV register. When the instruction to transmit with TFS is given (AX0 = TX0), the state of the serial clock is checked. The DSP waits until the SCLK has gone high, low, and high again before starting the transmission. If the timer and SCLK are chosen so that the instruction to transmit occurs on or near the rising edge of SCLK, data can be transmitted immediately or at the next clock edge. For example, the ADSP-2111 has a master clock frequency of 16 MHz. If the SCLKDIV register is loaded with the value 3, an SCLK of 2 MHz is obtained, and eight master clock periods elapse for every one SCLK period. If the timer registers are loaded with the value 803, 100.5 SCLKs occur between interrupts and, subsequently, between transmit instructions. This situation leads to nonequidistant sampling because the transmit instruction occurs on an SCLK edge. If the number of SCLKs between interrupts is an integer of N, equidistant sampling is implemented by the DSP.
AD7321 TO ADSP-21xx
The ADSP-21xx family of DSPs interface directly to the AD7321 without requiring glue logic. The VDRIVE pin of the AD7321 takes the same supply voltage as that of the ADSP-21xx. This allows the ADC to operate at a higher supply voltage than its serial interface. The SPORT0 on the ADSP-21xx should be configured as shown in Table 14. Table 14. SPORT0 Control Register Setup
Setting TFSW = RFSW = 1 INVRFS = INVTFS = 1 DTYPE = 00 SLEN = 1111 ISCLK = 1 TFSR = RFSR = 1 IRFS = 0 ITFS = 1 Description Alternative framing Active low frame signal Right justify data 16-bit data-word Internal serial clock Frame every word
AD7321 TO ADSP-BF53x
The ADSP-BF53x family of DSPs interfaces directly to the AD7321 without requiring glue logic, as shown in Figure 52. The SPORT0 Receive Configuration 1 register should be set up as outlined in Table 15.
AD73211
SCLK CS DIN DOUT VDRIVE
The connection diagram is shown in Figure 51. The ADSP-21xx has TFS0 and RFS0 tied together. TFS0 is set as an output, and RFS0 is set as an input. The DSP operates in alternative framing mode, and the SPORT0 control register is set up as described in Table 14. The frame synchronization signal generated on the TFS is tied to CS, and, as with all signal processing applications, requires equidistant sampling. However, as in this example, the timer interrupt is used to control the sampling rate of the ADC, and under certain conditions equidistant sampling cannot be achieved.
AD73211
SCLK CS
ADSP-BF53x1
RSCLK0 RFS0 DT0 DR0
VDD
1ADDITIONAL
SCLK0 TFS0 RFS0 DT0 DR0
PINS OMITTED FOR CLARITY.
Figure 52. Interfacing the AD7321 to the ADSP-BF53x
Table 15. SPORT0 Receive Configuration 1 Register
Setting RCKFE = 1 LRFS = 1 RFSR = 1 IRFS = 1 RLSBIT = 0 RDTYPE = 00 IRCLK = 1 RSPEN = 1 SLEN = 1111 TFSR = RFSR = 1 Description Sample data with falling edge of RSCLK Active low frame signal Frame every word Internal RFS used Receive MSB first Zero fill Internal receive clock Receive enable 16-bit data-word
DIN DOUT VDRIVE
VDD
1ADDITIONAL PINS OMITTED FOR CLARITY.
Figure 51. Interfacing the AD7321 to the ADSP-21xx
The timer registers are loaded with a value that provides an interrupt at the required sampling interval. When an interrupt is received, a value is transmitted with TFS/DT (ADC control word). The TFS is used to control the RFS, and hence the reading of data.
05399-037
Rev. 0 | Page 32 of 36
05399-038
ADSP-21xx1
AD7321 APPLICATION HINTS
LAYOUT AND GROUNDING
The printed circuit board that houses the AD7321 should be designed so that the analog and digital sections are confined to certain areas of the board. This design facilitates the use of ground planes that can easily be separated. To provide optimum shielding for ground planes, a minimum etch technique is generally best. All AGND pins on the AD7321 should be connected to the AGND plane. Digital and analog ground pins should be joined in only one place. If the AD7321 is in a system where multiple devices require an AGND and DGND connection, the connection should still be made at only one point. A star point should be established as close as possible to the ground pins on the AD7321. Good connections should be made to the power and ground planes. This can be done with a single via or multiple vias for each supply and ground pin. Avoid running digital lines under the AD7321 device because this couples noise onto the die. However, the analog ground plane should be allowed to run under the AD7321 to avoid noise coupling. The power supply lines to the AD7321 device should use as large a trace as possible to provide low impedance paths and reduce the effects of glitches on the power supply line. To avoid radiating noise to other sections of the board, components, such as clocks, with fast switching signals should be shielded with digital ground and never run near the analog inputs. Avoid crossover of digital and analog signals. To reduce the effects of feedthrough within the board, traces should be run at right angles to each other. A microstrip technique is the best method, but its use may not be possible with a double-sided board. In this technique, the component side of the board is dedicated to ground planes, and signals are placed on the other side. Good decoupling is also important. All analog supplies should be decoupled with 10 F tantalum capacitors in parallel with 0.1 F capacitors to AGND. To achieve the best results from these decoupling components, they must be placed as close as possible to the device, ideally right up against the device. The 0.1 F capacitors should have a low effective series resistance (ESR) and low effective series inductance (ESI), such as is typical of common ceramic and surface mount types of capacitors. These low ESR, low ESI capacitors provide a low impedance path to ground at high frequencies to handle transient currents due to internal logic switching.
Rev. 0 | Page 33 of 36
AD7321 OUTLINE DIMENSIONS
5.10 5.00 4.90
14
8
4.50 4.40 4.30
1 7
6.40 BSC
PIN 1 1.05 1.00 0.80 0.65 BSC 1.20 MAX 0.15 0.05 0.30 0.19
0.20 0.09
SEATING COPLANARITY PLANE 0.10
8 0
0.75 0.60 0.45
COMPLIANT TO JEDEC STANDARDS MO-153-AB-1
Figure 53. 14-Lead Thin Shrink Small Outline Package [TSSOP] (RU-14) Dimensions show in millimeters
ORDERING GUIDE
Model AD7321BRUZ 1 AD7321BRUZ-REEL1 AD7321BRUZ-REEL71 EVAL-AD7321CB 2 EVAL-CONTROL BRD2 3
1 2 3
Temperature Range -40C to +85C -40C to +85C -40C to +85C
Package Description 14-Lead TSSOP 14-Lead TSSOP 14-Lead TSSOP Evaluation Board Controller Board
Package Option RU-14 RU-14 RU-14
Z = Pb-free part. This can be used as a standalone evaluation board or in conjunction with the EVAL-CONTROL board for evaluation/demonstration purposes. This board is a complete unit allowing a PC to control and communicate with all Analog Devices evaluation boards ending in the CB designators. To order a complete evaluation kit, the particular ADC evaluation board (for example, EVAL-AD7321CB), the EVAL-CONTROL BRD2, and a 12 V transformer must be ordered. See the relevant evaluation board technical note for more information.
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AD7321 NOTES
Rev. 0 | Page 35 of 36
AD7321 NOTES
(c)2006 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. D05399-0-1/06(0)
Rev. 0 | Page 36 of 36

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