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16-Bit 100 kSPS PulSARTM Unipolar ADC with Reference AD7651
FEATURES
Throughput: 100 kSPS 16-bit resolution Analog input voltage range: 0 V to 2.5 V No pipeline delay Parallel and serial 5 V/3 V interface SPI(R)/QSPITM/MICROWIRETM/DSP compatible Single 5 V supply operation Power dissipation 16 mW typ, 160 W @ 1 kSPS without REF 38 mW typ with REF 48-lead LQFP and 48-lead LFCSP packages Pin-to-pin compatible with PulSAR ADCs
FUNCTIONAL BLOCK DIAGRAM
REFBUFIN REF REFGND AGND AVDD REF SWITCHED CAP DAC PARALLEL INTERFACE CLOCK CONTROL LOGIC AND CALIBRATION CIRCUITRY DVDD DGND OVDD SERIAL PORT 16 OGND
AD7651
IN INGND PDREF PDBUF PD RESET
DATA[15:0] BUSY RD CS SER/P A R OB/2 C BYTESWAP
CNVST
02964-0-001
APPLICATIONS
Data acquisition Instrumentation Digital signal processing Spectrum analysis Medical instruments Battery-powered systems Process control
Figure 1. Functional Block Diagram
Table 1. PulSAR Selection
Type/kSPS PseudoDifferential True Bipolar True Differential 18-Bit Multichannel/ Simultaneous 100-250 AD7651 AD7660/AD7661 AD7663 AD7675 AD7678 500-570 AD7650/AD7652 AD7664/AD7666 AD7665 AD7676 AD7679 AD7654 AD7655 800- 1000 AD7653 AD7667 AD7671 AD7677 AD7674
GENERAL DESCRIPTION
The AD7651* is a 16-bit, 100 kSPS, charge redistribution SAR analog-to-digital converter that operates from a single 5 V power supply. The part contains a high speed 16-bit sampling ADC, an internal conversion clock, internal reference, error correction circuits, and both serial and parallel system interface ports. The AD7651 is fabricated using Analog Devices' high performance, 0.6 micron CMOS process, with correspondingly low cost, and is available in a 48-lead LQFP and a tiny 48-lead LFCSP with operation specified from -40C to +85C.
*
PRODUCT HIGHLIGHTS
1. Fast Throughput. The AD7651 is a 100 kSPS, charge redistribution, 16-bit SAR ADC with internal error correction circuitry. Internal Reference. The AD7651 has an internal reference with a typical temperature drift of 7 ppm/C. Single-Supply Operation. The AD7651 operates from a single 5 V supply. Its power dissipation decreases with throughput. Serial or Parallel Interface. Versatile parallel or 2-wire serial interface arrangement is compatible with both 3 V and 5 V logic.
2.
Patent Pending.
3.
4.
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: 781.326.8703 (c) 2003 Analog Devices, Inc. All rights reserved.
AD7651 TABLE OF CONTENTS
Specifications..................................................................................... 3 Timing Specifications....................................................................... 5 Absolute Maximum Ratings............................................................ 7 Pin Configuration and Function Descriptions............................. 8 Definitions of Specifications ......................................................... 11 Typical Performance Characteristics ........................................... 12 Circuit Information ........................................................................ 15 Converter Operation.................................................................. 15 Typical Connection Diagram.................................................... 17 Power Dissipation versus Throughput .................................... 19 Conversion Control.................................................................... 19 Digital Interface .......................................................................... 20 Parallel Interface ......................................................................... 20 Serial Interface ............................................................................ 20 Master Serial Interface ............................................................... 21 Slave Serial Interface .................................................................. 22 Microprocessor Interfacing....................................................... 24 Application Hints............................................................................ 25 Bipolar and Wider Input Ranges .............................................. 25 Layout .......................................................................................... 25 Evaluating the AD7651's Performance .................................... 25 Outline Dimensions ....................................................................... 26 Ordering Guide........................................................................... 26
REVISION HISTORY
Revision 0, Initial Version.
Rev. 0 | Page 2 of 28
AD7651 SPECIFICATIONS
Table 2. -40C to +85C, AVDD = DVDD = 5 V, OVDD = 2.7 V to 5.25 V, unless otherwise noted
Parameter RESOLUTION ANALOG INPUT Voltage Range Operating Input Voltage Analog Input CMRR Input Current Input Impedance1 THROUGHPUT SPEED Complete Cycle Throughput Rate DC ACCURACY Integral Linearity Error No Missing Codes Differential Linearity Error Transition Noise Unipolar Zero Error, TMIN to TMAX3 Unipolar Zero Error Temperature Drift3 Full-Scale Error, TMIN to TMAX 3 Full-Scale Error Temperature Drift Power Supply Sensitivity AC ACCURACY Signal-to-Noise Spurious Free Dynamic Range Total Harmonic Distortion Signal-to-(Noise + Distortion) -3 dB Input Bandwidth SAMPLING DYNAMICS Aperture Delay Aperture Jitter Transient Response REFERENCE Internal Reference Voltage Internal Reference Temperature Drift Line Regulation Turn-On Settling Time Temperature Pin Voltage Output @ 25C Temperature Sensitivity Output Resistance External Reference Voltage Range External Reference Current Drain Conditions Min 16 0 -0.1 -0.1 65 1.1 Typ Max Unit Bits V V V dB A
VIN - VINGND VIN VINGND fIN = 10 kHz 100 kSPS Throughput
VREF +3 +0.5
0 -6 15 -2 0.7
10 100 +6 +3 5 0.25
s kSPS LSB2 Bits LSB LSB LSB ppm/C % of FSR ppm/C LSB dB4 dB dB dB dB dB kHz ns ps rms s V ppm/C ppm/V ms mV mV/C k V A
REF = 2.5 V AVDD = 5 V 5% fIN = 45 kHz fIN = 45 kHz fIN = 10 kHz fIN = 45 kHz fIN = 45 kHz -60 dB Input, fIN = 45 kHz 0.6 2 86 98 -98 -98 86 30 800 2 5 Full-Scale Step VREF @ 25C -40C to +85C AVDD = 5 V 5% CREF = 10 F 2.48 2.5 7 24 5 300 1 4.3 2.5 35
0.12
8.75 2.52
2.3 100 kSPS Throughput
AVDD - 1.85
Rev. 0 | Page 3 of 28
AD7651
Parameter DIGITAL INPUTS Logic Levels VIL VIH IIL IIH DIGITAL OUTPUTS Data Format5 Pipeline Delay6 VOL VOH POWER SUPPLIES Specified Performance AVDD DVDD OVDD Operating Current AVDD8 AVDD9 DVDD10 OVDD10 Power Dissipation without REF10 Power Dissipation with REF10 TEMPERATURE RANGE11 Specified Performance Conditions Min Typ Max Unit
-0.3 2.0 -1 -1
+0.8 DVDD + 0.3 +1 +1
V V A A
ISINK = 1.6 mA ISOURCE = -500 A
0.4 OVDD - 0.6
V V
4.75 4.75 2.7 100 kSPS Throughput With Reference and Buffer Reference and Buffer Alone
5 5
5.25 5.25 5.257
V V V mA mA mA A mW W mW C
100 kSPS Throughput 1 kSPS Throughput 100 kSPS Throughput TMIN to TMAX -40
6.2 3 1.5 18 16 160 38
25 45 +85
1 2
See Analog Input section. LSB means least significant bit. With the 0 V to 2.5 V input range, 1 LSB is 38.15 V. 3 See Definitions of Specifications section. These specifications do not include the error contribution from the external reference. 4 All specifications in dB are referred to a full-scale input FS. Tested with an input signal at 0.5 dB below full-scale, unless otherwise specified. 5 Parallel or Serial 16-Bit. 6 Conversion results are available immediately after completed conversion. 7 The max should be the minimum of 5.25 V and DVDD + 0.3 V. 8 With REF, PDREF and PDBUF are LOW; without REF, PDREF and PDBUF are HIGH. 9 With PDREF, PDBUF LOW and PD HIGH. 10 Tested in Parallel Reading Mode 11 Consult factory for extended temperature range.
Rev. 0 | Page 4 of 28
AD7651 TIMING SPECIFICATIONS
Table 3. -40C to +85C, AVDD = DVDD = 5 V, OVDD = 2.7 V to 5.25 V, unless otherwise noted
Parameter Refer to Figure 26 and Figure 27 Convert Pulse Width Time between Conversions CNVST LOW to BUSY HIGH Delay BUSY HIGH All Modes Except Master Serial Read after Convert Aperture Delay End of Conversion to BUSY LOW Delay Conversion Time Acquisition Time RESET Pulse Width Refer to Figure 28, Figure 29, and Figure 30 (Parallel Interface Modes) CNVST LOW to DATA Valid Delay DATA Valid to BUSY LOW Delay Bus Access Request to DATA Valid Bus Relinquish Time Refer to Figure 32 and Figure 33 (Master Serial Interface Modes)1 CS LOW to SYNC Valid Delay CS LOW to Internal SCLK Valid Delay1 CS LOW to SDOUT Delay CNVST LOW to SYNC Delay SYNC Asserted to SCLK First Edge Delay Internal SCLK Period2 Internal SCLK HIGH2 Internal SCLK LOW2 SDOUT Valid Setup Time2 SDOUT Valid Hold Time2 SCLK Last Edge to SYNC Delay2 CS HIGH to SYNC HI-Z CS HIGH to Internal SCLK HI-Z CS HIGH to SDOUT HI-Z BUSY HIGH in Master Serial Read after Convert2 CNVST LOW to SYNC Asserted Delay SYNC Deasserted to BUSY LOW Delay Refer to Figure 34 and Figure 35 (Slave Serial Interface Modes)1 External SCLK Setup Time External SCLK Active Edge to SDOUT Delay SDIN Setup Time SDIN Hold Time External SCLK Period External SCLK HIGH External SCLK LOW Symbol t1 t2 t3 t4 t5 t6 t7 t8 t9 t10 t11 t12 t13 t14 t15 t16 t17 t18 t19 t20 t21 t22 t23 t24 t25 t26 t27 t28 t29 t30 t31 t32 t33 t34 t35 t36 t37 Min 10 10 35 1.25 2 10 1.25 8.75 10 1.25 12 5 45 15 10 10 10 525 3 25 12 7 4 2 3 40 Typ Max Unit ns s ns s ns ns s s ns s ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns s ns ns ns ns ns ns ns ns
10 10 10 See Table 4 1.25 25 5 3 5 5 25 10 10
18
1 2
In serial interface modes, the SYNC, SCLK, and SDOUT timings are defined with a maximum load CL of 10 pF; otherwise, the load is 60 pF maximum. In Serial Master Read during Convert Mode. See Table 4 for serial master read after convert mode.
Rev. 0 | Page 5 of 28
AD7651
Table 4. Serial Clock Timings in Master Read after Convert
DIVSCLK[1] DIVSCLK[0] SYNC to SCLK First Edge Delay Minimum Internal SCLK Period Minimum Internal SCLK Period Maximum Internal SCLK HIGH Minimum Internal SCLK LOW Minimum SDOUT Valid Setup Time Minimum SDOUT Valid Hold Time Minimum SCLK Last Edge to SYNC Delay Minimum BUSY HIGH Width Maximum Symbol t18 t19 t19 t20 t21 t22 t23 t24 t24 0 0 3 25 40 12 7 4 2 3 2 0 1 17 50 70 22 21 18 4 55 2.5 1 0 17 100 140 50 49 18 30 130 3.5 1 1 17 200 280 100 99 18 80 290 5.75 Unit ns ns ns ns ns ns ns ns s
Rev. 0 | Page 6 of 28
AD7651 ABSOLUTE MAXIMUM RATINGS
Table 5. AD7651 Stress Ratings1
IN2, TEMP2,REF, REFBUFIN, INGND, REFGND to AGND Ground Voltage Differences AGND, DGND, OGND Supply Voltages AVDD, DVDD, OVDD AVDD to DVDD, AVDD to OVDD DVDD to OVDD Digital Inputs PDREF, PDBUF3 Internal Power Dissipation4 Internal Power Dissipation5 Junction Temperature Storage Temperature Range Lead Temperature Range (Soldering 10 sec)
1
AVDD + 0.3 V to AGND - 0.3 V
TO OUTPUT PIN
1.6mA
IOL
1.4V CL 60pF* 500A IOH
0.3 V -0.3 V to +7 V 7 V -0.3 V to +7 V -0.3 V to DVDD + 0.3 V 20 mA 700 mW 2.5 W 150C -65C to +150C 300C
* IN SERIAL INTERFACE MODES,THE SYNC, SCLK, AND SDOUT TIMINGS ARE DEFINED WITH A MAXIMUM LOAD CL OF 10pF; OTHERWISE,THE LOAD IS 60pF MAXIMUM.
02964-0-006
Figure 2. Load Circuit for Digital Interface Timing, SDOUT, SYNC, SCLK Outputs CL = 10 pF
2V 0.8V
tDELAY
2V 0.8V
tDELAY
2V 0.8V
02965-0-007
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 listed in the operational sections of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. 2 See Analog Input section. 3 See Voltage Reference Input section. 4 Specification is for the device in free air: 48-Lead LQFP; JA = 91C/W, JC = 30C/W 5 Specification is for the device in free air: 48-Lead LFCSP; JA = 26C/W.
Figure 3. Voltage Reference Levels for Timing
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 7 of 28
AD7651 PIN CONFIGURATION AND FUNCTION DESCRIPTIONS
REFBUFIN
48 47 46 45 44 43 42 41 40 39 38 37
AGND 1 AVDD 2 NC 3 BYTESWAP 4 OB/2C 5 NC 6 NC 7 SER/PAR 8 D0 9 D1 10 D2/DIVSCLK0 11 D3/DIVSCLK1 12 NC = NO CONNECT
REFGND REF
PDBUF
PDREF
INGND
AGND
AGND
AVDD IN
TEMP
NC
PIN 1 IDENTIFIER
36 35 34 33
AGND CNVST PD RESET CS RD DGND BUSY D15 D14 D13 D12
AD7651
TOP VIEW (Not to Scale)
32 31 30 29 28 27 26 25
13 14 15 16 17 18 19 20 21 22 23 24
D5/INVSYNC D6/INVSCLK
D11/RDERROR
02965-0-002
D8/SDOUT
D7/RDC/SDIN
Figure 4. 48-Lead LQFP (ST-48) and 48-Lead LFCSP (CP-48)
Table 6. Pin Function Descriptions
Pin No. 1, 36, 41, 42 2, 44 3, 6, 7, 40 4 5 Mnemonic AGND AVDD NC BYTESWAP OB/2C Type1 P P Description Analog Power Ground Pin. Input Analog Power Pin. Nominally 5 V. No Connect. Parallel Mode Selection (8-/16-bit). When LOW, the LSB is output on D[7:0] and the MSB is output on D[15:8]. When HIGH, the LSB is output on D[15:8] and the MSB is output on D[7:0]. Straight Binary/Binary Twos Complement. When OB/2C is HIGH, the digital output is straight binary; when LOW, the MSB is inverted, resulting in a twos complement output from its internal shift register. Serial/Parallel Selection Input. When LOW, the parallel port is selected; when HIGH, the serial interface mode is selected and some bits of the DATA bus are used as a serial port. Bit 0 and Bit 1 of the Parallel Port Data Output Bus. When SER/PAR is HIGH, these outputs are in high impedance. When SER/PAR is LOW, these outputs are used as Bit 2 and Bit 3 of the parallel port data output bus. When SER/PAR is HIGH, EXT/INT is LOW, and RDC/SDIN is LOW (serial master read after convert), these inputs, part of the serial port, are used to slow down, if desired, the internal serial clock that clocks the data output. In other serial modes, these pins are not used. When SER/PAR is LOW, this output is used as Bit 4 of the parallel port data output bus. When SER/PAR is HIGH, this input, part of the serial port, is used as a digital select input for choosing the internal data clock or an external data clock. With EXT/INT tied LOW, the internal clock is selected on the SCLK output. With EXT/INT set to a logic HIGH, output data is synchronized to an external clock signal connected to the SCLK input. When SER/PAR is LOW, this output is used as Bit 5 of the parallel port data output bus. When SER/PAR is HIGH, this input, part of the serial port, is used to select the active state of the SYNC signal. It is active in both master and slave modes. When LOW, SYNC is active HIGH. When HIGH, SYNC is active LOW. When SER/PAR is LOW, this output is used as Bit 6 of the parallel port data output bus. When SER/PAR is HIGH, this input, part of the serial port, is used to invert the SCLK signal. It is active in both master and slave modes.
DI DI
8 9, 10 11, 12
SER/PAR D[0:1] D[2:3]or DIVSCLK[0:1]
DI DO DI/O
13
D4 or EXT/INT
DI/O
14
D5 or INVSYNC
DI/O
15
D6 or INVSCLK
DI/O
D4/EXT/INT
Rev. 0 | Page 8 of 28
D9/SCLK D10/SYNC
OGND
DVDD DGND
OVDD
AD7651
Pin No. 16 Mnemonic D7 or RDC/SDIN Type1 DI/O Description When SER/PAR is LOW, this output is used as Bit 7 of the parallel port data output bus. When SER/PAR is HIGH, this input, part of the serial port, is used as either an external data input or a read mode selection input depending on the state of EXT/INT. When EXT/INT is HIGH, RDC/SDIN could be used as a data input to daisy-chain the conversion results from two or more ADCs onto a single SDOUT line. The digital data level on SDIN is output on DATA with a delay of 16 SCLK periods after the initiation of the read sequence. When EXT/INT is LOW, RDC/SDIN is used to select the read mode. When RDC/SDIN is HIGH, the data is output on SDOUT during conversion. When RDC/SDIN is LOW, the data can be output on SDOUT only when the conversion is complete. Input/Output Interface Digital Power Ground. Input/Output Interface Digital Power. Nominally at the same supply as the host interface (5 V or 3 V). Digital Power. Nominally at 5 V. Digital Power Ground. When SER/PAR is LOW, this output is used as Bit 8 of the parallel port data output bus. When SER/PAR is HIGH, this output, part of the serial port, is used as a serial data output synchronized to SCLK. Conversion results are stored in an on-chip register. The AD7651 provides the conversion result, MSB first, from its internal shift register. The DATA format is determined by the logic level of OB/2C. In serial mode when EXT/INT is LOW, SDOUT is valid on both edges of SCLK. In serial mode when EXT/INT is HIGH, if INVSCLK is LOW, SDOUT is updated on the SCLK rising edge and valid on the next falling edge; if INVSCLK is HIGH, SDOUT is updated on the SCLK falling edge and valid on the next rising edge. When SER/PAR is LOW, this output is used as Bit 9 of the parallel port data or SCLK output bus. When SER/PAR is HIGH, this pin, part of the serial port, is used as a serial data clock input or output, depending upon the logic state of the EXT/INT pin. The active edge where the data SDOUT is updated depends upon the logic state of the INVSCLK pin. When SER/PAR is LOW, this output is used as Bit 10 of the parallel port data output bus. When SER/PAR is HIGH, this output, part of the serial port, is used as a digital output frame synchronization for use with the internal data clock (EXT/INT = logic LOW). When a read sequence is initiated and INVSYNC is LOW, SYNC is driven HIGH and remains HIGH while the SDOUT output is valid. When a read sequence is initiated and INVSYNC is HIGH, SYNC is driven LOW and remains LOW while the SDOUT output is valid. When SER/PAR is LOW, this output is used as Bit 11 of the parallel port data output bus. When SER/PAR and EXT/INT are HIGH, this output, part of the serial port, is used as an incomplete read error flag. In slave mode, when a data read is started and not complete when the following conversion is complete, the current data is lost and RDERROR is pulsed HIGH. Bit 12 to Bit 15 of the Parallel Port Data Output Bus. These pins are always outputs regardless of the state of SER/PAR. Busy Output. Transitions HIGH when a conversion is started and remains HIGH until the conversion is complete and the data is latched into the on-chip shift register. The falling edge of BUSY could be used as a data ready clock signal. Must Be Tied to Digital Ground. Read Data. When CS and RD are both LOW, the interface parallel or serial output bus is enabled. Chip Select. When CS and RD are both LOW, the interface parallel or serial output bus is enabled. CS is also used to gate the external clock. Reset Input. When set to a logic HIGH, this pin resets the AD7651 and the current conversion, if any, is aborted. If not used, this pin could be tied to DGND. Power-Down Input. When set to a logic HIGH, power consumption is reduced and conversions are inhibited after the current one is completed. Start Conversion. If CNVST is HIGH when the acquisition phase (t8) is complete, the next falling edge on CNVST puts the internal sample/hold into the hold state and initiates a conversion. The mode is most appropriate if low sampling jitter is desired. If CNVST is LOW when the acquisition phase (t8) is complete, the internal sample/hold is put into the hold state and a conversion is immediately started. Reference Input Voltage. On-chip reference output voltage. Reference Input Analog Ground. Analog Input Ground.
Rev. 0 | Page 9 of 28
17 18 19 20 21
OGND OVDD DVDD DGND D8 or SDOUT
P P P P DO
22
D9 or SCLK
DI/O
23
D10 or SYNC
DO
24
D11 or RDERROR
DO
25-28 29
D[12:15] BUSY
DO DO
30 31 32 33 34 35
DGND RD CS RESET PD CNVST
P DI DI DI DI DI
37 38 39
REF REFGND INGND
AI/O AI AI
AD7651
Pin No. 43 45 46 47 Mnemonic IN TEMP REFBUFIN PDREF Type1 AI AO AI/O DI Description Primary Analog Input with a Range of 0 V to 2.5 V. Temperature Sensor Voltage Output. Reference Input Voltage. The reference output and the reference buffer input. This pin allows the choice of internal or external voltage references. When LOW, the on-chip reference is turned on. When HIGH, the internal reference is switched off and an external reference must be used. This pin allows the choice of buffering an internal or external reference with the internal buffer. When LOW, the buffer is selected. When HIGH, the buffer is switched off.
48
PDBUF
DI
1
AI = Analog Input; AI/O = Bidirectional Analog; AO = Analog Output; DI = Digital Input; DI/O = Bidirectional Digital; DO = Digital Output; P = Power.
Rev. 0 | Page 10 of 28
AD7651 DEFINITIONS OF SPECIFICATIONS
Integral Nonlinearity Error (INL)
Linearity error refers to the deviation of each individual code from a line drawn from negative full scale through positive full scale. The point used as negative full scale occurs 1/2 LSB before the first code transition. Positive full scale is defined as a level 11/2 LSB beyond the last code transition. The deviation is measured from the middle of each code to the true straight line.
Total Harmonic Distortion (THD)
THD is the ratio of the rms sum of the first five harmonic components to the rms value of a full-scale input signal, and is expressed in decibels.
Signal-to-Noise Ratio (SNR)
SNR is the ratio of the rms value of the actual input signal to the rms sum of all other spectral components below the Nyquist frequency, excluding harmonics and dc. The value for SNR is expressed in decibels.
Differential Nonlinearity Error (DNL)
In an ideal ADC, code transitions are 1 LSB apart. Differential nonlinearity is the maximum deviation from this ideal value. It is often specified in terms of resolution for which no missing codes are guaranteed.
Signal-to-(Noise + Distortion) Ratio (S/[N+D])
S/(N+D) is the ratio of the rms value of the actual input signal to the rms sum of all other spectral components below the Nyquist frequency, including harmonics but excluding dc. The value for S/(N+D) is expressed in decibels.
Full-Scale Error
The last transition (from 011...10 to 011...11 in twos complement coding) should occur for an analog voltage 11/2 LSB below the nominal full scale (2.49994278 V for the 0 V to 2.5 V range). The full-scale error is the deviation of the actual level of the last transition from the ideal level.
Aperture Delay
Aperture delay is a measure of the acquisition performance and is measured from the falling edge of the CNVST input to when the input signal is held for a conversion.
Unipolar Zero Error
The first transition should occur at a level 1/2 LSB above analog ground (19.073 V for the 0 V to 2.5 V range). Unipolar zero error is the deviation of the actual transition from that point.
Transient Response
Transient response is the time required for the AD7651 to achieve its rated accuracy after a full-scale step function is applied to its input.
Spurious-Free Dynamic Range (SFDR)
SFDR is the difference, in decibels (dB), between the rms amplitude of the input signal and the peak spurious signal.
Overvoltage Recovery
Overvoltage recovery is the time required for the ADC to recover to full accuracy after an analog input signal 150% of the full-scale value is reduced to 50% of the full-scale value.
Effective Number Of Bits (ENOB)
ENOB is a measurement of the resolution with a sine wave input. It is related to S/(N+D) by the following formula: ENOB = (S/[N+D]dB - 1.76)/6.02 and is expressed in bits.
Reference Voltage Temperature Coefficient
Reference voltage temperature coefficient is the change of internal reference voltage output voltage V over the operating temperature range and normalized by the output voltage at 25C, expressed in ppm/C. The equation follows:
TCV ( ppm / C ) = V (T 2) - V (T 1) x 106 V (25C) x (T 2 - T 1)
where: V(25C) = V at +25C V(T2) = V at Temperature 2 (+85C) V(T1) = V at Temperature 1 (-40C)
Rev. 0 | Page 11 of 28
AD7651 TYPICAL PERFORMANCE CHARACTERISTICS
4 3 2 1 0 -1 -2 -3 -4
DNL (LSB)
2.0
1.5
1.0
INL (LSB)
0.5
0
-0.5
0
16384
32768 CODE
49152
65536
02964-0-026
-1.0
0
16384
32768 CODE
49152
65536
02964-0-023
Figure 5. Integral Nonlinearity vs. Code
Figure 8. Differential Nonlinearity vs. Code
140000 120000 100000 114156 114686
180000 160000 140000 120000 155528
COUNTS
COUNTS
80000 60000 40000 20000 0 0 0 479 20890 10822 137 0 0
100000 80000 60000 40000 20000 0 0 19 2896 1903 8002 5 8003 0 8004
02964-0-028
51585
49184
7FFB 7FFC 7FFD 7FFE 7FFF 8000 8001 8002 8003 8004 CODE IN HEX 02964-0-027
7FFC 7FFD 7FFE 7FFF 8000 8001 CODE IN HEX
Figure 6. Histogram of 261,120 Conversions of a DC Input at the Code Transition
Figure 9. Histogram of 261,120 Conversions of a DC Input at the Code Center
0 -20 AMPLITUDE (dB of Full Scale) -40 -60 -80 -100 -120 -140 -160 -180
90
15.5
fS = 100kSPS fIN = 45.7kHz
89 88 87 15.0
SNR, S/[N+D] (dB)
SNR = 86.7dB THD = 102.5dB SFDR = 103.6dB S/[N+D] = 86.6dB
SNR
85 84 83 82 81 ENOB S/[N+D] 13.5 14.0
0
10
20 30 FREQUENCY (kHz)
40
50
02964-0-029
80 1 100 10 FREQUENCY (kHz)
13.0 1000
02964-0-030
Figure 7. FFT Plot
Figure 10. SNR, S/(N+D), and ENOB vs. Frequency
Rev. 0 | Page 12 of 28
ENOB (Bits)
86
14.5
AD7651
-70 -75 -80 SFDR 120 110 100 90
-100
THD, HARMONICS (dB)
-85 -90 -95 -100 THD -105 -110 -115 -120 1 100 10 FREQUENCY (kHz) THIRD HARMONIC SECOND HARMONIC
THD, HARMONICS (dB)
-105 THD -110 SECOND HARMONIC
70 60 50 40 30 20 1000
02964-0-031
SFDR (dB)
80
-115
THIRD HARMONIC
-120 -55
-35
-15
5
25
45
65
85
105
125
TEMPERATURE (C)
02964-0-034
Figure 11. THD, Harmonics, and SFDR vs. Frequency
Figure 14. THD and Harmonics vs. Temperature
90
SNR, S/[N+D] REFERRED TO FULL SCALE (dB)
10000 1000
AVDD
89
OPERATING CURRENT (A)
100 10 1 0.1 0.01
DVDD
88 SNR 87 S/[N+D] 86
OVDD
85 -60
-50
-40
-30
-20
-10
0
02964-0-032
0.001
PDREF = PDBUF = HIGH 10 100 1000 SAMPLE RATE (SPS) 10000 100000
02964-0-035
INPUT LEVEL (dB)
Figure 12. SNR and S/(N+D) vs. Input Level (Referred to Full Scale)
Figure 15. Operating Current vs. Sample Rate
89
15.5
6 5
88 SNR, S/[N+D] (dB)
ZERO ERROR, FULL SCALE (LSB)
4 3 2 1 0 -1 -2 -3 -4 -5 ZERO ERROR FULL SCALE
15.0
87
SNR 14.5
86
S/[N+D] 14.0
85
ENOB (Bits)
-55
-35
-15
5
25
45
65
85
105
13.5 125
-6 -55
-35
-15
5
25
45
65
85
105
125
TEMPERATURE (dB)
02964-0-033
TEMPERATURE (C)
02964-0-036
Figure 13. SNR, S/(N+D), and ENOB vs. Temperature
Figure 16. Zero Error, Full Scale with Reference vs. Temperature
Rev. 0 | Page 13 of 28
AD7651
2.5042 2.5040 40 2.5038 OVDD = 2.7V @ 25C 50 OVDD = 2.7V @ 85C
t12 DELAY (ns)
VREF (V)
2.5036 2.5034 2.5032
30 OVDD = 5V @ 85C 20 OVDD = 5V @ 25C
10 2.5030 2.5028 -40 0
-20
0
20
40
60
80
100
120
02964-0-037
0
50
100 CL (pF)
150
200
02964-0-039
TEMPERATURE (C)
Figure 17. Typical Reference Output Voltage vs. Temperature
Figure 19. Typical Delay vs. Load Capacitance CL
25
20
NUMBER OF UNITS
15
10
5
0
-30 -26 -22 -18 -14 -10 -6 -2
2
6
10 14 18 22 26 30
02965-0-040
REFERENCE DRIFT (ppm/C)
Figure 18. Reference Voltage Temperature Coefficient Distribution (100 Units)
Rev. 0 | Page 14 of 28
AD7651 CIRCUIT INFORMATION
IN REF REFGND MSB 32,768C 16,384C 4C 2C C C BUSY COMP INGND 65,536C SWB CNVST
02964-0-005
LSB
SWA
SWITCHES CONTROL
CONTROL LOGIC OUTPUT CODE
Figure 20. ADC Simplified Schematic
The AD7651 is a very fast, low power, single supply, precise 16-bit analog-to-digital converter (ADC). The AD7651 provides the user with an on-chip track/hold, successive approximation ADC that does not exhibit any pipeline or latency, making it ideal for multiple multiplexed channel applications. The AD7651 can be operated from a single 5 V supply and can be interfaced to either 5 V or 3 V digital logic. It is housed in either a 48-lead LQFP or a 48-lead LFCSP that saves space and allows flexible configurations as either a serial or parallel interface. The AD7651 is pin-to-pin compatible with PulSAR ADCs.
During the acquisition phase, the common terminal of the array tied to the comparator's positive input is connected to AGND via SWA. All independent switches are connected to the analog input IN. Thus, the capacitor array is used as a sampling capacitor and acquires the analog signal on IN. Similarly, the dummy capacitor acquires the analog signal on INGND. When CNVST goes LOW, a conversion phase is initiated. When the conversion phase begins, SWA and SWB are opened. The capacitor array and dummy capacitor are then disconnected from the inputs and connected to REFGND. Therefore, the differential voltage between IN and INGND captured at the end of the acquisition phase is applied to the comparator inputs, causing the comparator to become unbalanced. By switching each element of the capacitor array between REFGND and REF, the comparator input varies by binary weighted voltage steps (VREF/2, VREF/4, ...VREF/65536). The control logic toggles these switches, starting with the MSB, to bring the comparator back into a balanced condition. After this process is completed, the control logic generates the ADC output code and brings the BUSY output LOW.
CONVERTER OPERATION
The AD7651 is a successive-approximation ADC based on a charge redistribution DAC. Figure 20 shows a simplified schematic of the ADC. The capacitive DAC consists of an array of 16 binary weighted capacitors and an additional LSB capacitor. The comparator's negative input is connected to a dummy capacitor of the same value as the capacitive DAC array.
Rev. 0 | Page 15 of 28
AD7651
Transfer Functions
Using the OB/2C digital input, the AD7651 offers two output codings: straight binary and twos complement. The LSB size is VREF/65536, which is about 38.15 V. The AD7651's ideal transfer characteristic is shown in Figure 21 and Table 7.
1 LSB = V REF /65536
ADC CODE (Straight Binary)
Table 7. Output Codes and Ideal Input Voltages
Analog Input 2.499962 V 2.499923 V 1.250038 V 1.25 V 1.249962 V 38 V 0V Digital Output Code (Hex) Straight Twos Binary Complement FFFF1 7FFF1 FFFE 7FFE 8001 0001 8000 0000 7FFF FFFF 0001 8001 00002 80002
111...111 111...110 111...101
Description FSR -1 LSB FSR - 2 LSB Midscale + 1 LSB Midscale Midscale - 1 LSB -FSR + 1 LSB -FSR
1
2
This is also the code for overrange analog input (VIN - VINGND above VREF - VREFGND). This is also the code for underrange analog input (VIN below VINGND).
000...010 000...001 000...000 0V 1 LSB 0.5 LSB VREF - 1 LSB VREF - 1.5 LSB ANALOG INPUT
02964-0-003
Figure 21. ADC Ideal Transfer Function
ANALOG SUPPLY (5V)
20 + 10F 100nF + 10F 100nF 100nF + 10F
DIGITAL SUPPLY (3.3V OR 5V)
AVDD
AGND
DGND
DVDD
OVDD
OGND SCLK
SERIAL PORT
REF CR4 100nF REFBUFIN1 REFGND SDOUT
BUSY
C/P/DSP
AD7651
U12 ANALOG INPUT (0V TO 2.5V) CC INGND PDREF PD PDBUF RESET CS IN
CNVST
D3
OB/2C SER/PAR DVDD
RD BYTESWAP
CLOCK
NOTES 1THE CONFIGURATION SHOWN IS USING THE INTERNAL REFERENCE AND INTERNAL BUFFER. 2THE AD8021 IS RECOMMENDED. SEE DRIVER AMPLIFIER CHOICE SECTION. 3OPTIONAL LOW JITTER. 4A 10F CERAMIC CAPACITOR (X5R, 1206 SIZE) IS RECOMMENDED (e.g., PANASONIC ECJ3YB0J106M). SEE VOLTAGE REFERENCE INPUT SECTION.
02964-0-004
Figure 22. Typical Connection Diagram
Rev. 0 | Page 16 of 28
AD7651
TYPICAL CONNECTION DIAGRAM
Figure 22 shows a typical connection diagram for the AD7651.
Driver Amplifier Choice
Although the AD7651 is easy to drive, the driver amplifier needs to meet the following requirements: * The driver amplifier and the AD7651 analog input circuit must be able to settle for a full-scale step of the capacitor array at a 16-bit level (0.0015%). In the amplifier's data sheet, settling at 0.1% to 0.01% is more commonly specified. This could differ significantly from the settling time at a 16-bit level and should be verified prior to driver selection. The tiny op amp OP184, which combines ultra low noise and high gain-bandwidth, meets this settling time requirement. The noise generated by the driver amplifier needs to be kept as low as possible in order to preserve the SNR and transition noise performance of the AD7651. The noise coming from the driver is filtered by the AD7651 analog input circuit 1-pole low-pass filter made by R1 and C2 or by the external filter, if one is used. The driver needs to have a THD performance suitable to that of the AD7651.
Analog Input
Figure 23 shows an equivalent circuit of the input structure of the AD7651. The two diodes, D1 and D2, provide ESD protection for the analog inputs IN and INGND. Care must be taken to ensure that the analog input signal never exceeds the supply rails by more than 0.3 V. This will cause these diodes to become forward-biased and start conducting current. These diodes can handle a forward-biased current of 100 mA maximum. For instance, these conditions could eventually occur when the input buffer's (U1) supplies are different from AVDD. In such a case, an input buffer with a short-circuit current limitation can be used to protect the part.
AVDD D1 C1 D2 R1 C2
*
IN OR INGND AGND
*
02965-0-008
Figure 23. Equivalent Analog Input Circuit
This analog input structure allows the sampling of the differential signal between IN and INGND. Unlike other converters, INGND is sampled at the same time as IN. By using this differential input, small signals common to both inputs are rejected. For instance, by using INGND to sense a remote signal ground, ground potential differences between the sensor and the local ADC ground are eliminated. During the acquisition phase, the impedance of the analog input IN can be modeled as a parallel combination of capacitor C1 and the network formed by the series connection of R1 and C2. C1 is primarily the pin capacitance. R1 is typically 3250 and is a lumped component made up of some serial resistors and the on resistance of the switches. C2 is typically 60 pF and is mainly the ADC sampling capacitor. During the conversion phase, where the switches are opened, the input impedance is limited to C1. R1 and C2 make a 1-pole low-pass filter that reduces undesirable aliasing effect and limits the noise. When the source impedance of the driving circuit is low, the AD7651 can be driven directly. Large source impedances will significantly affect the ac performance, especially total harmonic distortion.
The OP184, OP162 or AD8519 meet these requirements and are usually appropriate for almost all applications. As an alternative, in very high speed and noise-sensitive applications, the AD8021 with an external 10 pF compensation capacitor can be used. This capacitor should have good linearity as an NPO ceramic or mica type. Moreover, the use of a noninverting +1 gain arrangement is recommended and helps to obtain the best signal-to-noise ratio. The AD8022 could also be used if a dual version is needed and gain of 1 is present. The AD829 is an alternative in applications where high frequency (above 100 kHz) performance is not required. In gain of 1 applications, it requires an 82 pF compensation capacitor. The AD8610 is an option when low bias current is needed in low frequency applications.
Rev. 0 | Page 17 of 28
AD7651
Voltage Reference Input
The AD7651 allows the choice of either a very low temperature drift internal voltage reference or an external 2.5 V reference. Unlike many ADCs with internal references, the internal reference of the AD7651 provides excellent performance and can be used in almost all applications. To use the internal reference along with the internal buffer, PDREF and PDBUF should both be LOW. This will produce a 1.207 V voltage on REFBUFIN which, amplified by the buffer, will result in a 2.5 V reference on the REF pin. The output impedance of REFBUFIN is 11 k (minimum) when the internal reference is enabled. It is useful to decouple REFBUFIN with a 100 nF ceramic capacitor. Thus, the 100 nF capacitor provides an RC filter for noise reduction. To use an external reference along with the internal buffer, PDREF should be HIGH and PDBUF should be LOW. This powers down the internal reference and allows the 2.5 V reference to be applied to REFBUFIN. To use an external reference directly on REF pin, PDREF and PDBUF should both be HIGH. PDREF and PDBUF respectively power down the internal reference and the internal reference buffer. Note that the PDREF and PDBUF input current should never exceed 20 mA. This could eventually occur when input voltage is above AVDD (for instance at power up). In this case, a 100 series resistor is recommended. The internal reference is temperature compensated to 2.5 V 20 mV. The reference is trimmed to provide a typical drift of 7 ppm/C. This typical drift characteristic is shown in Figure 17. For improved drift performance, an external reference such as the AD780 can be used. The AD7651 voltage reference input REF has a dynamic input impedance; it should therefore be driven by a low impedance source with efficient decoupling between the REF and REFGND inputs. This decoupling depends on the choice of the voltage reference but usually consists of a low ESR tantalum capacitor connected to REF and REFGND with minimum parasitic inductance. A 10 F (X5R, 1206 size) ceramic chip capacitor (or 47 F tantalum capacitor) is appropriate when using either the internal reference or one of these recommended reference voltages: * * * The low noise, low temperature drift ADR421 and AD780 The low power ADR291 The low cost AD1582 For applications that use multiple AD7651s, it is more effective to use the internal buffer to buffer the reference voltage. Care should be taken with the voltage reference's temperature coefficient, which directly affects the full-scale accuracy if this parameter matters. For instance, a 15 ppm/C temperature coefficient of the reference changes full scale by 1 LSB/C. Note that VREF can be increased to AVDD - 1.85 V. Since the input range is defined in terms of VREF, this would essentially increase the range to 0 V to 3 V with an AVDD above 4.85 V. The AD780 can be selected with a 3 V reference voltage. The TEMP pin, which measures the temperature of the AD7651, can be used as shown in Figure 24. The output of TEMP pin is applied to one of the inputs of the analog switch (e.g., ADG779), and the ADC itself is used to measure its own temperature. This configuration is very useful for improving the calibration accuracy over the temperature range.
ADG779 TEMP TEMPERATURE SENSOR
ANALOG INPUT (UNIPOLAR) AD8021 CC
IN
AD7651
02964-0-024
Figure 24. Temperature Sensor Connection Diagram
Power Supply
The AD7651 uses three power supply pins: an analog 5 V supply AVDD, a digital 5 V core supply DVDD, and a digital input/ output interface supply OVDD. OVDD allows direct interface with any logic between 2.7 V and DVDD + 0.3 V. To reduce the supplies needed, the digital core (DVDD) can be supplied through a simple RC filter from the analog supply, as shown in Figure 22. The AD7651 is independent of power supply sequencing once OVDD does not exceed DVDD by more than 0.3 V, and is thus free of supply voltage induced latch-up.
Rev. 0 | Page 18 of 28
AD7651
POWER DISSIPATION VERSUS THROUGHPUT
Operating currents are very low during the acquisition phase, allowing significant power savings when the conversion rate is reduced (see Figure 25). The AD7651 automatically reduces its power consumption at the end of each conversion phase. This makes the part ideal for very low power battery applications. The digital interface and the reference remain active even during the acquisition phase. To reduce operating digital supply currents even further, digital inputs need to be driven close to the power supply rails (i.e., DVDD or DGND), and OVDD should not exceed DVDD by more than 0.3 V.
100000
The CNVST trace should be shielded with ground and a low value serial resistor (i.e., 50 ) termination should be added close to the output of the component that drives this line. For applications where SNR is critical, the CNVST signal should have very low jitter. This may be achieved by using a dedicated oscillator for CNVST generation, or to clock CNVST with a high frequency, low jitter clock, as shown in Figure 22.
t2 t1
CNVST
POWER DISSIPATION (W)
10000
BUSY
t3 t5
1000
MODE ACQUIRE
t4 t6
CONVERT ACQUIRE CONVERT
t7
t8
02964-0-011
100
Figure 26. Basic Conversion Timing
t9
RESET
10 10
PDREF = PDBUF = PDHIGH 100 1k 10k SAMPLING RATE (SPS) 100k
02964-0-038
Figure 25. Power Dissipation vs. Sampling Rate
BUSY
CONVERSION CONTROL
Figure 26 shows the detailed timing diagrams of the conversion process. The AD7651 is controlled by the CNVST signal, which initiates conversion. Once initiated, it cannot be restarted or aborted, even by the power-down input PD, until the conversion is complete. CNVST operates independently of CS and RD. Conversions can be automatically initiated with the AD7651. If CNVST is held LOW when BUSY is LOW, the AD7651 controls the acquisition phase and automatically initiates a new conversion. By keeping CNVST LOW, the AD7651 keeps the conversion process running by itself. It should be noted that the analog input must be settled when BUSY goes LOW. Also, at power-up, CNVST should be brought LOW once to initiate the conversion process. In this mode, the AD7651 can run slightly faster than the guaranteed 100 kSPS. Although CNVST is a digital signal, it should be designed with special care with fast, clean edges, and levels with minimum overshoot and undershoot or ringing.
DATA
t8
CNVST
02964-0-011
Figure 27. RESET Timing
CS = RD = 0
t1
CNVST
t 10
BUSY
t3
DATA BUS
t4 t11
PREVIOUS CONVERSION DATA NEW DATA
02964-0-012
Figure 28. Master Parallel Data Timing for Reading (Continuous Read)
Rev. 0 | Page 19 of 28
AD7651
DIGITAL INTERFACE
The AD7651 has a versatile digital interface; it can be interfaced with the host system by using either a serial or a parallel interface. The serial interface is multiplexed on the parallel data bus. The AD7651 digital interface also accommodates both 3 V and 5 V logic by simply connecting the OVDD supply pin of the AD7651 to the host system interface digital supply. Finally, by using the OB/2C input pin, both twos complement or straight binary coding can be used. The two signals, CS and RD, control the interface. CS and RD have a similar effect because they are OR'd together internally. When at least one of these signals is HIGH, the interface outputs are in high impedance. Usually CS allows the selection of each AD7651 in multicircuit applications and is held low in a single AD7651 design. RD is generally used to enable the conversion result on the data bus.
CS
RD
BUSY
DATA BUS
CURRENT CONVERSION
t12
t 13
02964-0-013
Figure 29. Slave Parallel Data Timing for Reading (Read after Convert)
CS = 0 CNVST, RD
t1
PARALLEL INTERFACE
The AD7651 is configured to use the parallel interface when SER/PARis held LOW. The data can be read either after each conversion, which is during the next acquisition phase, or during the following conversion, as shown in Figure 29 and Figure 30, respectively. When the data is read during the conversion, however, it is recommended that it is read only during the first half of the conversion phase. This avoids any potential feedthrough between voltage transients on the digital interface and the most critical analog conversion circuitry. The BYTESWAP pin allows a glueless interface to an 8-bit bus. As shown in Figure 31, the LSB byte is output on D[7:0] and the MSB is output on D[15:8] when BYTESWAP is LOW. When BYTESWAP is HIGH, the LSB and MSB bytes are swapped and the LSB is output on D[15:8] and the MSB is output on D[7:0]. By connecting BYTESWAP to an address line, the 16-bit data can be read in two bytes on either D[15:8] or D[7:0].
BUSY
t4 t3
DATA BUS
PREVIOUS CONVERSION
t 12
t13
02964-0-014
Figure 30. Slave Parallel Data Timing for Reading (Read during Convert)
CS
RD
BYTESWAP
SERIAL INTERFACE
The AD7651 is configured to use the serial interface when SER/PAR is held HIGH. The AD7651 outputs 16 bits of data, MSB first, on the SDOUT pin. This data is synchronized with the 16 clock pulses provided on the SCLK pin. The output data is valid on both the rising and falling edges of the data clock.
PINS D[15:8] HI-Z HIGH BYTE LOW BYTE HI-Z
t12
PINS D[7:0] HI-Z LOW BYTE
t12
HIGH BYTE
t13
HI-Z
02964-0-025
Figure 31. 8-Bit Parallel Interface
Rev. 0 | Page 20 of 28
AD7651
MASTER SERIAL INTERFACE
Internal Clock
The AD7651 is configured to generate and provide the serial data clock SCLK when the EXT/INT pin is held LOW. The AD7651 also generates a SYNC signal to indicate to the host when the serial data is valid. The serial clock SCLK and the SYNC signal can be inverted if desired. Depending on the RDC/SDIN input, the data can be read after each conversion or during the following conversion. Figure 32 and Figure 33 show detailed timing diagrams of these two modes. Usually, because the AD7651 has a longer acquisition phase than the conversion phase, the data is read immediately after conversion. This makes the Master Read After Conversion the most recommended serial mode when it can be used. In this mode, it should be noted that unlike in other modes, the BUSY signal returns LOW after the 16 data bits are pulsed out and not at the end of the conversion phase, which results in a longer BUSY width. In the Read During Conversion mode, the serial clock and data toggle at appropriate instants, which minimize potential feedthrough between digital activity and critical conversion decisions
INVSCLK = INVSYNC = 0
EXT/INT = 0
CS, RD
RDC/SDIN = 0
t3
CNVST
BUSY
t28 t29 t30 t 25 t18 t 19
SYNC
t14 t20
SCLK 1
t21
2 3 14 15
t 24
16
t26
t15 t 27
SDOUT X D15 D14 D2 D1 D0
t16
t22
t23
02964-0-015
Figure 32. Master Serial Data Timing for Reading (Read after Convert)
EXT/INT = 0
CS, RD
RDC/SDIN = 1
INVSCLK = INVSYNC = 0
t1
CNVST
t3
BUSY
t17
SYNC
t 25 t19 t20 t21 t 24
2 3 14 15 16
t14
SCLK
t15 t18
t 26
1
t 27
D15 D14 D2 D1 D0
SDOUT
X
t 16
t22
t23
02964-0-016
Figure 33. Master Serial Data Timing for Reading (Read Previous Conversion during Convert)
Rev. 0 | Page 21 of 28
AD7651
SLAVE SERIAL INTERFACE
External Clock
The AD7651 is configured to accept an externally supplied serial data clock on the SCLK pin when the EXT/INT pin is held HIGH. In this mode, several methods can be used to read the data. The external serial clock is gated by CS. When CS and RD are both LOW, the data can be read after each conversion or during the following conversion. The external clock can be either a continuous or a discontinuous clock. A discontinuous clock can be either normally HIGH or normally LOW when inactive. Figure 34 and Figure 35 show the detailed timing diagrams of these methods. Usually, because the AD7651 has a longer acquisition phase than conversion phase, the data are read immediately after conversion. While the AD7651 is performing a bit decision, it is important that voltage transients be avoided on digital input/output pins or degradation of the conversion result could occur. This is particularly important during the second half of the conversion phase because the AD7651 provides error correction circuitry that can correct for an improper bit decision made during the first half of the conversion phase. For this reason, it is recommended that when an external clock is being provided, it is a discontinuous clock that is toggling only when BUSY is LOW, or, more importantly, that it does not transition during the latter half of BUSY HIGH.
RD
EXT/INT = 1
INVSCLK = 0
RD = 0
BUSY
t36
SCLK 1
t35 t37
2 3 14 15 16 17 18
t31
SDOUT X D15
t32
D14 D13 D1 D0 X15 X14
t16
SDIN
t 34
X15 X14 X13 X1 X0 Y15 Y14
02964-0-017
t33
Figure 34. Slave Serial Data Timing for Reading (Read after Convert)
CS
EXT/INT = 1
INVSCLK = 0
RD = 0
CNVST
BUSY
t3
t35 t36 t37
1 2 3 14 15 16
SCLK
t31
SDOUT X D15
t32
D14 D13 D1 D0
02965-0-018
t16
Figure 35. Slave Serial Data Timing for Reading (Read Previous Conversion during Convert)
Rev. 0 | Page 22 of 28
AD7651
External Discontinuous Clock Data Read After Conversion
Though the maximum throughput cannot be achieved using this mode, it is the most recommended of the serial slave modes. Figure 34 shows the detailed timing diagrams of this method. After a conversion is complete, indicated by BUSY returning LOW, the conversion's result can be read while both CS and RD are LOW. Data is shifted out MSB first with 16 clock pulses and is valid on the rising and falling edges of the clock. Among the advantages of this method is the fact that conversion performance is not degraded because there are no voltage transients on the digital interface during the conversion process. Another advantage is the ability to read the data at any speed up to 40 MHz, which accommodates both the slow digital host interface and the fastest serial reading. Finally, in this mode only, the AD7651 provides a daisy-chain feature using the RDC/SDIN pin for cascading multiple converters together. This feature is useful for reducing component count and wiring connections when desired, as, for instance, in isolated multiconverter applications. An example of the concatenation of two devices is shown in Figure 36. Simultaneous sampling is possible by using a common CNVST signal. It should be noted that the RDC/SDIN input is latched on the opposite edge of SCLK of the one used to shift out the data on SDOUT. Therefore, the MSB of the "upstream" converter just follows the LSB of the "downstream" converter on the next SCLK cycle.
BUSY OUT BUSY BUSY
External Clock Data Read During Conversion
Figure 35 shows the detailed timing diagrams of this method. During a conversion, while both CS and RD are LOW, the result of the previous conversion can be read. The data is shifted out MSB first with 16 clock pulses, and is valid on both the rising and falling edges of the clock. The 16 bits must be read before the current conversion is complete; otherwise, RDERROR is pulsed HIGH and can be used to interrupt the host interface to prevent incomplete data reading. There is no daisy-chain feature in this mode and the RDC/SDIN input should always be tied either HIGH or LOW. To reduce performance degradation due to digital activity, a fast discontinuous clock of at least 18 MHz is recommended to ensure that all the bits are read during the first half of the conversion phase. It is also possible to begin to read data after conversion and continue to read the last bits after a new conversion has been initiated. This allows the use of a slower clock speed like 14 MHz.
#2 (UPSTREAM) RDC/SDIN SDOUT
CNVST CS
AD7651
#1 (DOWNSTREAM) RDC/SDIN SDOUT
CNVST CS
AD7651
DATA OUT
SCLK
SCLK
SCLK IN CS IN CNVST IN
02964-0-019
Figure 36. Two AD7651s in a Daisy-Chain Configuration
Rev. 0 | Page 23 of 28
AD7651
MICROPROCESSOR INTERFACING
The AD7651 is ideally suited for traditional dc measurement applications supporting a microprocessor, and for ac signal processing applications interfacing to a digital signal processor. The AD7651 is designed to interface either with a parallel 8-bit or 16-bit wide interface, or with a general-purpose serial port or I/O ports on a microcontroller. A variety of external buffers can be used with the AD7651 to prevent digital noise from coupling into the ADC. The following section discusses the use of an AD7651 with an ADSP-219x SPI equipped DSP. (MSTR) = 1, Clock Polarity bit (CPOL) = 0, Clock Phase bit (CPHA) = 1, and SPI Interrupt Enable (TIMOD) = 00--by writing to the SPI control register (SPICLTx). To meet all timing requirements, the SPI clock should be limited to 17 Mbps, which allows it to read an ADC result in less than 1 s. When a higher sampling rate is desired, use of one of the parallel interface modes is recommended.
DVDD
AD7651*
SER/PAR EXT/INT BUSY CS RD INVSCLK SDOUT SCLK CNVST PFx
ADSP-219x*
SPI Interface (ADSP-219x)
Figure 37 shows an interface diagram between the AD7651 and the SPI equipped ADSP-219x. To accommodate the slower speed of the DSP, the AD7651 acts as a slave device and data must be read after conversion. This mode also allows the daisychain feature. The convert command can be initiated in response to an internal timer interrupt. The reading process can be initiated in response to the end-of-conversion signal (BUSY going LOW) using an interrupt line of the DSP. The serial interface (SPI) on the ADSP-219x is configured for master mode--
SPIxSEL (PFx) MISOx SCKx PFx or TFSx
* ADDITIONAL PINS OMITTED FOR CLARITY
02964-0-021
Figure 37. Interfacing the AD7651 to an SPI Interfac
Rev. 0 | Page 24 of 28
AD7651 APPLICATION HINTS
BIPOLAR AND WIDER INPUT RANGES
In some applications, it is desirable to use a bipolar or wider analog input range such as 10 V, 5 V, or 0 V to 5 V. Although the AD7651 has only one unipolar range, simple modifications of input driver circuitry allow bipolar and wider input ranges to be used without any performance degradation. Figure 38 shows a connection diagram that allows this. Component values required and resulting full-scale ranges are shown in Table 8. When desired, accurate gain and offset can be calibrated by acquiring a ground and voltage reference using an analog multiplexer (U2), as shown in Figure 38.
CF R1 ANALOG INPUT R2 U1 IN
be established as close as possible to the AD7651. Running digital lines under the device should be avoided since these will couple noise onto the die. The analog ground plane should be allowed to run under the AD7651 to avoid noise coupling. Fast switching signals like CNVST or clocks should be shielded with digital ground to avoid radiating noise to other sections of the board, and should never run near analog signal paths. Crossover of digital and analog signals should be avoided. Traces on different but close layers of the board should run at right angles to each other. This will reduce the effect of crosstalk through the board. The power supply lines to the AD7651 should use as large a trace as possible to provide low impedance paths and reduce the effect of glitches on the power supply lines. Good decoupling is also important to lower the supply's impedance presented to the AD7651 and to reduce the magnitude of the supply spikes. Decoupling ceramic capacitors, typically 100 nF, should be placed on each power supply pin--AVDD, DVDD, and OVDD--close to, and ideally right up against these pins and their corresponding ground pins. Additionally, low ESR 10 F capacitors should be located near the ADC to further reduce low frequency ripple. The DVDD supply of the AD7651 can be a separate supply or can come from the analog supply AVDD or the digital interface supply OVDD. When the system digital supply is noisy or when fast switching digital signals are present, if no separate supply is available, the user should connect DVDD to AVDD through an RC filter (see Figure 22) and the system supply to OVDD and the remaining digital circuitry. When DVDD is powered from the system supply, it is useful to insert a bead to further reduce high frequency spikes. The AD7651 has five different ground pins: INGND, REFGND, AGND, DGND, and OGND. INGND is used to sense the analog input signal. REFGND senses the reference voltage and, because it carries pulsed currents, should be a low impedance return to the reference. AGND is the ground to which most internal ADC analog signals are referenced; it must be connected with the least resistance to the analog ground plane. DGND must be tied to the analog or digital ground plane depending on the configuration. OGND is connected to the digital system ground.
AD7651
U2 R3 R4 100nF INGND REF CREF REFGND
02964-0-022
Figure 38. Using the AD7651 in 16-Bit Bipolar and/or Wider Input Ranges
Table 8. Component Values and Input Ranges
Input Range 10 V 5 V 0 V to -5 V R1 () 500 500 500 R2 (k) 4 2 1 R3 (k) 2.5 2.5 None R4 (k) 2 1.67 0
LAYOUT
The AD7651 has very good immunity to noise on the power supplies. However, care should still be taken with regard to grounding layout. The printed circuit board that houses the AD7651 should be designed so the analog and digital sections are separated and confined to certain areas of the board. This facilitates the use of ground planes that can be separated easily. Digital and analog ground planes should be joined in only one place, preferably underneath the AD7651, or as close as possible to the AD7651. If the AD7651 is in a system where multiple devices require analog-to-digital ground connections, the connection should still be made at one point only, a star ground point that should
EVALUATING THE AD7651'S PERFORMANCE
A recommended layout for the AD7651 is outlined in the EVAL-AD7651 evaluation board for the AD7651. The evaluation board package includes a fully assembled and tested evaluation board, documentation, and software for controlling the board from a PC via the EVAL-CONTROL BRD2.
Rev. 0 | Page 25 of 28
AD7651 OUTLINE DIMENSIONS
0.75 0.60 0.45 1.60 MAX
48 1
PIN 1
9.00 BSC SQ
37 36
1.45 1.40 1.35
10 6 2
SEATING PLANE
0.20 0.09 7 3.5 0 0.10 MAX COPLANARITY
TOP VIEW
(PINS DOWN )
7.00 BSC SQ
VIEW A
12 13 24 25
0.15 0.05
SEATING PLANE
VIEW A
ROTATED 90 CCW
0.50 BSC
0.27 0.22 0.17
COMPLIANT TO JEDEC STANDARDS MS-026BBC
Figure 39. 48-Lead Quad Flatpack (LQFP) [ST-48] Dimensions shown in millimeters
0.30 0.23 0.18
48 1
7.00 BSC SQ
0.60 MAX 0.60 MAX
37 36
PIN 1 INDICATOR
PIN 1 INDICATOR
TOP VIEW
6.75 BSC SQ
BOTTOM VIEW
5.25 5.10 SQ 4.95
0.50 0.40 0.30
25 24
12 13
0.25 MIN 1.00 0.85 0.80 0.80 MAX 0.65 TYP 0.05 MAX 0.02 NOM 0.50 BSC SEATING PLANE 0.20 REF COPLANARITY 0.08 5.50 REF PADDLE CONNECTED TO AGND. THIS CONNECTION IS NOT REQUIRED TO MEET THE ELECTRICAL PERFORMANCES
12 MAX
COMPLIANT TO JEDEC STANDARDS MO-220-VKKD-2
Figure 40. 48-Lead Frame Chip Scale Package (LFCSP) [CP-48] Dimensions shown in millimeters
ORDERING GUIDE
Model AD7651AST AD7651ASTRL AD7651ACP AD7651ACPRL EVAL-AD7651CB1 EVAL-CONTROL BRD22 Temperature Range -40C to +85C -40C to +85C -40C to +85C -40C to +85C Package Description Quad Flatpack (LQFP) Quad Flatpack (LQFP) Lead Frame Chip Scale (LFCSP) Lead Frame Chip Scale (LFCSP) Evaluation Board Controller Board Package Option ST-48 ST-48 CP-48 CP-48
1 2
This board can be used as a standalone evaluation board or in conjunction with the EVAL-CONTROL BRD2 for evaluation/demonstration purposes. This board allows a PC to control and communicate with all Analog Devices evaluation boards ending in the CB designators.
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AD7651
NOTES
Rev. 0 | Page 27 of 28
AD7651
NOTES
(c) 2003 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective companies. C02964-0-9/03(0)
Rev. 0 | Page 28 of 28
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