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16-bit, 4-channel/8-channel, 250 ksps pulsar adc AD7682/ad7689 rev. 0 information furnished by analog devices is believed to be accurate and reliable. however, no responsibility is assumed by analog devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. specifications subject to change without notice. no license is granted by implication or otherwise under any patent or patent rights of analog devices. trademarks and registered trademarks are the property of their respective owners. one technology way, p.o. box 9106, norwood, ma 02062-9106, u.s.a. tel: 781.329.4700 www.analog.com fax: 781.461.3113 ?2008 analog devices, inc. all rights reserved. features 16-bit resolution with no missing codes 4-channel (AD7682)/8-channel (ad7689) multiplexer with choice of inputs unipolar single ended differential (gnd sense) pseudobipolar throughput: 250 ksps inl: 0.5 lsb typical, 1 .5 lsb maximum (23 ppm or fsr) dynamic range: 93.8 db sinad: 92.5 db @ 20 khz thd: ?100 db @ 20 khz analog input range: 0 v to v ref with v ref up to vdd multiple reference types internal selectable 2.5 v or 4.096 v external buffered (up to 4.096 v) external (up to vdd) internal temperature sensor channel sequencer, selectable 1-pole filter, busy indicator no pipeline delay, sar architecture single-supply 2.7 v to 5.5 v operation with 1.8 v to 5 v logic interface serial interface compatible with spi, microwire, qspi, and dsp power dissipation 3.5 mw @ 2.5 v/200 ksps 12 mw @ 5 v/250 ksps standby current: 50 na 20-lead 4 mm 4 mm lfcsp package applications battery-powered equipment medical instruments: ecg/ekg mobile communications: gps personal digital assistants power line monitoring data acquisition seismic data acquisition systems instrumentation process control functional block diagram AD7682/ ad7689 ref gnd vdd vio din sck sdo cnv 1.8v to vdd 2.7v to 5 v sequencer spi serial interface mux 16-bit sar adc band gap ref temp sensor refin in0 in1 in4 in5 in6 in7 in3 in2 com 0.5v to vdd 22f one-pole lpf 0.5v to 4.096v 0.1f 07353-001 figure 1. table 1. multichannel 14-/16-bit pulsar? adc type channels 250 ksps 500 ksps adc driver 14-bit 8 ad7949 ada4841-x 16-bit 4 AD7682 ada4841-x 16-bit 8 ad7689 ad7699 ada4841-x general description the AD7682/ad7689 are 4-channel/8-channel, 16-bit, charge redistribution successive approximation register (sar) analog- to-digital converters (adcs) that operate from a single power supply, vdd. the AD7682/ad7689 contain all components for use in a multichannel, low power data acquisition system, including a true 16-bit sar adc with no missing codes; a 4-channel (AD7682) or 8-channel (ad7689), low crosstalk multiplexer useful for configuring the inputs as single ended (with or without ground sense), differential, or bipolar; an internal low drift reference (selectable 2.5 v or 4.096 v) and buffer; a temperature sensor; a selectable one-pole filter; and a sequencer that is useful when channels are continuously scanned in order. the AD7682/ad7689 use a simple spi interface for writing to the configuration register and receiving conversion results. the spi interface uses a separate supply, vio, which is set to the host logic level. power dissipation scales with throughput. each AD7682/ad7689 is housed in a tiny 20-lead lfcsp with operation specified from ?40c to +85c.
AD7682/ad7689 rev. 0 | page 2 of 28 table of contents features .............................................................................................. 1 applications....................................................................................... 1 functional block diagram .............................................................. 1 general description ......................................................................... 1 revision history ............................................................................... 2 specifications..................................................................................... 3 timing specifications....................................................................... 6 absolute maximum ratings............................................................ 8 esd caution.................................................................................. 8 pin configurations and function descriptions ........................... 9 typical performance characteristics ........................................... 11 terminology .................................................................................... 14 theory of operation ...................................................................... 15 overview...................................................................................... 15 converter operation.................................................................. 15 transfer functions...................................................................... 16 typical connection diagrams.................................................. 17 analog inputs ............................................................................. 18 driver amplifier choice ........................................................... 19 voltage reference output/input .............................................. 20 power supply............................................................................... 21 supplying the adc from the reference.................................. 21 digital interface .............................................................................. 22 configuration register, cfg .................................................... 22 read/write spanning conversion without a busy indicator............................................................................. 24 read/write spanning conversion with a busy indicator..... 25 application hints ........................................................................... 26 layout .......................................................................................... 26 evaluating AD7682/ad7689 performance ............................ 26 outline dimensions ....................................................................... 27 ordering guide .......................................................................... 27 revision history 5/08revision 0: initial version
AD7682/ad7689 rev. 0 | page 3 of 28 specifications vdd = 2.5 v to 5.5 v, vio = 2.3 v to vdd, v ref = vdd, all specifications t min to t max , unless otherwise noted. table 2. AD7682b/ad7689b parameter conditions/comments min typ max unit resolution 16 bits analog input voltage range unipolar mode 0 +v ref v bipolar mode ?v ref /2 +v ref /2 absolute input voltage positive input, unipolar and bipolar modes ?0.1 v ref + 0.1 v negative or com input, unipolar mode ?0.1 +0.1 negative or com input, bipolar mode v ref /2 ? 0.1 v ref /2 v ref /2 + 0.1 analog input cmrr f in = 250 khz 68 db leakage current at 25c acquisition phase 1 na input impedance 1 throughput conversion rate full bandwidth 2 vdd = 4.5 v to 5.5 v 0 250 ksps vdd = 2.7 v to 4.5 v 0 200 ksps vdd = 2.3 v to 2.7 v 0 190 ksps ? bandwidth 2 vdd = 4.5 v to 5.5 v 0 60 ksps vdd = 2.7 v to 4.5 v 0 50 ksps vdd = 2.3 v to 2.7 v 0 47 ksps transient response full-scale step, full bandwidth 1.8 s full-scale step, ? bandwidth 14.5 s accuracy no missing codes 16 bits integral linearity error ?1.5 0.5 +1.5 lsb 3 differential linearity error ?1 0.25 +1.5 lsb transition noise ref = vdd = 5 v 0.5 lsb gain error 4 ?30 2 +30 lsb gain error match ?2 0.5 +2 lsb gain error temperature drift 1 ppm/c offset error 4 2 lsb offset error match ?2 0.5 +2 lsb offset error temperature drift 1 ppm/c power supply sensitivity vdd = 5 v 5% 1.5 lsb
AD7682/ad7689 rev. 0 | page 4 of 28 AD7682b/ad7689b parameter conditions/comments min typ max unit ac accuracy 5 dynamic range 93.8 db 6 signal-to-noise f in = 20 khz, vref = 5 v 92.5 93.5 db f in = 20 khz, vref = 4.096 v internal ref 91 92.3 db f in = 20 khz, vref = 2.5 v internal ref 87.5 88.8 db sinad f in = 20 khz, vref = 5 v 91 92.5 db f in = 20 khz, vref = 5 v ?60 db input 33.5 db f in = 20 khz, vref = 4.096 v internal ref 90 91 db f in = 20 khz, vref = 2.5 v internal ref 87 88.4 db total harmonic distortion f in = 20 khz ?100 db spurious-free dynamic range f in = 20 khz 110 db channel-to-channel crosstalk f in = 100 khz on adjacent channel(s) ?125 db sampling dynamics ?3 db input bandwidth selectable 0.425 1.7 mhz aperture delay vdd = 5 v 2.5 ns internal reference ref output voltage 2.5 v, @ 25c 2.490 2.500 2.510 v 4.096 v, @ 25c 4.086 4.096 4.106 v refin output voltage 7 2.5 v, @ 25c 1.2 v 4.096 v, @ 25c 2.3 v ref output current 300 a temperature drift 10 ppm/c line regulation vdd = 5 v 5% 15 ppm/v long-term drift 1000 hours 50 ppm turn-on settling time cref = 10 f 5 ms external reference voltage range ref input 0.5 vdd + 0.3 v refin input (buffered) 0.5 vdd ? 0.2 v current drain 250 ksps, ref = 5 v 50 a temperature sensor output voltage 8 @ 25c 283 mv temperature sensitivity 1 mv/c digital inputs logic levels v il ?0.3 +0.3 vio v v ih 0.7 vio vio + 0.3 v i il ?1 +1 a i ih ?1 +1 a digital outputs data format 9 pipeline delay 10 v ol isink = +500 a 0.4 v v oh isource = ?500 a vio ? 0.3 v
AD7682/ad7689 rev. 0 | page 5 of 28 AD7682b/ad7689b parameter conditions/comments min typ max unit power supplies vdd specified performance 2.3 5.5 v vio specified performance 2.3 vdd + 0.3 v operating range 1.8 vdd + 0.3 v standby current 11 , 12 vdd and vio = 5 v, @ 25c 50 na power dissipation vdd = 2.5 v, 100 sps throughput 1.7 w vdd = 2.5 v, 100 ksps throughput 1.75 2.1 mw vdd = 2.5 v, 200 ksps throughput 3.5 4.1 mw vdd = 5 v , 250 ksps throughput 12.5 15.9 mw vdd = 5 v, 250 ksps throughput with internal reference 15.5 19.2 mw energy per conversion 50 nj temperature range 13 specified performance t min to t max ?40 +85 c 1 see the analog inputs section. 2 the bandwidth is set with the configuration register 3 lsb means least significant bit. with the 5 v input range, one lsb is 76.3 v. 4 see the terminology section. these specif ications include full temperature range variation but not the error contribution from the external reference. 5 with vdd = 5 v, unless otherwise noted. 6 all specifications expressed in decibels are referred to a full-scale input fsr and tested with an input signal at 0.5 db belo w full scale, unless otherwise specified. 7 this is the output from the internal band gap. 8 the output voltage is internal and pr esent on a dedicated multiplexer input. 9 unipolar mode: serial 16-bit straight binary. bipolar mode: serial 16- bit twos complement. 10 conversion results available immediately after completed conversion. 11 with all digital inputs forced to vio or gnd as required. 12 during acquisition phase. 13 contact an analog devices, inc., sales representative for the extended temperature range.
AD7682/ad7689 rev. 0 | page 6 of 28 timing specifications vdd = 4.5 v to 5.5 v, vio = 2.3 v to vdd, all specifications t min to t max , unless otherwise noted. table 3. 1 parameter symbol min typ max unit conversion time: cnv rising edge to data available t conv 2.2 s acquisition time t acq 1.8 s time between conversions t cyc 4 s cnv pulse width t cnvh 10 ns data write/read during conversion t data 1.4 s sck period t sck 15 ns sck low time t sckl 7 ns sck high time t sckh 7 ns sck falling edge to data remains valid t hsdo 4 ns sck falling edge to data valid delay t dsdo vio above 4.5 v 16 ns vio above 3 v 17 ns vio above 2.7 v 18 ns vio above 2.3 v 19 ns cnv low to sdo d15 msb valid t en vio above 4.5 v 15 ns vio above 3 v 17 ns vio above 2.7 v 18 ns vio above 2.3 v 22 ns cnv high or last sck falling edge to sdo high impedance t dis 25 ns cnv low to sck rising edge t clsck 10 ns din valid setup time from sck falling edge t sdin 4 ns din valid hold time from sck falling edge t hdin 4 ns 1 see figure 2 and figure 3 for load conditions.
AD7682/ad7689 rev. 0 | page 7 of 28 vdd = 2.5 v to 4.5 v , vio = 2.3 v to vdd, all specifications t min to t max , unless otherwise noted. table 4. 1 parameter symbol min typ max unit conversion time: cnv rising edge to data available vdd = 2.7 v to 4.5 v t conv 3.2 s vdd = 2.3 v to 2.7 v t conv 3.4 s acquisition time t acq 1.8 s time between conversions s vdd = 2.7 v to 4.5 v t cyc 5 s vdd = 2.3 v to 2.7 v t cyc 5.2 s cnv pulse width t cnvh 10 ns data write/read during conversion t data 1.4 s sck period t sck 25 ns sck low time t sckl 12 ns sck high time t sckh 12 ns sck falling edge to data remains valid t hsdo 5 ns sck falling edge to data valid delay t dsdo vio above 3 v 24 ns vio above 2.7 v 30 ns vio above 2.3 v 37 ns cnv low to sdo d15 msb valid t en vio above 3 v 21 ns vio above 2.7 v 27 ns vio above 2.3 v 35 ns cnv high or last sck falling edge to sdo high impedance t dis 50 ns cnv low to sck rising edge t clsck 10 ns sdi valid setup time from sck falling edge t sdin 5 ns sdi valid hold time from sck falling edge t hdin 5 ns 1 see figure 2 and figure 3 for load conditions. i ol 500a 500a i oh 1.4v t osdo c l 50pf 07353-002 figure 2. load circuit fo r digital interface timing 3 0% vio 70% vio 2v or vio ? 0.5v 1 0.8v or 0.5v 2 0.8v or 0.5v 2 2v or vio ? 0.5v 1 t delay t delay 1 2v if vio above 2.5v, vio ? 0.5v if vio below 2.5v. 2 0.8v if vio above 2.5v, 0.5v if vio below 2.5v. 07353-003 figure 3. voltage levels for timing
AD7682/ad7689 rev. 0 | page 8 of 28 absolute maximum ratings table 5. parameter rating analog inputs inx, 1 com 1 gnd ? 0.3 v to vdd + 0.3 v or vdd 130 ma ref, refin gnd ? 0.3 v to vdd + 0.3 v supply voltages vdd, vio to gnd ?0.3 v to +7 v vdd to vio 7 v din, cnv, sck to gnd 2 ?0.3 v to vio + 0.3 v sdo to gnd ?0.3 v to vio + 0.3 v storage temperature range ?65c to +150c junction temperature 150c ja thermal impedance (lfcsp) 47.6c/w jc thermal impedance (lfcsp) 4.4c/w 1 see the analog inputs section. 2 cnv must be low at power up. see the power supply section. 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. esd caution
AD7682/ad7689 rev. 0 | page 9 of 28 pin configurations and function descriptions pin 1 indicator 1 vdd 2 ref nc = no connect 3 refin 4 gnd 5 gnd 13 sck 14 sdo 15 vio 12 din 11 cnv 6 n c 7 i n 2 8 n c 0 1 c o m 9 i n 3 8 1 i n 1 9 1 n c 0 2 v d d 7 1 n c 6 1 i n 0 top view (not to scale) AD7682 0 7353-004 figure 4. AD7682 20-lead lfcsp (qfn) pin configuration pin 1 indicator 1 vdd 2 ref 3 refin 4 gnd 5 gnd 13 sck 14 sdo 15 vio 12 din 11 cnv 6 i n 4 7 i n 5 8 i n 6 0 1 c o m 9 i n 7 8 1 i n 2 9 1 i n 3 0 2 v d d 7 1 i n 1 6 1 i n 0 top view (not to scale) ad7689 07353-005 figure 5. ad7689 20-lead lfcsp (qfn) pin configuration table 6. pin function descriptions pin no. AD7682 mnemonic ad7689 mnemonic type 1 description 1, 20 vdd vdd p power supply. nominally 2.5 v to 5.5 v when using an external reference and decoupled with 10 f and 100 nf capacitors. when using the internal reference for 2.5 v output, the minimum should be 2.7 v. when using the internal reference for 4.096 v output, the minimum should be 4.5 v. 2 ref ref ai/o reference input/output. see the voltage reference output/input section. when the internal reference is enabled, this pin produces a selectable system reference = 2.5 v or 4.096 v. when the internal reference is disabled an d the buffer is enabled, ref produces a buffered version of the voltage present on the refin pin (vdd C 0.3 v maximum) useful when using low cost, low power references. for improved drift performance, connect a precision reference to ref (0.5 v to vdd). for any reference method, this pin needs deco upling with an external 10 f capacitor connected as close to ref as possible. see the reference decoupling section. 3 refin refin ai/o internal reference output/reference buffer input. see the voltage reference output/input section. when using the internal reference, the internal unbuffered reference voltage is present and needs decoupling with a 0.1f capacitor. when using the internal reference buffer, apply a source between 0.5 v and 4.096 v that is buffered to the ref pin as described above. 4, 5 gnd gnd p power supply ground. 6 nc in4 ai AD7682: no connection. ad7689: analog input channel 4. 7 in2 in5 ai AD7682: analog input channel 2. ad7689: analog input channel 5. 8 nc in6 ai AD7682: no connection. ad7689: analog input channel 6. 9 in3 in7 ai AD7682: analog input channel 3. ad7689: analog input channel 7. 10 com com ai common channel input. all channels [3:0] or [7:0] can be referenced to a common mode point of 0 v or v ref /2 v. 11 cnv cnv di convert input. on the rising edge, cnv initiates the conversion. during conversion, if cnv is held high, the busy indictor is enabled. 12 din din di data input. this input is used for writin g to the 14-bit configuration register. the configuration register can be written to during and after conversion. 13 sck sck di serial data clock input. this input is used to clock out the data on ado and clock in data on din in an msb first fashion.
AD7682/ad7689 rev. 0 | page 10 of 28 pin no. AD7682 mnemonic ad7689 mnemonic type 1 description 14 sdo sdo do serial data output. the conversion result is output on this pin synchronized to sck. in unipolar modes, conversion results are straight binary; in bipolar modes, conversion results are twos complement. 15 vio vio p input/output interface digital power. nominally at the same supply as the host interface (1.8 v, 2.5 v, 3 v, or 5 v ). 16 in0 in0 ai analog input channel 0. 17 nc in1 ai AD7682: no connection. ad7689: analog input channel 1. 18 in1 in2 ai AD7682: analog input channel 1. ad7689: analog input channel 2. 19 nc in3 ai AD7682: no connection. ad7689: analog input channel 3. 1 ai = analog input, ai/o = analog input/output, di = digital input, do = digital output, and p = power.
AD7682/ad7689 rev. 0 | page 11 of 28 typical performance characteristics vdd = 2.5 v to 5.5 v, vref = 2.5 v to 5 v, vio = 2.3 v to vdd 1.5 1.0 0.5 0 ?0.5 ?1.5 ?1.0 inl (lsb) codes 07353-009 0 16,384 32,768 49,152 65,536 figure 6. integral nonlinearity vs. code, vref = vdd = 5 v 200k 180k 160k 140k 120k 100k 80k 60k 40k 20k 0 7ffa counts code in hex 7ffb 7ffc 7ffd 7ffe 7fff 8000 8001 8002 0 0 487 619 0 0 0 = 0.50 v ref = vdd = 5v 135,326 124,689 07353-007 figure 7. histogram of a dc input at code center 0 ?20 ?40 ?60 ?80 ?100 ?120 ?140 ?160 ?180 05 0 25 75 100 125 amplitude (db of full-scale) frequency (khz) v ref = vdd = 5v f s = 250ksps f in = 19.9khz snr = 92.9db sinad = 92.4db thd = ?102db sfdr = 103db second harmonic = ?111db third harmonic = ?104db 07353-008 figure 8. 20 khz fft, vref = vdd = 5 v 1.5 1.0 0.5 0 ?0.5 ?1.0 0 16,384 32,768 49,152 65,536 dnl (lsb) codes 07353-006 figure 9. differential nonlineari ty vs. code, vref = vdd = 5 v 160k 140k 120k 100k 80k 60k 40k 20k 0 counts code in hex 7ffb 7ffc 7ffd 7ffe 7fff 8000 8001 8002 8003 178 6649 51,778 4090 60 1 = 0.78 v ref = v dd = 2.5v 63,257 135,207 07353-010 figure 10. histogram of a dc input at code center 0 ?20 ?40 ?60 ?80 ?100 ?120 ?140 ?160 ?180 05 0 25 75 100 amplitude (db of full-scale) frequency (khz) v ref = vdd = 2.5v f s = 200ksps f in = 19.9khz snr = 88.0db sinad = 87.0db thd = ?89db sfdr = 89db second harmonic = ?105db third harmonic = ?90db 07353-011 figure 11. 20 khz fft, vref = vdd = 2.5 v
AD7682/ad7689 rev. 0 | page 12 of 28 100 95 90 85 80 75 70 65 60 0 50 100 150 200 snr (db) frequency (khz) vdd = v ref = 5v, ?0.5db vdd = v ref = 5v, ?10db vdd = v ref = 2.5v, ?0.5db vdd = v ref = 2.5v, ?10db 07353-041 figure 12. snr vs. frequency 96 94 92 90 88 86 84 82 80 17.0 16.5 16.0 15.5 15.0 14.5 14.0 13.5 13.0 1.0 snr, sinad (db) enob (bits) reference voltage (v) 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 snr @ 2khz sinad @ 2khz snr @ 20khz sinad @ 20khz enob @ 2khz enob @ 20khz 07353-013 figure 13. snr, sinad, and enob vs. reference voltage 96 94 92 90 88 86 84 ?55 snr (db) temperature (c) ?35 ?15 5 25 45 65 85 105 125 f in = 20khz vdd = v ref = 5v vdd = v ref = 2.5v 07353-014 figure 14. snr vs. temperature 100 95 90 85 80 75 70 65 60 0 50 100 150 200 sinad (db) frequency (khz) vdd = v ref = 5v, ?0.5db vdd = v ref = 5v, ?10db vdd = v ref = 2.5v, ?0.5db vdd = v ref = 2.5v, ?10db 07353-012 figure 15. sinad vs. frequency 130 125 120 115 110 105 100 95 90 85 80 75 70 1.0 sfdr (db) ? 60 ?65 ?70 ?75 ?80 ?85 ?90 ?95 ?100 ?105 ?110 ?115 ?120 thd (db) reference voltage (v) 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 sfdr = 2khz sfdr = 20khz thd = 2khz thd = 20khz 07353-016 figure 16. sfdr and thd vs. reference voltage ? 90 ?95 ?100 ?105 ?110 ?55 thd (db) temperature (c) ?35 ?15 5 25 45 65 85 105 125 f in = 20khz vdd = v ref = 5v vdd = v ref = 2.5v 07353-017 figure 17. thd vs. temperature
AD7682/ad7689 rev. 0 | page 13 of 28 ? 60 ?70 ?80 ?90 ?100 ?110 ?120 0 50 100 150 200 thd (db) frequency (khz) vdd = v ref = 5v, ?0.5db vdd = v ref = 2.5v, ?0.5db vdd = v ref = 2.5v, ?10db vdd = v ref = 5v, ?10db 07353-015 figure 18. thd vs. frequency 95 94 93 92 91 90 89 88 87 86 85 ?10 snr (db) input level (db) ?8 ?6 ?2 0 ?4 f in = 20khz vdd = v ref = 5v vdd = v ref = 2.5v 07353-018 figure 19. snr vs. input level 3 2 1 0 ?1 ?2 ?3 offset error and gain error (lsb) ?55 temperature (c) ?35 ?15 5 25 45 65 85 105 125 unipolar zero unipolar gain bipolar zero bipolar gain 07353-020 figure 20. offset and gain errors vs. temperature 3000 2750 2500 2250 2000 1750 1500 1250 1000 100 90 80 70 60 50 40 30 20 2.5 3.0 3.5 4.0 4.5 5.0 5.5 vdd current (a) vio current (a) vdd supply (v) 2.5v internal ref 4.096v internal ref internal buffer, temp on internal buffer, temp off external ref, temp on external ref, temp off vio f s = 200ksps 07353-021 figure 21. operating currents vs. supply 3000 2750 2500 2250 2000 1750 1500 1250 1000 180 160 140 120 100 80 60 40 20 vdd current (a) ?55 temperature (c) ?35 ?15 5 25 45 65 85 105 125 f s = 200ksps vdd = 5v, internal 4.096v ref vdd = 5v, external ref vdd = 2.5, external ref vio 07353-022 figure 22. operating currents vs. temperature 07353-023 sdo capacitive load (pf) 120 0 20406080100 t dsdo del a y (ns) 25 20 15 10 5 0 vdd = 2.5v, 85c vdd = 3.3v, 25c vdd = 3.3v, 85c vdd = 5v, 85c vdd = 5v, 25c vdd = 2.5v, 25c figure 23. t dsdo delay vs. sdo capacitance load and supply
AD7682/ad7689 rev. 0 | page 14 of 28 terminology least significant bit (lsb) the lsb is the smallest increment that can be represented by a converter. for an analog-to-digital converter with n bits of resolution, the lsb expressed in volts is n ref v lsb 2 (v) = integral nonlinearity error (inl) inl 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 ? lsb before the first code transition. positive full scale is defined as a level 1? lsb beyond the last code transition. the deviation is measured from the middle of each code to the true straight line (see figure 25 ). differential nonlinearity error (dnl) in an ideal adc, code transitions are 1 lsb apart. dnl is the maximum deviation from this ideal value. it is often specified in terms of resolution for which no missing codes are guaranteed. offset error the first transition should occur at a level ? lsb above analog ground. the unipolar offset error is the deviation of the actual transition from that point. gain error the last transition (from 111 10 to 111 11) should occur for an analog voltage 1? lsb below the nominal full scale. the gain error is the deviation in lsb (or percentage of full-scale range) of the actual level of the last transition from the ideal level after the offset error is adjusted out. closely related is the full-scale error (also in lsb or percentage of full-scale range), which includes the effects of the offset error. aperture delay aperture delay is the measure of the acquisition performance. it is the time between the rising edge of the cnv input and the point at which the input signal is held for a conversion. transient resp onse transient response is the time required for the adc to accurately acquire its input after a full-scale step function is applied. dynamic range dynamic range is the ratio of the rms value of the full scale to the total rms noise measured with the inputs shorted together. the value for dynamic range 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. signal-to-(noise + distortion) ratio (sinad) sinad 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 sinad is expressed in decibels. 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. spurious-free dynamic range (sfdr) sfdr is the difference, in decibels, 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. it is related to sinad by the formula enob = ( sinad db ? 1.76)/6.02 and is expressed in bits. channel-to-channel crosstalk channel-to-channel crosstalk is a measure of the level of crosstalk between any two adjacent channels. it is measured by applying a dc to the channel under test and applying a full-scale, 100 khz sine wave signal to the adjacent channel(s). the crosstalk is the amount of signal that leaks into the test channel and is expressed in decibels. reference voltage temperature coefficient reference voltage temperature coefficient is derived from the typical shift of output voltage at 25c on a sample of parts at the maximum and minimum reference output voltage (v ref ) meas- ured at t min , t (25c), and t max . it is expressed in ppm/c as 6 10 )C()c25( )(C)( )cppm/( = min max ref ref ref ref tt v minvmaxv tcv where: v ref ( max ) = maximum v ref at t min , t (25c), or t max . v ref ( min ) = minimum v ref at t min , t (25c), or t max . v ref (25 c ) = v ref at 25c. t max = +85c. t min = C40c.
AD7682/ad7689 rev. 0 | page 15 of 28 theory of operation sw+ msb 16,384c inx+ lsb comp control logic switches control busy output code cnv ref gnd inx? or com 4c 2c c c 32,768c sw? msb 16,384c lsb 4c 2c c c 32,768c 07353-026 figure 24. adc simplified schematic overview the AD7682/ad7689 are 4-channel/8-channel, 16-bit, charge redistribution successive approximation register (sar) analog-to- digital converters (adcs). these devices are capable of converting 250,000 samples per second (250 ksps) and power down between conversions. for example, when operating with an external reference at 1 ksps, they consumes 17 w typically, ideal for battery-powered applications. the AD7682/ad7689 contain all of the components for use in a multichannel, low power data acquisition system, including ? 16-bit sar adc with no missing codes ? 4-channel/8-channel, low crosstalk multiplexer ? internal low drift reference and buffer ? temp er ature s e ns or ? selectable one-pole filter ? channel sequencer these components are configured through an spi-compatible, 14-bit register. conversion results, also spi compatible, can be read after or during conversions with the option for reading back the current configuration. the AD7682/ad7689 provide the user with an on-chip track- and-hold and do not exhibit pipeline delay or latency. the AD7682/ad7689 are specified from 2.3 v to 5.5 v and can be interfaced to any 1.8 v to 5 v digital logic family. they are housed in a 20-lead, 4 mm 4 mm lfcsp that combines space savings and allows flexible configurations. they are pin-for-pin compatible with the 16-bit ad7699 and 14-bit ad7949 . converter operation the AD7682/ad7689 are successive approximation adcs based on a charge redistribution dac. figure 24 shows the simplified schematic of the adc. the capacitive dac consists of two identical arrays of 16 binary-weighted capacitors, which are connected to the two comparator inputs. during the acquisition phase, terminals of the array tied to the comparator input are connected to gnd 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 the inx+ and inx? (or com) inputs. when the acquisition phase is complete and the cnv input goes high, 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 gnd input. therefore, the differential voltage between the inx+ and inx? (or com) 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 gnd and cap, the comparator input varies by binary-weighted voltage steps (v ref /2, v ref /4, ... v ref /32,768). the control logic toggles these switches, starting with the msb, to bring the comparator back into a balanced condition. after the completion of this process, the part returns to the acquisition phase, and the control logic generates the adc output code and a busy signal indicator. because the AD7682/ad7689 have an on-board conversion clock, the serial clock, sck, is not required for the conversion process.
AD7682/ad7689 rev. 0 | page 16 of 28 transfer functions with the inputs configured for unipolar range (single ended, com with ground sense, or paired differentially with inx? as ground sense), the data output is straight binary. with the inputs configured for bipolar range (com = v ref /2 or paired differentially with inx? = v ref /2), the data outputs are twos complement. the ideal transfer characteristic for the AD7682/ad7689 is shown in figure 25 and for both unipolar and bipolar ranges with the internal 4.096 v reference. 100...000 100...001 100...010 011...101 011...110 011...111 twos complement straight binary 000...000 000...001 000...010 111...101 111...110 111...111 adc code analog input +fsr ? 1.5lsb +fsr ? 1lsb ?fsr + 1lsb ?fsr ?fsr + 0.5lsb 07353-027 figure 25. adc ideal transfer function table 7. output codes and ideal input voltages description unipolar analog input 1 v ref = 4.096 v digital output code (straight binary hex) bipolar analog input 2 v ref = 4.096 v digital output code (twos complement hex) fsr ? 1 lsb 4.095938 v 0xffff 2.047938 v 0x7fff midscale + 1 lsb 2.048063 v 0x8001 62.5 v 0x0001 midscale 2.048 v 0x8000 0 v 0x0000 4 midscale ? 1 lsb 2.047938 v 0x7fff ?62.5 v 0xffff 3 ?fsr + 1 lsb 62.5 v 0x0001 ?2.047938 v 0x8001 ?fsr 0 v 0x0000 ?2.048 v 0x8000 1 with com or inx? = 0 v or all inx referenced to gnd. 2 with com or inx? = v ref /2. 3 this is also the code for an overranged analog input ((inx+) ? (inx?), or com, above v ref ? v gnd ). 4 this is also the code for an underranged analog input ((inx+) ? (inx?), or com, below v gnd ).
AD7682/ad7689 rev. 0 | page 17 of 28 typical connection diagrams AD7682/ad7689 ref gnd vdd vio din mosi miso ss sck sck sdo cnv 100nf 100nf 5 v 10f 2 v+ v? 1.8v to vdd 0v to v ref 0v to v ref v+ v? ada4841-x 3 ada4841-x 3 1. internal reference shown. see voltage reference output/input section for reference selection. notes: 2. c ref is usually a 10f ceramic capacitor (x5r). 3. see driver amplifier choice section for additional recommended amplifiers. 4. see the digital interface section for configuring and reading conversion data. in0 inx com refin 100nf 0v or v ref /2 07353-028 figure 26. typical application diagram with multiple supplies ref gnd vdd vio din mosi miso ss sck sck sdo cnv 100nf 100nf 5 v 10f 2 v+ 1.8v to vdd v+ notes: 1. internal reference shown. see voltage reference output/input section for reference selection. 2. c ref is usually a 10f ceramic capacitor (x5r). 3. see driver amplifier choice section for additional recommended amplifiers. 4. see the digital interface section for configuring and reading conversion data. in0 inx com refin 100nf v ref /2 v ref p-p ada4841-x 3 ada4841-x 3 07353-029 AD7682/ad7689 figure 27. typical application diagram using bipolar input
AD7682/ad7689 rev. 0 | page 18 of 28 unipolar or bipolar figure 26 shows an example of the recommended connection diagram for the AD7682/ad7689 when multiple supplies are available. bipolar single supply figure 27 shows an example of a system with a bipolar input using single supplies with the internal reference (optional different vio supply). this circuit is also useful when the amplifier/signal conditioning circuit is remotely located with some common mode present. note that for any input config- uration, the inputs inx are unipolar and always referenced to gnd (no negative voltages even in bipolar range). for this circuit, a rail-to-rail input/output amplifier can be used; however, the offset voltage vs. input common-mode range should be noted and taken into consideration (1 lsb = 62.5 v with v ref = 4.096 v). note that the conversion results are in twos complement format when using the bipolar input configuration. refer to the an-581 application note for additional details about using single-supply amplifiers. analog inputs input structure figure 28 shows an equivalent circuit of the input structure of the AD7682/ad7689. the two diodes, d1 and d2, provide esd protection for the analog inputs, in[7:0] and com. care must be taken to ensure that the analog input signal does not exceed the supply rails by more than 0.3 v because this causes the diodes to become forward biased and to start conducting current. these diodes can handle a forward-biased current of 130 ma maximum. for instance, these conditions may eventually occur when the input buffer supplies are different from vdd. in such a case, for example, an input buffer with a short circuit, the current limitation can be used to protect the part. c in r in d1 d2 c pin inx+ or inx? or com gnd v dd 07353-030 figure 28. equivalent analog input circuit this analog input structure allows the sampling of the true differential signal between inx+ and com or inx+ and inx?. (com or inx? = gnd 0.1 v or v ref 0.1 v). by using these differential inputs, signals common to both inputs are rejected, as shown in figure 29 . 70 65 60 55 50 45 40 35 30 1 10k 10 cmrr (db) 100 1k frequency (khz) 07353-031 figure 29. analog input cmrr vs. frequency during the acquisition phase, the impedance of the analog inputs can be modeled as a parallel combination of the capacitor, c pin , and the network formed by the series connection of r in and c in . c pin is primarily the pin capacitance. r in is typically 3.5 k and is a lumped component made up of serial resistors and the on resistance of the switches. c in is typically 27 pf and is mainly the adc sampling capacitor. selectable low pass filter during the conversion phase, where the switches are opened, the input impedance is limited to c pin . while the AD7682/ ad7689 are acquiring, r in and c in make a one-pole, low-pass filter that reduces undesirable aliasing effects and limits the noise from the driving circuitry. the low-pass filter can be programmed for the full bandwidth or ? of the bandwidth with cfg[6] as shown in table 9 . note that the converters throughout must also be reduced by ? when using the filter. if the maximum throughput is used with the bw set to ?, the converter acquisition time, t acq , will be violated, resulting in increased thd. input configurations figure 30 shows the different methods for configuring the analog inputs with the configuration register (cfg[12:10]). refer to the configuration register, cfg , section for more details.
AD7682/ad7689 rev. 0 | page 19 of 28 gnd com ch0+ ch3+ ch1+ ch2+ ch4+ ch5+ ch6+ ch7+ ch0+ ch3+ ch1+ ch2+ ch4+ ch5+ ch6+ ch7+ com? gnd com in1 in0 in2 in3 in4 in5 in6 in7 in1 in0 in2 in3 in4 in5 in6 in7 in1 in0 in2 in3 in4 in5 in6 in7 in1 in0 in2 in3 in4 in5 in6 in7 a?8 channels, single ended b?8 channels, common refernce gnd com ch0+ (?) ch1+ (?) ch2+ (?) ch3+ (?) ch0? (+) ch1? (+) ch0+ (?) ch1+ (?) ch0? (+) ch1? (+) ch2? (+) ch3? (+) c?4 channels, differential gnd com ch2+ ch3+ ch4+ ch5+ d?combination com? 07353-032 figure 30. multiplexed analog input configuraitons the analog inputs can be configured as ? figure 30 a, single ended referenced to system ground; cfg[12:10] = 111 2 . ? figure 30 b, bipolar differential with a common reference point; com = v ref /2; cfg[12:10] = 010 2 . unipolar differential with com connected to a ground sense; cfg[12:10] = 110 2 . ? figure 30 c, bipolar differential pairs with inx? referenced to v ref /2; cfg[12:10] = 00x 2 . unipolar differential pairs with inx? referenced to a ground sense; cfg[12:10] = 10x 2 . in this configuration, the inx+ is identified by the channel in cfg[9:7]. example: for in0 = in1+ and in1 = in1?, cfg[9:7] = 000 2 ; for in1 = in1+ and in0 = in1?, cfg[9:7] = 001 2 . ? figure 30 d, inputs configured in any of the above combinations (showing that the AD7682/ad7689 can be configured dynamically). sequencer the AD7682/ad7689 include a channel sequencer useful for scanning channels in a in0 to inx fashion. channels are scanned as singles or pairs, with or without the temperature sensor, after the last channel is sequenced. the sequencer starts with in0 and finishes with inx set in cfg[9:7]. for paired channels, the channels are paired depending on the last channel set in cfg[9:7]. note that the channel pairs are always paired in (even) = inx+ and in (odd) = inx? regardless of cfg[7]. to enable the sequencer, cfg[2:1] are written to for initializing the sequencer. after cfg[13:0] are updated, din must be held low while reading data out (at least for bit 13), or the cfg will begin updating again. while operating in a sequence, the cfg can be changed by writing 01 2 to cfg[2:1]. however, if changing cfg11 (paired or single channel) or cfg[9:7] (last channel in sequence), the sequence reinitializes and converts in0 (or in1) after cfg is updated. examples only the bits for input and sequencer are highlighted. as a first example, scan all in[7:0] referenced to com = gnd with temperature sensor. 13 12 11 10 9 8 7 6 5 4 3 2 1 0 cfg incc inx bw ref seq rb - 1 1 0 1 1 1 - - - - 1 0 - as a second example, scan three paired channels without temperature sensor and referenced to v ref /2. 13 12 11 10 9 8 7 6 5 4 3 2 1 0 cfg incc inx bw ref seq rb - 0 0 x 1 0 x - - - - 1 1 - source resistance when the source impedance of the driving circuit is low, the AD7682/ad7689 can be driven directly. large source impedances significantly affect the ac performance, especially total harmonic distortion (thd). the dc performances are less sensitive to the input impedance. the maximum source imped- ance depends on the amount of thd that can be tolerated. the thd degrades as a function of the source impedance and the maximum input frequency. driver amplifier choice although the AD7682/ad7689 are easy to drive, the driver amplifier must meet the following requirements: ? the noise generated by the driver amplifier must be kept as low as possible to preserve the snr and transition noise performance of the AD7682/ad7689. note that the ad 7682/ ad7689 have a noise much lower than most of the other 16-bit adcs and, therefore, can be driven by a noisier amplifier to meet a given system noise specification. the noise from the amplifier is filtered by the AD7682/ad7689 analog input circuit low-pass filter made by r in and c in or by an external filter, if one is used. because the typical noise of the AD7682/ad7689 is 35 v rms (with v ref = 5 v), the snr degradation due to the amplifier is
AD7682/ad7689 rev. 0 | page 20 of 28 ? ? ? ? ? ? ? ? ? ? ? ? + = ? 2 3db 2 )(f 2 35 35 log20 n loss ne snr where: f C3db is the input bandwidth in megahertz of the AD7682/ad7689 (1.7 mhz in full bw or 425 khz in ? bw) or is the cutoff frequency of an input filter, if one is used. n is the noise gain of the amplifier (for example, 1 in buffer configuration). e n is the equivalent input noise voltage of the op amp, in nv/hz. ? for ac applications, the driver should have a thd performance commensurate with the AD7682/ad7689. figure 18 shows thd vs. frequency for the AD7682/ad7689. ? for multichannel, multiplexed applications on each input or input pair, the driver amplifier and the AD7682/ ad7689 analog input circuit must settle a full-scale step onto the capacitor array at a 16-bit level (0.0015%). in the amplifier data sheet, settling at 0.1% to 0.01% is more commonly specified. this may differ significantly from the settling time at a 16-bit level and should be verified prior to driver selection. table 8. recommended driver amplifiers amplifier typical application ada4841-x very low noise, small, and low power ad8655 5 v single supply, low noise ad8021 very low noise and high frequency ad8022 low noise and high frequency op184 low power, low noise, and low frequency ad8605 , ad8615 5 v single supply, low power voltage reference output/input the AD7682/ad7689 allow the choice of a very low temper- ature drift internal voltage reference, an external reference, or an external buffered reference. the internal reference of the AD7682/ad7689 provide excel- lent performance and can be used in almost all applications. there are six possible choices of voltage reference schemes briefly described in table 9 with more details in each of the following sections. internal reference/temperature sensor the internal reference can be set for either 2.5 v or a 4.096 v output as detailed in table 9 . with the internal reference enabled, the band gap voltage is also present on the refin pin, which requires an external 0.1 f capacitor. because the current output of refin is limited, it can be used as a source if followed by a suitable buffer, such as the ad8605 . enabling the reference also enables the internal temperature sensor, which measures the internal temperature of the AD7682/ad7689 and is thus useful for performing a system calibration. note that, when using the temperature sensor, the output is straight binary referenced from the AD7682/ad7689 gnd pin. the internal reference is temperature-compensated to within 15 mv. the reference is trimmed to provide a typical drift of 3 ppm/c. external reference and internal buffer for improved drift performance, an external reference can be used with the internal buffer. the external reference is con- nected to refin, and the output is produced on the ref pin. an external reference can be used with the internal buffer with or without the temperature sensor enabled. refer to table 9 for the register details. with the buffer enabled, the gain is unity and is limited to an input/output of 4.096 v. the internal reference buffer is useful in multiconverter appli- cations because a buffer is typically required in these applications. in addition, a low power reference can be used because the internal buffer provides the necessary performance to drive the sar architecture of the AD7682/ad7689. external reference in any of the six voltage reference schemes, an external reference can be connected directly on the ref pin because the output impedance of ref is >5 k. to reduce power consumption, the reference and buffer can be powered down independently or together for the lowest power consumption. however, for applications requiring the use of the temperature sensor, the reference must be active. refer to table 9 for register details. for improved drift performance, an external reference such as the adr43x or adr44x is recommended. reference decoupling whether using an internal or external reference, the AD7682/ ad7689 voltage reference output/input, ref, has a dynamic input impedance and should therefore be driven by a low impedance source with efficient decoupling between the ref and gnd pins. this decoupling depends on the choice of the voltage reference but usually consists of a low esr capacitor connected to ref and gnd with minimum parasitic inductance. a 10 f (x5r, 1206 size) ceramic chip capacitor is appropriate when using the internal reference, the adr43x / adr44x external reference, or a low impedance buffer such as the ad8031 or the ad8605 . the placement of the reference decoupling capacitor is also important to the performance of the AD7682/ad7689, as explained in the layout section. the decoupling capacitor should be mounted on the same side as the adc at the ref pin with a thick pcb trace. the gnd should also connect to the reference decoupling capacitor with the shortest distance and to the analog ground plane with several vias.
AD7682/ad7689 rev. 0 | page 21 of 28 if desired, smaller reference decoupling capacitor values down to 2.2 f can be used with a minimal impact on performance, especially on dnl. regardless, there is no need for an additional lower value ceramic decoupling capacitor (for example, 100 nf) between the ref and gnd pins. for applications that use multiple AD7682/ad7689s or other pulsar devices, it is more effective to use the internal reference buffer to buffer the external reference voltage, thus reducing sar conversion crosstalk. the voltage reference temperature coefficient (tc) directly impacts full scale; therefore, in applications where full-scale accuracy matters, care must be taken with the tc. for instance, a 15 ppm/c tc of the reference changes full scale by 1 lsb/c. power supply the AD7682/ad7689 use two power supply pins: an analog and digital core supply (vdd) and a digital input/output interface supply (vio). vio allows direct interface with any logic between 1.8 v and vdd. to reduce the supplies needed, the vio and vdd pins can be tied together. the AD7682/ad7689 are independent of power supply sequencing between vio and vdd. the only restriction is that cnv must be low when powering up the AD7682/ad7689. additionally, it is very insensitive to power supply variations over a wide frequency range, as shown in figure 31 . 75 70 65 60 55 50 45 40 35 30 1 10k 10 pssr (db) 100 1k frequency (khz) 07353-034 figure 31. psrr vs. frequency the AD7682/ad7689 power down automatically at the end of each conversion phase; therefore, the operating currents and power scale linearly with the sampling rate. this makes the part ideal for low sampling rates (even of a few hertz) and low battery-powered applications. 10000 1000 100 10 1 0.1 0.010 0.001 10 1m 100 operating current (a) 1k 10k 100k sampling rate (sps) 0 7353-040 vdd = 5v, internal ref vdd = 5v, external ref vdd = 2.5v, external ref vio figure 32. operating currents vs. sampling rate supplying the adc from the reference for simplified applications, the AD7682/ad7689, with their low operating current, can be supplied directly using the reference circuit as shown in figure 33 . the reference line can be driven by ? the system power supply directly ? a reference voltage with enough current output capability, such as the adr43x / adr44x ? a reference buffer, such as the ad8605 , which can also filter the system power supply, as shown in figure 33 ad8605 ad7689 vio ref vdd 10f 1f 0.1f 10 ? 10k ? 5v 5v 5v 1f 1 1 optional reference buffer and filter. 0.1f 0 7353-035 figure 33. example of an application circuit
AD7682/ad7689 rev. 0 | page 22 of 28 digital interface the AD7682/ad7689 use a simple 4-wire interface and are compatible with spi, microwire?, qspi?, digital hosts, and dsps, for example, blackfin? adsp-bf53x, sharc?, adsp- 219x, and adsp-218x. the interface uses the cnv, din, sck, and sdo signals and allows cnv, which initiates the conversion, to be independent of the readback timing. this is useful in low jitter sampling or simultaneous sampling applications. a 14-bit register, cfg[13:0], is used to configure the adc for the channel to be converted, the reference selection, and other components, which are detailed in the configuration register, cfg , section. when cnv is low, reading/writing can occur during conversion, acquisition, and spanning conversion (acquisition plus conversion), as detailed in the following sections. the cfg word is updated on the first 14 sck rising edges, and conversion results are read back on the first 15 (or 16 if busy mode is selected) sck falling edges. if the cfg readback is enabled, an additional 14 sck falling edges are required to read back the cfg word associated with the conversion results with the cfg msb following the lsb of the conversion result. a discontinuous sck is recommended because the part is selected with cnv low, and sck activity begins to write a new configuration word and clock out data. note that in the following sections, the timing diagrams indicate digital activity (sck, cnv, din, sdo) during the conversion. however, due to the possibility of performance degradation, digital activity should occur only prior to the safe data reading/writing time, t data , because the AD7682/ad7689 provide error correction circuitry that can correct for an incorrect bit during this time. from t data to t conv , there is no error correction and conversion results may be corrupted. the user should configure the AD7682/ad7689 and initiate the busy indicator (if desired) prior to t data . it is also possible to corrupt the sample by having sck or din transitions near the sampling instant. therefore, it is recommended to keep the digital pins quiet for approximately 30 ns before and 10 ns after the rising edge of cnv, using a discontinuous sck whenever possible to avoid any potential performance degradation. reading/writing during conversion, fast hosts when reading/writing during conversion (n), conversion results are for the previous (n ? 1) conversion, and writing the cfg is for the next (n + 1) acquisition and conversion. after the cnv is brought high to initiate conversion, it must be brought low again to allow reading/writing during conversion. reading/writing should only occur up to t data and, because this time is limited, the host must use a fast sck. the sck frequency required is calculated by data t edges sck number sck f __ the time between t data and t conv is a safe time when digital activity should not occur, or sensitive bit decisions may be corrupt. reading/writing during acquisition, any speed hosts when reading/writing during acquisition (n), conversion results are for the previous (n ? 1) conversion, and writing is for the (n + 1) acquisition. for the maximum throughput, the only time restriction is that the reading/writing take place during the t acq (min) time. for slow throughputs, the time restriction is dictated by throughput required by the user, and the host is free to run at any speed. thus for slow hosts, data access must take place during the acquisition phase. reading/writing spanning co nversion, any speed host when reading/writing spanning conversion, the data access starts at the current acquisition (n) and spans into the conversion (n). conversion results are for the previous (n ? 1) conversion, and writing the cfg is for the next (n + 1) acquisition and conversion. similar to reading/writing during conversion, reading/writing should only occur up to t data . for the maximum throughput, the only time restriction is that reading/writing take place during the t acq (min) + t data time. for slow throughputs, the time restriction is dictated by the users required throughput, and the host is free to run at any speed. similar to the reading/writing during acquisition, for slow hosts, the data access must take place during the acquisition phase with additional time into the conversion. note that data access spanning conversion requires the cnv to be driven high to initiate a new conversion, and data access is not allowed when cnv is high. thus, the host must perform two bursts of data access when using this method. configuration register, cfg the AD7682/ad7689 use a 14-bit configuration register (cfg[13:0]) as detailed in tabl e 9 for configuring the inputs, channel to be converted, one-pole filter bandwidth, reference, and channel sequencer. the cfg is latched (msb first) on din with 14 sck rising edges. the cfg update is edge dependent, allowing for asynchronous or synchronous hosts.
AD7682/ad7689 rev. 0 | page 23 of 28 the register can be written to during conversion, during acquisition, or spanning acquisition/conversion and is updated at the end of conversion, tconv (max). there is always a one deep delay when writing cfg. note that, at power up, the cfg is undefined and two dummy conversions are required to update the register. to preload the cfg with a factory setting, hold din high for two conversions. thus cfg[13:0] = 0x3fff. this sets the AD7682/ad7689 for ? in[7:0] unipolar referenced to gnd, sequenced in order ? full bandwidth for one-pole filter ? internal reference/temperature sensor disabled, buffer enabled ? no readback of cfg table 9 summarizes the configuration register bit details. see the theory of operation section for more details. 13 12 11 10 9 8 7 6 5 4 3 2 1 0 cfg incc incc incc inx inx inx bw ref ref ref seq seq rb table 9. configuration register description bit(s) name description [13] cfg configuration update. 0 = keep current configuration settings. 1 = overwrite contents of register. [12:10] incc input channel configuration. selection of pseudobipolar, pseudodifferentia l, pairs, single-ended or temperature se nsor. refer to the input configurations section. bit 12 bit 11 bit 10 function 0 0 x bipolar differential pairs; inx? referenced to v ref /2 0.1 v. 0 1 0 bipolar; inx referenced to com = v ref /2 0.1 v. 0 1 1 temperature sensor. 1 0 x unipolar differential pairs; inx? referenced to gnd 0.1 mv. 1 1 0 unipolar, in0 to in7 referenced to com = gnd 0.1 v (gnd sense). 1 1 1 unipolar, in0 to in7 referenced to gnd. [9:7] inx input channel selection in binary fashion. AD7682 ad7689 bit 9 bit 8 bit 7 channel bit 9 bit 8 bit 7 channel 0 0 x in0 0 0 0 in0 0 1 x in1 0 0 1 in1 1 0 x in2 1 1 x in3 1 1 1 in7 [6] bw select bandwidth for low-pass filter. refer to the selectable low pass filter section. 0 = ? of bw, uses an additional series resistor to further band width limit the noise. maximum throughout must be reduced to ? also. 1 = full bw. [5:3] ref reference/buffer selection. selection of internal, external, exte rnal buffered, and enabling of the on-chip temperature sensor. refer to the voltage reference output/input section. bit 5 bit 4 bit 3 function 0 0 0 internal reference, ref = 2.5 v output. 0 0 1 internal reference, ref = 4.096 v output. 0 1 0 external reference, temperature enabled. 0 1 1 external reference, internal buffer, temperature enabled. 1 1 0 external reference, temperature disabled. 1 1 1 external reference, internal buffer, temperature disabled. [2:1] seq channel sequencer. allows for scanning channe ls in an in0 to inx fashion. refer to the sequencer section. bit 2 bit 1 function 0 0 disable sequencer. 0 1 update configuration during sequence. 1 0 scan in0 to inx (set in cfg[9:7]), then temperature. 1 1 scan in0 to inx (set in cfg[9:7]). 0 rb read back the cfg register. 0 = read back current configuration at end of data. 1 = do not read back contents of configuration.
AD7682/ad7689 rev. 0 | page 24 of 28 read/write spanning conversion without a busy indicator this mode is used when the AD7682/ad7689 are connected to any host using an spi, serial port, or fpga. the connection diagram is shown in figure 34 , and the corresponding timing is given in figure 35 . for spi, the host should use cpha = cpol = 0. reading/writing spanning conversion is shown, which covers all three modes detailed in the digital interface section. a rising edge on cnv initiates a conversion, forces sdo to high impedance, and ignores data present on din. after a conversion is initiated, it continues until completion irrespective of the state of cnv. cnv must be returned high before the safe data transfer time, t data , and then held high beyond the conversion time, t conv , to avoid generation of the busy signal indicator. after the conversion is complete, the AD7682/ad7689 enter the acquisition phase and power down. when the host brings cnv low after t conv (max), the msb is enabled on sdo. the host also must enable the msb of cfg at this time (if necessary) to begin the cfg update. while cnv is low, both a cfg update and a data readback take place. the first 14 sck rising edges are used to update the cfg, and the first 15 sck falling edges clock out the conversion results starting with msb ? 1. the restriction for both configuring and reading is that they both occur before the t data time of the next conversion elapses. all 14 bits of cfg[13:0] must be written, or they are ignored. in addition, if the 16-bit conversion result is not read back before t data elapses, it is lost. the sdo data is valid on both sck edges. although the rising edge can be used to capture the data, a digital host using the sck falling edge allows a faster reading rate, provided it has an acceptable hold time. after the 16 th (or 30 th ) sck falling edge, or when cnv goes high (whichever occurs first), sdo returns to high impedance. if cfg readback is enabled, the cfg associ- ated with the conversion result ( n ? 1 ) is read back msb first following the lsb of the conversion result. a total of 30 sck falling edges is required to return sdo to high impedance if this is enabled. miso mosi sck ss cnv for spi use cpha = 0, cpol = 0. sck sdo din AD7682/ ad7689 digital host 07353-036 figure 34. connection diagram for the AD7682/ad7689 without a busy indicator update (n) cfg/sdo update (n + 1) cfg/sdo acquisition (n) acquisition (n + 1) acquisition (n - 1) msb msb ? 1 1 2 begin data (n ? 1) begin cfg (n + 1) cfg msb cfg msb ? 1 lsb + 1 14 15 see note see note notes: 1. the lsb is for conversion results or the configuration register cfg (n ? 1) if 15 sck falling edges = lsb of conversion results. 29 sck falling edges = lsb of configuration register. on the 16th or 30th sck falling edge, sdo is driven to high impendance. 16/ 30 conversion (n) return cnv high for no busy end data (n ? 1) end cfg (n + 1) cfg lsb x x > t conv lsb sck cnv din sdo lsb + 1 14 15 16/ 30 conversion (n ? 1) return cnv high for no busy end data (n ? 2) end cfg (n) cfg lsb x x t conv t data t cnvh t data t dis t dis t en t dsdo t hsdo t hdin t sdin t clsck t en t en t dis t dis t conv lsb 07353-037 t acq t cyc (quiet time) (quiet time) figure 35. serial interface timing for th e AD7682/ad7689 without a busy indicator
AD7682/ad7689 rev. 0 | page 25 of 28 read/write spanning conversion with a busy indicator this mode is used when the AD7682/ad7689 are connected to any host using an spi, serial port, or fpga with an interrupt input. the connection diagram is shown in figure 36 , and the corresponding timing is given in figure 37 . for spi, the host should use cpha = cpol = 1. reading/writing spanning conversion is shown, which covers all three modes detailed in the digital interface section. a rising edge on cnv initiates a conversion, forces sdo to high impedance, and ignores data present on din. after a conversion is initiated, it continues until completion irrespective of the state of cnv. cnv must be returned low before the safe data transfer time, t data , and then held low beyond the conversion time, t conv , to generate the busy signal indicator. when the conversion is complete, sdo transitions from high impedance to low with a pull-up to vio, which can be used to interrupt the host to begin data transfer. after the conversion is complete, the AD7682/ad7689 enter the acquisition phase and power down. the host must enable the msb of cfg at this time (if necessary) to begin the cfg update. while cnv is low, both a cfg update and a data readback take place. the first 14 sck rising edges are used to update the cfg, and the first 16 sck falling edges clock out the conversion results starting with the msb. the restriction for both configuring and reading is that they both occur before the t data time elapses for the next conversion. all 14 bits of cfg[13:0] must be written or they are ignored. also, if the 16-bit conversion result is not read back before t data elapses, it is lost. the sdo data is valid on both sck edges. although the rising edge can be used to capture the data, a digital host using the sck falling edge allows a faster reading rate, provided it has an acceptable hold time. after the optional 17 th sck falling edge, sdo returns to high impedance. note that, if the optional sck falling edge is not used, the busy feature cannot be detected if the lsb for the conversion is low. if cfg readback is enabled, the cfg associated with the conversion result ( n ? 1 ) is read back msb first following the lsb of the conversion result. a total of 31 sck falling edges is required to return sdo to high impedance if this is enabled. AD7682/ ad7689 miso mosi sck ss sdo v io for spi use cpha = 1, cpol = 1. sck cnv din digital host irq 07353-038 figure 36. connection diagram for the AD7682/ad7689 with a busy indicator sck acquisition (n) acquisition (n + 1) cnv din sdo msb msb ? 1 1 2 begin data (n ? 1) beign cfg (n + 1) cfg msb lsb + 1 lsb 15 15 see note see note notes: 1. the lsb is for conversion results or the configuration register cfg (n ? 1) if 16 sck falling edges = lsb of conversion results. 30 sck falling edges = lsb of configuration register. on the 17th or 31st sck falling edge, sdo is driven to high impendance. otherwise, the lsb remains active until the busy indicator is driven low. 16 16 17/ 31 17/ 31 conversion (n) conversion (n ? 1) (quiet time) end data (n ? 2) end data (n ? 1) end cfg (n + 1) end cfg (n) x xxx x x t data update (n + 1) cfg/sdo lsb + 1 lsb conversion (n ? 1) (quiet time) update (n) cfg/sdo t cyc t acq t hdin t hsdo t dsdo t sdin t data t conv t cnvh t dis t dis t dis t en t en t en cfg msb ?1 07353-039 figure 37. serial interface timing for the AD7682/ad7689 with a busy indicator
AD7682/ad7689 rev. 0 | page 26 of 28 application hints layout the printed circuit board that houses the AD7682/ad7689 should be designed so that the analog and digital sections are separated and confined to certain areas of the board. the pinout of the AD7682/ad7689, with all its analog signals on the left side and all its digital signals on the right side, eases this task. avoid running digital lines under the device because these couple noise onto the die unless a ground plane under the AD7682/ad7689 is used as a shield. fast switching signals, such as cnv or clocks, should not run near analog signal paths. crossover of digital and analog signals should be avoided. at least one ground plane should be used. it can be common or split between the digital and analog sections. in the latter case, the planes should be joined underneath the AD7682/ad7689. the AD7682/ad7689 voltage reference input ref has a dynamic input impedance and should be decoupled with minimal parasitic inductances. this is done by placing the reference decoupling ceramic capacitor close to, ideally right up against, the ref and gnd pins and connecting them with wide, low impedance traces. finally, the power supplies vdd and vio of the AD7682/ ad7689 should be decoupled with ceramic capacitors, typically 100 nf, placed close to the AD7682/ad7689 and connected using short, wide traces to provide low impedance paths and reduce the effect of glitches on the power supply lines. evaluating AD7682/ad7689 performance other recommended layouts for the AD7682/ad7689 are outlined in the documentation of the evaluation board for the AD7682/ad7689 ( eval-AD7682cb z/eval-ad7689cbz ). the evaluation board package includes a fully assembled and tested evaluation board, documentation, and software for controlling the board from a pc via the evaluation controller board, eval-control brd3 .
AD7682/ad7689 rev. 0 | page 27 of 28 outline dimensions 2.65 2.50 sq 2.35 3.75 bsc sq 4.00 bsc sq compliant to jedec standards mo-220-vggd-1 012508-b 1 0.50 bsc p i n 1 i n d i c a t o r 0.50 0.40 0.30 top view 12 max 0.80 max 0.65 typ seating plane pin 1 indi c ator coplanarity 0.08 1.00 0.85 0.80 0.30 0.23 0.18 0.05 max 0.02 nom 0.20 ref 20 6 16 10 11 15 5 exposed pad (bottom view) 0.60 max 0.60 max 0.25 min figure 38. 20-lead lead frame chip scale package (lfcsp_vq) 4 mm 4 mm body, very thin quad (cp-20-4) dimensions shown in millimeters ordering guide model integral nonlinearity no missing code temperature range package description package option ordering quantity AD7682bcpz 1 2 lsb max 16 bits C40c to +85c 20-lead qfn (lfcsp_vq) cp-20-4 tray, 490 AD7682bcpzrl7 1 2 lsb max 16 bits C40c to +85c 20-lead qfn (lfcsp_vq) cp-20-4 reel, 1,500 ad7689acpz 1 6 lsb max 15 bits C40c to +85c 20-lead qfn (lfcsp_vq) cp-20-4 tray, 490 ad7689acpzrl7 1 6 lsb max 15 bits C40c to +85c 20-lead qfn (lfcsp_vq) cp-20-4 reel, 1,500 ad7689bcpz 1 2 lsb max 16 bits C40c to +85c 20-lead qfn (lfcsp_vq) cp-20-4 tray, 490 ad7689bcpzrl7 1 2 lsb max 16 bits C40c to +85c 20-lead qfn (lfcsp_vq) cp-20-4 reel, 1,500 eval-AD7682cbz 1 evaluation board eval-ad7689cbz 1 evaluation board eval-control brd3 2 controller board 1 z = rohs compliant part. 2 this controller board allows a pc to control and communicate with all analog devices evaluation boards whose model numbers end in cb.
AD7682/ad7689 rev. 0 | page 28 of 28 notes ?2008 analog devices, inc. all rights reserved. trademarks and registered trademarks are the property of their respective owners. d07353-0-5/08(0)

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