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18-Bit, 2.5 LSB INL, 100 kSPS SAR ADC AD7678
FEATURES
18-bit resolution with no missing codes No pipeline delay (SAR architecture) Differential input range: VREF (VREF up to 5 V) Throughput: 100 kSPS INL: 2.5 LSB max (9.5 ppm of full scale) Dynamic range: 103 dB typ (VREF = 5 V) S/(N+D): 100 dB typ @ 2 kHz (VREF = 5 V) Parallel (18-,16-, or 8-bit bus) and serial 5 V/3 V interface SPI(R)/QSPITM/MICROWIRETM/DSP compatible On-board reference buffer Single 5 V supply operation Power dissipation: 18 mW @ 100 kSPS 180 W @ 1 kSPS 48-lead LQFP or 48-lead LFCSP package Pin-to-pin compatible upgrade of AD7674/AD7676/AD7679
FUNCTIONAL BLOCK DIAGRAM
PDBUF AGND AVDD REFBUFIN IN+ IN- SWITCHED CAP DAC PARALLEL INTERFACE CLOCK PD RESET CONTROL LOGIC AND CALIBRATION CIRCUITRY REF REFGND DVDD DGND OVDD SERIAL PORT 18 OGND
AD7678
D[17:0] BUSY RD CS MODE0 MODE1
CNVST
03084-0-001
Figure 1. Functional Block Diagram
APPLICATIONS
CT scanners High dynamic data acquisition Geophone and hydrophone sensors - replacement (low power, multichannel) Instrumentation Spectrum analysis Medical instruments
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 AD7678 is an 18-bit, 100 kSPS, charge redistribution SAR, fully differential analog-to-digital converter that operates on a single 5 V power supply. The part contains a high speed 18-bit sampling ADC, an internal conversion clock, an internal reference buffer, error correction circuits, and both serial and parallel system interface ports. The part is available in 48-lead LQFP or 48-lead LFCSP packages with operation specified from -40C to +85C.
PRODUCT HIGHLIGHTS
1. High Resolution, Fast Throughput. The AD7678 is a 100 kSPS, charge redistribution, 18-bit SAR ADC (no latency). Excellent Accuracy. The AD7678 has a maximum integral nonlinearity of 2.5 LSB with no missing 18-bit codes. Serial or Parallel Interface. Versatile parallel (18-, 16-, or 8-bit bus) or 2-wire serial interface arrangement compatible with both 3 V and 5 V logic.
2.
3.
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 companies.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. www.analog.com Tel: 781.329.4700 Fax: 781.326.8703 (c) 2003 Analog Devices, Inc. All rights reserved.
AD7678 TABLE OF CONTENTS
Specifications..................................................................................... 3 Timing Specifications....................................................................... 5 Absolute Maximum Ratings............................................................ 7 Pin Configuration and Functional Descriptions.......................... 8 Definition 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 Layout .......................................................................................... 25 Evaluating the AD7678's Performance.................................... 25 Outline Dimensions ....................................................................... 26 Ordering Guide .......................................................................... 26
REVISION HISTORY
Revision 0: Initial Version
Rev. 0 | Page 2 of 28
AD7678 SPECIFICATIONS
Table 2. -40C to +85C, VREF = 4.096 V, 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 Differential Linearity Error No Missing Codes Transition Noise Zero Error, TMIN to TMAX3 Zero Error Temperature Drift Gain Error, TMIN to TMAX3 Gain Error Temperature Drift Power Supply Sensitivity AC ACCURACY Signal-to-Noise Conditions Min 18 -VREF -0.1 65 4 Typ Max Unit Bits V V dB A
VIN+ - VIN- VIN+, VIN- to AGND fIN = 100 kHz 100 kSPS Throughput
+VREF AVDD + 0.1
0 -2.5 -1 18 VREF = 5 V -40 -0.048 AVDD = 5 V 5% fIN = 2 kHz, VREF = 5 V VREF = 4.096 V fIN = 10 kHz, VREF = 4.096 V fIN = 45 kHz, VREF = 4.096 V VIN+ = VIN- = VREF/2 = 2.5 V fIN = 2 kHz fIN = 10 kHz fIN = 45 kHz fIN = 2 kHz fIN = 10 kHz fIN = 45 kHz fIN = 2 kHz fIN = 2 kHz, -60 dB Input 0.5 See Note 3 1.6 4 101 100 99.5 98 103 120 117 110 -118 -115 -110 100 41 900 2 5 Full-Scale Step 0.7
10 100 +2.5 +1.75
s kSPS LSB2 LSB Bits LSB LSB ppm/C % of FSR ppm/C LSB dB4 dB dB dB dB dB dB dB dB dB dB dB dB kHz ns ps rms s s V V V A A
40 +0.048
98
Dynamic Range Spurious-Free Dynamic Range
Total Harmonic Distortion
Signal-to-(Noise + Distortion) -3 dB Input Bandwidth SAMPLING DYNAMICS Aperture Delay Aperture Jitter Transient Response Overvoltage Recovery REFERENCE External Reference Voltage Range REF Voltage with Reference Buffer Reference Buffer Input Voltage Range REFBUFIN Input Current REF Current Drain
8.5 8.5 3 4.05 1.8 -1 4.096 4.096 2.5 42 AVDD + 0.1 4.15 2.6 +1
REF REFBUFIN = 2.5 V REFBUFIN 100 kSPS Throughput
Rev. 0 | Page 3 of 28
AD7678
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 AVDD DVDD8 OVDD8 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 PDBUF High
5 5
5.25 5.25 DVDD + 0.37
V V V mA mA A mW W mW C
PDBUF High @ 100 kSPS PDBUF High @ 1 kSPS PDBUF Low @ 100 kSPS TEMPERATURE RANGE9 Specified Performance TMIN to TMAX -40
2.6 1 40 18 180 31
26
+85
1 2
See Analog Inputs section. LSB means Least Significant Bit. With the 4.096 V input range, 1 LSB is 31.25 V. 3 See Definition of Specifications section. The nominal gain error is not centered at zero and is +0.273% of FSR. This specification is the deviation from this nominal value. These specifications do not include the error contribution from the external reference, but do include the error contribution from the reference buffer if used. 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 Data format parallel or serial 18-bit. 6 Conversion results are available immediately after completed conversion. 7 The maximum should be the minimum of 5.25 V and DVDD + 0.3 V. 8 Tested in Parallel Reading mode. 9 Contact factory for extended temperature range.
Rev. 0 | Page 4 of 28
AD7678 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 27 and Figure 28 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 Pulsewidth Refer to Figure 29, Figure 30, and Figure 31 (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 33 and Figure 34 (Master Serial Interface Modes)1 CS LOW to SYNC Valid Delay CS LOW to Internal SCLK Valid Delay CS LOW to SDOUT Delay CNVST LOW to SYNC Delay SYNC Asserted to SCLK First Edge Delay2 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 35 and Figure 36 (Slave Serial Interface Modes) 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 1.5 2 10 1.5 8.5 10 1.5 20 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 1.5 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
AD7678
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 t28 0 0 3 25 40 12 7 4 2 3 2.25 0 1 17 60 80 22 21 18 4 60 3 1 0 17 120 160 50 49 18 30 140 4.5 1 1 17 240 320 100 99 18 89 300 7.5 Unit ns ns ns ns ns ns ns ns s
Rev. 0 | Page 6 of 28
AD7678 ABSOLUTE MAXIMUM RATINGS
Table 5. AD7678 Absolute Maximum Ratings1
Parameter Analog Inputs IN+2, IN-2, REF, REFBUFIN, 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 Internal Power Dissipation3 Internal Power Dissipation4 Junction Temperature Storage Temperature Range Lead Temperature Range (Soldering 10 sec) Rating AVDD + 0.3 V to AGND - 0.3 V 0.3 V
500A IOH 1.6mA IOL
TO OUTPUT PIN
CL 60pF1
1.4V
-0.3 V to +7 V 7 V -0.3 V to +7 V -0.3 V to DVDD + 0.3 V 700 mW 2.5 W 150C -65C to +150C 300C
1 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.
03084-0-002
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
03084-0-003
1
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. 2 See Analog Inputs section. 3 Specification is for device in free air: 48-Lead LQFP: JA = 91C/W, JC = 30C/W. 4 Specification is for device in free air: 48-Lead LFCSP: JA = 26C/W.
Figure 3. Voltage Reference Levels for Timing
Rev. 0 | Page 7 of 28
AD7678 PIN CONFIGURATION AND FUNCTIONAL DESCRIPTIONS
AVDD REFBUFIN
48 47 46 45 44 43 42 41 40 39 38 37
AGND 1 AVDD 2 MODE0 3 MODE1 4 D0/OB/2C 5 NC 6 NC 7 D1/A0 8 D2/A1 9 D3 10 D4/DIVSCLK[0] 11 D5/DIVSCLK[1] 12 NC = NO CONNECT
REFGND REF
PDBUF
NC AGND
IN+ NC
NC NC IN-
PIN 1 IDENTIFIER
36 35 34
AGND
CNVST PD 33 RESET
AD7678
TOP VIEW (Not to Scale)
32 31 30
CS
RD DGND 29 BUSY 28 D17
27 26 25
D16 D15 D14
13 14 15 16 17 18 19 20 21 22 23 24
D6/EXT/INT D7/INVSYNC D8/INVSCLK D9/RDC/SDIN OGND
D12/SYNC D13/RDERROR
03084-0-004
OVDD DVDD DGND D10/SDOUT
Figure 4. 48-Lead LQFP and 48-Lead LFCSP (ST-48 and CP-48)
Table 6. Pin Function Descriptions
Pin No. 1, 44 2, 47 3 4 Mnemonic AGND AVDD MODE0 MODE1 Type1 P P DI DI Description Analog Power Ground Pin. Input Analog Power Pins. Nominally 5 V. Data Output Interface Mode Selection. Data Output Interface Mode Selection: Interface MODE # MODE1 MODE0 Description 0 0 0 18-Bit Interface 1 0 1 16-Bit Interface 2 1 0 Byte Interface 3 1 1 Serial Interface When MODE = 0 (18-bit interface mode), this pin is Bit 0 of the parallel port data output bus and the data coding is straight binary. In all other modes, this pin allows choice of 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. No Connect.
5
D0/OB/2C
DI/O
6, 7, 40-42, 45 8 9 10 11, 12
NC
D1/A0 D2/A1 D3 D[4:5]or DIVSCLK[0:1]
DI/O DI/O DO DI/O
When MODE = 0 (18-bit interface mode), this pin is Bit 1 of the parallel port data output bus. In all other modes, this input pin controls the form in which data is output, as shown in Table 7. When MODE = 0 or 1 (18-bit or 16-bit interface mode), this pin is Bit 2 of the parallel port data output bus. In all other modes, this input pin controls the form in which data is output, as shown in Table 7. In all modes except MODE = 3, this output is used as Bit 3 of the parallel port data output bus. This pin is always an output, regardless of the interface mode. In all modes except MODE = 3, these pins are Bit 4 and Bit 5 of the parallel port data output bus. When MODE = 3 (serial mode), 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.
Rev. 0 | Page 8 of 28
D11/SCLK
AD7678
Pin No. 13 Mnemonic D6 or EXT/INT Type1 DI/O Description In all modes except MODE = 3, this output is used as Bit 6 of the parallel port data output bus. When MODE = 3 (serial mode), 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. In all modes except MODE = 3, this output is used as Bit 7 of the parallel port data output bus. When MODE = 3 (serial mode), this input, part of the serial port, is used to select the active state of the SYNC signal. When LOW, SYNC is active HIGH. When HIGH, SYNC is active LOW. In all modes except MODE = 3, this output is used as Bit 8 of the parallel port data output bus. When MODE = 3 (serial mode), this input, part of the serial port, is used to invert the SCLK signal. It is active in both master and slave modes. In all modes except MODE = 3, this output is used as Bit 9 of the parallel port data output bus. When MODE = 3 (serial mode), 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 SDOUT with a delay of 18 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. Output Interface Digital Power. Nominally at the same supply as the host interface (5 V or 3 V). Should not exceed DVDD by more than 0.3 V. Digital Power. Nominally at 5 V. Digital Power Ground. In all modes except MODE = 3, this output is used as Bit 10 of the parallel port data output bus. When MODE = 3 (serial mode), 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 AD7678 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 and INVSCLK is LOW, SDOUT is updated on the SCLK rising edge and is valid on the next falling edge; if INVSCLK is HIGH, SDOUT is updated on the SCLK falling edge and is valid on the next rising edge. In all modes except MODE = 3, this output is used as Bit 11 of the parallel port data output bus. When MODE = 3 (serial mode), 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. In all modes except MODE = 3, this output is used as Bit 12 of the parallel port data output bus. When MODE = 3 (serial mode), 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 SDOUT output is valid. In all modes except MODE = 3, this output is used as Bit 13 of the parallel port data output bus. In MODE = 3 (serial mode) and when EXT/INT is 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 14 to Bit 17 of the Parallel Port Data Output Bus. These pins are always outputs regardless of the interface mode. Busy Output. Transitions HIGH when a conversion is started. 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, reset the AD7678. Current conversion, if any, is aborted. If not used, this pin could be tied to DGND.
Rev. 0 | Page 9 of 28
14
D7 or INVSYNC D8 or INVSCLK D9 or RDC/SDIN
DI/O
15
DI/O
16
DI/O
17 18 19 20 21
OGND OVDD DVDD DGND D10 or SDOUT
P P P P DO
22
D11 or SCLK
DI/O
23
D12 or SYNC
DO
24
D13 or RDERROR
DO
25-28 29
D[14:17] BUSY
DO DO
30 31 32 33
DGND RD CS RESET
P DI DI DI
AD7678
Pin No. 34 35 Mnemonic PD CNVST Type1 DI DI Description 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 held 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. If CNVST is held LOW when the acquisition phase is complete, the internal sample/hold is put into the hold state and a conversion is started immediately. Must Be Tied to Analog Ground. Reference Input Voltage and Internal Reference Buffer Output. Apply an external reference on this pin if the internal reference buffer is not used. Should be decoupled effectively with or without the internal buffer. Reference Input Analog Ground. Differential Negative Analog Input. Differential Positive Analog Input. Reference Buffer Input Voltage. The internal reference buffer has a fixed gain. It outputs 4.096 V typically when 2.5 V is applied on this pin. Allows Choice of Buffering Reference. When LOW, buffer is selected. When HIGH, buffer is switched off.
36 37
AGND REF
P AI
38 39 43 46 48
REFGND IN- IN+ REFBUFIN PDBUF
AI AI AI AI DI
1
AI = Analog Input; AO = Analog Output; DI = Digital Input; DI/O = Bidirectional Digital; DO = Digital Output; P = Power.
Table 7. Data Bus Interface Definitions
MODE 0 1 1 2 2 2 2 3 MODE1 0 0 0 1 1 1 1 1 MODE0 0 1 1 0 0 0 0 1 D0/OB/2C R[0] OB/2C OB/2C OB/2C OB/2C OB/2C OB/2C OB/2C D1/A0 R[1] A0:0 A0:1 A0:0 A0:0 A0:1 A0:1 D2/A1 R[2] R[2] R[0] A1:0 A1:1 A1:0 A1:1 All Hi-Z D[3] R[3] R[3] R[1] All Hi-Z All Hi-Z All Hi-Z All Hi-Z R[2:3] R[0:1] All Zeros Serial Interface D[4:9] R[4:9] R[4:9] D[10:11] R[10:11] R[10:11] R[10:11] D[12:15] R[12:15] R[12:15] R[12:15] R[4:7] D[16:17] R[16:17] R[16:17] R[16:17] R[8:9] All Zeros R[0:1] Description 18-Bit Parallel 16-Bit High Word 16-Bit Low Word 8-Bit HIGH Byte 8-Bit MID Byte 8-Bit LOW Byte 8-Bit LOW Byte Serial Interface
All Zeros
R[0:17] is the 18-bit ADC value stored in its output register.
Rev. 0 | Page 10 of 28
AD7678 DEFINITION 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.
Dynamic Range
Dynamic range is the ratio of the rms value of the full scale to the rms noise measured with the inputs shorted together. The value for dynamic range 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 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.
Gain Error
The first transition (from 000...00 to 000...01) should occur for an analog voltage 1/2 LSB above the nominal -full scale (-4.095991 V for the 4.096 V range). The last transition (from 111...10 to 111...11) should occur for an analog voltage 11/2 LSB below the nominal full scale (4.095977 V for the 4.096 V range). The gain error is the deviation of the difference between the actual level of the last transition and the actual level of the first transition from the difference between the ideal levels.
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.
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.
Zero Error
The zero error is the difference between the ideal midscale input voltage (0 V) from the actual voltage producing the midscale output code.
Transient Response
Transient response is the time required for the AD7678 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.
Effective Number of Bits (ENOB)
ENOB is a measurement of the resolution with a sine wave input, and is expressed in bits. It is related to S/(N+D) by the following formula: ENOB = (S/[N+D]dB - 1.76)/6.02
Rev. 0 | Page 11 of 28
AD7678 TYPICAL PERFORMANCE CHARACTERISTICS
2.5 2.0 1.5 1.0 0.5 0 -0.5 -1.0 -1.5 -2.0 -2.5 0 65536 131072 CODE 196608 262144
03084-0-005
2.0
1.5
DNL-LSB (18-Bit)
INL-LSB (18-Bit)
1.0
0.5
0
-0.5
-1.0
0
65536
131072 CODE
196608
262144
03084-0-008
Figure 5. Integral Nonlinearity vs. Code
70000 60000 50000 60000 60158 59966 VREF = 5V 90000 80000 70000
Figure 8. Differential Nonlinearity vs. Code
83610 VREF = 5V
COUNTS
COUNTS
40000 30000 20000
50000 40000 30000 20000 23000
21862
10000 0 0 0 32
5919
3931 42 0 0
10000 0 0 0 522 1053 1 0 2001B 2001C 2001D 2001E 2001F 20020 20021 20022 20023 CODE IN HEX
20015 20016 20017 20018 20019 2001A2001B2001C2001D 2001E CODE IN HEX
03084-0-006
03084-0-009
Figure 6. Histogram of 131,072 Conversions of a DC Input at the Code Transition
0 -20 fS = 100kSPS fIN = 11kHz VREF = 4.096V SNR = 99.6dB THD = -116dB SFDR = 116.2dB S/(N+D) = 99.5dB 102
Figure 9. Histogram of 131,072 Conversions of a DC Input at the Code Center
16.6
AMPLITUDE (dB of Full Scale)
-40 -60 -80 -100 -120 -140 -160 -180 0 5 10 15
100 SNR AND S/[N+D] (dB)
16.4
98
SNR S/(N+D)
16.2
96 ENOB
16.0
20 25 30 35 FREQUENCY (kHz)
40
45
50
94
0
10
20
30
40
15.8 50
03084-0-015
03084-0-012
FREQUENCY (kHz)
Figure 7. FFT (11 kHz Tone)
Figure 10. SNR, S/(N+D), and ENOB vs. Frequency
Rev. 0 | Page 12 of 28
ENOB (Bits)
AD7678
-80 -90 -100
THD, HARMONICS (dB)
THD, HARMONICS (dB)
-100 -110 THD -120 -130 -140 -150 THIRD HARMONIC SECOND HARMONIC
-110 THD THIRD HARMONIC SECOND HARMONIC -130
-120
0
10
20 30 FREQUENCY (kHz)
40
50
03084-0-016
-140 -55
-35
-15
5
25
45
65
85
105
125
TEMPERATURE (C)
03084-0-019
Figure 11. THD and Harmonics vs. Frequency
104
Figure 14. THD and Harmonics vs. Temperature
10000
SNR REFERRED TO FULL SCALE (dB)
VREF = 4.096V 103
OPERATING CURRENT (A)
1000 100 10 1 0.1 0.01 0.001 OVDD AVDD DVDD
102 SNR 101 S/(N+D)
100
99
98 -60
-50
-40
-30
-20
-10
0
03084-0-017
1
10
INPUT LEVEL (dB)
100 1k SAMPLING RATE (SPS)
10k
100k
03084-0-020
Figure 12. SNR and S/(N+D) vs. Input Level
101
Figure 15. Operating Current vs. Sampling Rate
16.5 1000
POWER-DOWN OPERATING CURRENTS (nA)
SNR
800 DVDD 600
100
16.0 S/(N+D)
SNR, S/[N+D] (dB)
99
ENOB
15.5
400 AVDD 200 OVDD
98
15.0
97 -55
-35
-15
5
25
45
65
85
105
14.5 125
0 -55
-35
-15
TEMPERATURE (C)
03084-0-018
5 25 45 65 TEMPERATURE (C)
85
105
125
03084-0-021
Figure 13. SNR, S/(N+D), and ENOB vs. Temperature
Figure 16. Power-Down Operating Currents vs. Temperature
Rev. 0 | Page 13 of 28
AD7678
50 40 50 OVDD = 2.7V @ 85C GAIN ERROR 40
ZERO ERROR, GAIN ERROR (LSB)
30 20
t12 DELAY (ns)
10 0 -10 -20 -30 -40 -50 -55 -35 -15 5 25 45 65 85 105 125 ZERO ERROR
30
20 OVDD = 2.7V @ 25C OVDD = 5V @ 85C 10 OVDD = 5V @ 25C
0
0
50
100 CL (pF)
150
200
03084-0-024
TEMPERATURE (C)
03083-0-022
Figure 17. Zero Error and Gain Error vs. Temperature
Figure 18. Typical Delay vs. Load Capacitance CL
Rev. 0 | Page 14 of 28
AD7678 CIRCUIT INFORMATION
IN+ MSB 262,144C 131,072C REF REFGND 262,144C 131,072C MSB 4C 2C C C LSB SW- CNVST LSB 4C 2C C C BUSY COMP CONTROL LOGIC OUTPUT CODE SW+ SWITCHES CONTROL
IN-
03084-0-025
Figure 19. ADC Simplified Schematic
The AD7678 is a very fast, low power, single-supply, precise 18-bit analog-to-digital converter (ADC) using successive approximation architecture. The AD7678's linearity and dynamic range are similar or better than many - ADCs. With the advantages of its successive architecture, which ease multiplexing and reduce power with throughput, it can be advantageous in applications that normally use - ADCs. The AD7678 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 AD7678 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 a 48-lead LQFP, or a tiny 48-lead LFCSP package that offers space savings and allows for flexible configurations as either a serial or parallel interface. The AD7678 is pin-to-pin compatible with the AD7674, AD7676, and AD7679.
CONVERTER OPERATION
The AD7678 is a successive approximation ADC based on a charge redistribution DAC. Figure 19 shows the simplified schematic of the ADC. The capacitive DAC consists of two identical arrays of 18 binary weighted capacitors, which are connected to the two comparator inputs. During the acquisition phase, terminals of the array tied to the comparator's input are connected to AGND via SW+ and SW-. All independent switches are connected to the analog inputs. Thus, the capacitor arrays are used as sampling capacitors and acquire the analog signal on IN+ and IN- inputs. When the acquisition phase is complete and the CNVST input goes low, a conversion phase is initiated. When the conversion phase begins, SW+ and SW- are opened first. The two capacitor arrays are then disconnected from the inputs and connected to the REFGND input. Therefore, the differential voltage between the IN+ and IN- inputs 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/262144). The control logic toggles these switches, starting with the MSB first, to bring the comparator back into a balanced condition. After completing this process, the control logic generates the ADC output code and brings the BUSY output low.
Rev. 0 | Page 15 of 28
AD7678
Transfer Functions
Except in 18-bit interface mode, the AD7678 offers straight binary and twos complement output coding when using OB/2C. See Figure 20 and Table 8 for the ideal transfer characteristic. Table 8. Output Codes and Ideal Input Voltages
Analog Input VREF = 4.096 V 4.095962 V 4.095924 V 31.25 V 0V -31.25 V -4.095962 V -4.096 V Straight Binary (Hex) 3FFFF1 3FFFE 20001 20000 1FFFF 00001 000002 Twos Complement (Hex) 1FFFF1 1FFFE 00001 00000 3FFFF 20001 200002
ADC CODE (Straight Binary)
111...111 111...110 111...101
Description FSR -1 LSB FSR - 2 LSB Midscale + 1 LSB Midscale Midscale - 1 LSB -FSR + 1 LSB -FSR
000...010 000...001 000...000 -FS -FS + 1 LSB
1
+FS - 1 LSB +FS - 1.5 LSB
2
-FS + 0.5 LSB ANALOG INPUT
This is also the code for overrange analog input (VIN+ - VIN- above VREF - VREFGND). This is also the code for underrange analog input (VIN+ - VIN- below -VREF + VREFGND).
03084-0-026
Figure 20. ADC Ideal Transfer Function
DVDD ANALOG SUPPLY (5V) 20 +
NOTE 5
DIGITAL SUPPLY (3.3V OR 5V) + 10 F 100nF 100nF + 10 F
10F
100nF
ADR421
2.5V REF
NOTE 1
AVDD 1M 100nF
NOTE 3
AGND
DGND
DVDD
OVDD
OGND SCLK SDOUT SERIAL PORT
REFBUFIN 50k 100nF
CREF 10F
NOTE 2
REF BUSY REFGND C/P/DSP CNVST
50 - U1 + CC
NOTE 4
IN+
ANALOG INPUT+
AD7678
MODE1 MODE0 OB/2C
DVDD
AD8021
50 - U2 + CC
PDBUF CS RD IN- RESET PD
CLOCK
NOTE 4
ANALOG INPUT-
AD8021
NOTES 1. SEE VOLTAGE REFERENCE SECTION. 2. CREF is 10F CERAMIC CAPACITOR OR LOW ESR TANTALUM. CERAMIC SIZE 1206 PANASONIC ECJ-3xB0J106 IS RECOMMENDED. SEE VOLTAGE REFERENCE SECTION. 3. OPTIONAL CIRCUITRY FOR HARDWARE GAIN CALIBRATION. 4.THE AD8021 IS RECOMMENDED. SEE DRIVER AMPLIFIER CHOICE SECTION. 5. OPTION, SEE POWER SUPPLY SECTION.
03084-0-027
Figure 21. Typical Connection Diagram (Internal Reference Buffer, Serial Interface)
Rev. 0 | Page 16 of 28
AD7678
TYPICAL CONNECTION DIAGRAM
Figure 21 shows a typical connection diagram for the AD7678. Different circuitry shown on this diagram is optional and is discussed later in this data sheet. During the acquisition phase for ac signals, the AD7678 behaves like a 1-pole RC filter consisting of the equivalent resistance, R+, R-, and CS. Resistors R+ and R- are typically 3 k and are lumped components made up of a serial resistor and the on resistance of the switches. CS is typically 60 pF and mainly consists of the ADC sampling capacitor. This 1-pole filter with a -3 dB cutoff frequency of 900 kHz typ reduces any undesirable aliasing effect and limits the noise coming from the inputs. Because the input impedance of the AD7678 is very high, the part can be driven directly by a low impedance source without gain error.
Analog Inputs
Figure 22 shows a simplified analog input section of the AD7678. The diodes shown in Figure 22 provide ESD protection for the inputs. Care must be taken to ensure that the analog input signal never exceeds the absolute ratings on these inputs. This will cause these diodes to become forward-biased and start conducting current. These diodes can handle a forward-biased current of 120 mA max. This condition could eventually occur when the input buffer's U1 or U2 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
Driver Amplifier Choice
Although the AD7678 is easy to drive, the driver amplifier needs to meet the following requirements: * The driver amplifier and the AD7678 analog input circuit have to be able to settle for a full-scale step of the capacitor array at an 18-bit level (0.0004%). In the amplifier's data sheet, settling at 0.1% or 0.01% is more commonly specified. This could differ significantly from the settling time at an 18-bit level and, therefore, should be verified prior to driver selection. The tiny op amp AD8021, which combines ultralow 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 AD7678. The noise coming from the driver is filtered by the AD7678 analog input circuit 1-pole low-pass filter made by R+, R-, and CS. The SNR degradation due to the amplifier is
SNRLOSS = 20 log
IN+
R+ = 3k CS CS R- = 3k
IN-
AGND
03084-0-028
*
Figure 22. Simplified Analog Input
This analog input structure is a true differential structure. By using these differential inputs, signals common to both inputs are rejected as shown in Figure 23, which represents typical CMRR over frequency.
80
25 625 + f -3dB (Ne N ) 2
75

70
65
60
where: f-3dB is the -3 dB input bandwidth in MHz of the AD7678 (0.9 MHz). N is the noise factor of the amplifiers (1 if in buffer configuration). eN is the equivalent input noise voltage of each op amp in nV/Hz. For instance, for a driver with an equivalent input noise of 6 nV/Hz (e.g., AD8610) configured as a buffer, thus with a noise gain of +1, the SNR degrades by only 0.65 dB. * The driver needs to have a THD performance suitable to that of the AD7678.
CMRR (dB)
55
50
1
10
100 FREQUECY (kHz)
1000
10000
03084-0-029
Figure 23. Analog Input CMRR vs. Frequency
Rev. 0 | Page 17 of 28
AD7678
The AD8021 meets these requirements and is usually appropriate for almost all applications. The AD8021 needs a 10 pF external compensation capacitor, which should have good linearity as an NPO ceramic or mica type. The AD8022 could 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 another option when low bias current is needed in low frequency applications. To use the internal reference buffer, PDBUF should be LOW. A 2.5 V reference voltage applied on the REFBUFIN input will result in a 4.096 V reference on the REF pin. In both cases, the voltage reference input REF has a dynamic input impedance and therefore requires an efficient decoupling between REF and REFGND inputs. The decoupling consists of a low ESR 47 F tantalum capacitor connected to the REF and REFGND inputs with minimum parasitic inductance. Care should also be taken with the reference temperature coefficient of the voltage reference, which directly affects the full-scale accuracy if this parameter matters. For instance, a 4 ppm/C temperature coefficient of the reference changes the full scale by 1 LSB/C.
Single-to-Differential Driver
For applications using unipolar analog signals, a single-endedto-differential driver will allow for a differential input into the part. The schematic is shown in Figure 24. When provided an input signal of 0 to VREF, this configuration will produce a differential VREF with midscale at VREF/2. If the application can tolerate more noise, the AD8138 differential driver can be used.
Power Supply
The AD7678 uses three sets of power supply pins: an analog 5 V supply (AVDD), a digital 5 V core supply (DVDD), and a digital output interface supply (OVDD). The OVDD supply defines the output logic level and allows direct interface with any logic working between 2.7 V and DVDD + 0.3 V. To reduce the number of supplies needed, the digital core (DVDD) can be supplied through a simple RC filter from the analog supply, as shown in Figure 21. The AD7678 is independent of power supply sequencing once OVDD does not exceed DVDD by more than 0.3 V, and is therefore free from supply voltage induced latch-up. Additionally, it is very insensitive to power supply variations over a wide frequency range, as shown in Figure 25.
65
ANALOG INPUT (UNIPOLAR 0V TO 4.096V)
U1
AD8021
10pF 590
590
IN+
AD7678
AD8021
IN- REF REFBUFIN
1.82k
8.25k
U2
100nF
10pF
10F
2.5V
03084-0-030
60
Figure 24. Single-Ended-to-Differential Driver Circuit (Internal Reference Buffer Used)
Voltage Reference
The AD7678 allows the use of an external voltage reference with or without the internal reference buffer. Using the internal reference buffer is recommended when sharing a common reference voltage between multiple ADCs is desired. However, the advantages of using the external reference voltage directly are * The SNR and dynamic range improvement (about 1.7 dB) resulting from the use of a reference voltage very close to the supply (5 V) instead of a typical 4.096 V reference when the internal buffer is used. The power saving when the internal reference buffer is powered down (PDBUF HIGH).
Rev. 0 | Page 18 of 28
PSRR (dB)
55
50
45
40 1
10
100 FREQUECY (kHz)
1000
10000
03084-0-031
Figure 25. PSRR vs. Frequency
*
AD7678
POWER DISSIPATION VERSUS THROUGHPUT
The AD7678 automatically reduces its power consumption at the end of each conversion phase. During the acquisition phase, the operating currents are very low, which allows for a significant power savings when the conversion rate is reduced, as shown in Figure 26. This feature makes the AD7678 ideal for very low power battery applications. It should be noted that the digital interface remains active even during the acquisition phase. To reduce the operating digital supply currents even further, the digital inputs need to be driven close to the power rails (DVDD and DGND), and OVDD should not exceed DVDD by more than 0.3 V.
100000
CNVST
t1
t2
BUSY
t4 t3 t5 t6
CONVERT ACQUIRE CONVERT
MODE
ACQUIRE
t7
t8
03084-0-033
Figure 27. Basic Conversion Timing
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. For other applications, conversions can be automatically initiated. If CNVST is held low when BUSY is low, the AD7678 controls the acquisition phase and then automatically initiates a new conversion. By keeping CNVST low, the AD7678 keeps the conversion process running by itself. It should be noted that the analog input has to 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 AD7678 could sometimes run slightly faster than the guaranteed limits of 100 kSPS.
10000
POWER DISSIPATION (mW)
1000
100
10
1 PDBUF HIGH 0.1 1 10 100 1k SAMPLING RATE (SPS) 10k 100k
03084-0-032
t9
RESET
Figure 26. Power Dissipation vs. Sample Rate
CONVERSION CONTROL
Figure 27 shows the detailed timing diagrams of the conversion process. The AD7678 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. The CNVST signal operates independently of the CS and RD signals.
BUSY
DATA BUS
t8
CNVST
03084-0-034
Figure 28. RESET Timing
Rev. 0 | Page 19 of 28
AD7678
DIGITAL INTERFACE
The AD7678 has a versatile digital interface; it can be interfaced with the host system by using either a serial or parallel interface. The serial interface is multiplexed on the parallel data bus. The AD7678 digital interface also accommodates both 3 V and 5 V logic by simply connecting the AD7678's OVDD supply pin to the host system interface digital supply. Finally, by using the OB/2C input pin in any mode except 18-bit interface mode, both twos complement and straight binary coding can be used. The two signals, CS and RD, control the interface. When at least one of these signals is high, the interface outputs are in high impedance. Usually, CS allows the selection of each AD7678 in multicircuit applications, and is held low in a single AD7678 design. RD is generally used to enable the conversion result on the data bus.
CS = RD = 0
CS
RD
BUSY DATA BUS CURRENT CONVERSION
t12
t13
03084-0-036
Figure 30. Slave Parallel Data Timing for Reading (Read after Convert)
CS = 0 CNVST, RD
t1
BUSY
t4 t3
t1
CNVST
t10
BUSY
DATA BUS
PREVIOUS CONVERSION
t4 t3 t11
PREVIOUS CONVERSION DATA NEW DATA
03084-0-035
t12
t13
03084-0-037
Figure 31. Slave Parallel Data Timing for Reading (Read during Convert)
DATA BUS
CS
Figure 29. Master Parallel Data Timing for Reading (Continuous Read)
RD
PARALLEL INTERFACE
The AD7678 is configured to use the parallel interface with an 18-bit, a 16-bit, or an 8-bit bus width, according to Table 7. 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 30 and Figure 31, 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. Refer to Table 7 for a detailed description of the different options available.
A0, A1 HI-Z HI-Z
PINS D[15:8]
HIGH BYTE
LOW BYTE
t12
PINS D[7:0] HI-Z LOW BYTE
t12
HIGH BYTE
t13
HI-Z
03084-0-038
Figure 32. 8-Bit and 16-Bit Parallel Interface
SERIAL INTERFACE
The AD7678 is configured to use the serial interface when MODE0 and MODE1 are held high. The AD7678 outputs 18 bits of data, MSB first, on the SDOUT pin. This data is synchronized with the 18 clock pulses provided on the SCLK pin. The output data is valid on both the rising and falling edge of the data clock.
Rev. 0 | Page 20 of 28
AD7678
MASTER SERIAL INTERFACE
Internal Clock
The AD7678 is configured to generate and provide the serial data clock SCLK when the EXT/INT pin is held low. The AD7678 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 33 and Figure 34 show the detailed timing diagrams of these two modes. Usually, because the AD7678 is used with a fast throughput, the mode master read during conversion is the most recommended serial mode. In Read during Conversion mode, the serial clock and data toggle at appropriate instants, minimizing potential feedthrough between digital activity and critical conversion decisions. In Read after Conversion mode, it should be noted that unlike in other modes, the BUSY signal returns low after the 18 data bits are pulsed out and not at the end of the conversion phase, which results in a longer BUSY width. To accommodate slow digital hosts, the serial clock can be slowed down by using DIVSCLK.
CS, RD
EXT/INT = 0
RDC/SDIN = 0
INVSCLK = INVSYNC = 0
t3
CNVST
BUSY
t28 t29 t30 t25 t18 t19 t20 t21
1 2 3 16 17
SYNC
t14
t24
18
t26
SCLK
t15
SDOUT X D17 D16 D2 D1 D0
t27
t16
t22
t23
03084-0-039
Figure 33. Master Serial Data Timing for Reading (Read after Convert)
Rev. 0 | Page 21 of 28
AD7678
CS, RD EXT/INT = 0 RDC/SDIN = 1 INVSCLK = INVSYNC = 0
t1
CNVST
t3
BUSY
t17
SYNC
t25 t19 t20 t21
1 2 3 16 17 18
t14
t24
SCLK
t15 t18
t26
t27
D17 D16 D2 D1 D0
SDOUT
X
t16
t22
t23
03084-0-040
Figure 34. Master Serial Data Timing for Reading (Read Previous Conversion during Convert)
SLAVE SERIAL INTERFACE
External Clock
The AD7678 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 35 and Figure 36 show the detailed timing diagrams of these methods. While the AD7678 is performing a bit decision, it is important that voltage transients not occur 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 AD7678 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 only toggles when BUSY is low or, more importantly, that it does not transition during the latter half of BUSY high.
External Discontinuous Clock Data Read after Conversion
This mode is the most recommended of the serial slave modes. Figure 35 shows the detailed timing diagrams of this method. After a conversion is complete, indicated by BUSY returning low, the result of this conversion can be read while both CS and RD are low. Data is shifted out MSB first with 18 clock pulses, and is valid on both the rising and falling edge of the clock. Among the advantages of this method, the conversion performance is not degraded because there are no voltage transients on the digital interface during the conversion process. Also, data can be read at speeds up to 40 MHz, accommodating both slow digital host interface and the fastest serial reading. Finally, in this mode only, the AD7678 provides a daisy-chain feature using the RDC/SDIN input pin to cascade multiple converters together. This feature is useful for reducing component count and wiring connections when desired (for instance, in isolated multiconverter applications). An example of the concatenation of two devices is shown in Figure 37. Simultaneous sampling is possible by using a common CNVST signal. It should be noted that the RDC/SDIN input is latched on the edge of SCLK opposite the one used to shift out data on SDOUT. Thus, the MSB of the upstream converter follows the LSB of the downstream converter on the next SCLK cycle.
Rev. 0 | Page 22 of 28
AD7678
CS
EXT/INT = 1
INVSCLK = 0
RD = 0
BUSY
t36
SCLK 1
t35 t37
2 3 16 17 18 19 20
t31
SDOUT X D17
t32
D16 D15 D1 D0 X17 X16
t16
t34
X17 X16 X15 X1 X0 Y17 Y16
SDIN
t33
03084-0-041
Figure 35. Slave Serial Data Timing for Reading (Read after Convert)
EXT/INT = 1
CS
INVSCLK = 0
RD = 0
CNVST
BUSY
t3 t36
SCLK 1
t35 t37
2 3 16 17 18
t31
SDOUT X D17
t32
D16 D15 D1 D0
t16
03084-0-042
Figure 36. Slave Serial Data Timing for Reading (Read Previous Conversion during Convert)
Rev. 0 | Page 23 of 28
AD7678
BUSY OUT BUSY BUSY
MICROPROCESSOR INTERFACING
The AD7678 is ideally suited for traditional dc measurement applications supporting a microprocessor, and for ac signal processing applications interfacing to a digital signal processor. The AD7678 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 AD7678 to prevent digital noise from coupling into the ADC. The following section illustrates the use of the AD7678 with an SPI equipped DSP, the ADSP-219x.
#2 (UPSTREAM)
AD7678
#1 (DOWNSTREAM)
AD7678
RDC/SDIN
SDOUT CNVST CS SCLK
RDC/SDIN
SDOUT CNVST CS SCLK
DATA OUT
SCLK IN CS IN CNVST IN
03084-0-043
Figure 37. Two AD7678s in a Daisy-Chain Configuration
SPI Interface (ADSP-219x)
Figure 38 shows an interface diagram between the AD7678 and the SPI equipped ADSP-219x. To accommodate the slower speed of the DSP, the AD7678 acts as a slave device, and data must be read after conversion. This mode also allows the daisychain feature. The convert command could be initiated in response to an internal timer interrupt. The 18-bit output data are read with 3-byte SPI access. The reading process could 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 (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). It should be noted that to meet all timing requirements, the SPI clock should be limited to 17 Mbits/s, which allow it to read an ADC result in about 1.1 s.
DVDD
External Clock Data Read during Conversion
Figure 36 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 18 clock pulses, and is valid on both the rising and falling edge of the clock. The 18 bits have to be read before the current conversion is complete. If that is not done, RDERROR is pulsed high and can be used to interrupt the host interface to prevent incomplete data reading. There is no daisychain 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 is recommended to ensure that all bits are read during the first half of the conversion phase. It is also possible to begin to read the data after conversion and continue to read the last bits even after a new conversion has been initiated.
AD7678*
SER/PAR EXT/INT BUSY CS RD INVSCLK SDOUT SCLK CNVST PFx
ADSP-219x*
SPIxSEL (PFx) MISOx SCKx PFx or TFSx
* ADDITIONAL PINS OMITTED FOR CLARITY
03084-0-044
Figure 38. Interfacing the AD7678 to an SPI Interface
Rev. 0 | Page 24 of 28
AD7678 APPLICATION HINTS
LAYOUT
The AD7678 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 AD7678 should be designed so that 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 easily separated. Digital and analog ground planes should be joined in only one place, preferably underneath the AD7678, or at least as close to the AD7678 as possible. If the AD7678 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 be established as close to the AD7678 as possible. The user should avoid running digital lines under the device, because these will couple noise onto the die. The analog ground plane should be allowed to run under the AD7678 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 feedthrough through the board. The power supply lines to the AD7678 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 AD7678 and to reduce the magnitude of the supply spikes. Decoupling ceramic capacitors, typically 100 nF, should be placed close to and ideally right up against each power supply pin (AVDD, DVDD, and OVDD) 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 AD7678 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, and if no separate supply is available, the user should connect the DVDD digital supply to the analog supply AVDD through an RC filter, (see Figure 21), and connect the system supply to the interface digital supply 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 AD7678 has four different ground pins: REFGND, AGND, DGND, and OGND. REFGND senses the reference voltage and should be a low impedance return to the reference because it carries pulsed currents. AGND is the ground to which most internal ADC analog signals are referenced. This ground 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. The layout of the decoupling of the reference voltage is important. The decoupling capacitor should be close to the ADC and should be connected with short and large traces to minimize parasitic inductances.
EVALUATING THE AD7678'S PERFORMANCE
A recommended layout for the AD7678 is outlined in the documentation of the EVAL-AD7678CB evaluation board for the AD7678. The evaluation board package includes a fully assembled and tested evaluation board, documentation, and software for controlling the board from a PC via the EVALCONTROL BRD2.
Rev. 0 | Page 25 of 28
AD7678 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
(PINS DOWN )
TOP VIEW
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)
7.00 BSC SQ 0.30 0.23 0.18
48 1
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)
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 the AD7678 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.
ORDERING GUIDE
Model AD7678AST AD7678ASTRL AD7678ACP AD7678ACPRL EVAL-AD7678CB1 EVAL-CONTROL BRD22
1 2
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
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.
Rev. 0 | Page 26 of 28
AD7678 NOTES
Rev. 0 | Page 27 of 28
AD7678 NOTES
(c) 2003 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective companies. C03084-0-8/03(0)
Rev. 0 | Page 28 of 28
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