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16-bit, 1 msps, integrated data acquisition subsystem data sheet adaq7980 / adaq7988 rev. 0 document feedback 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 ?2017 analog devices, inc. all rights reserved. technical support www.analog.com features easy to use integrated data acquisition subsystem all active components designed by analog devices, inc. on-board adc driver and reference buffer 50% pcb area savings includes critical passive components spi-/qspi-/microwire?-/dsp-compatible serial interface daisy-chain multiple adaq7980 / adaq7988 devices versatile supply configuration with 1.8 v/2.5 v/3 v/5 v logic interface pseudo differential adc input structure high performance 16-bit resolution with no missing codes throughput: 1 msps ( adaq7980 ) and 500 ksps ( adaq7988 ) inl: 8 ppm typical and 20 ppm maximum snr: 91.5 db typical at 10 khz (unity gain) thd: ?105 db at 10 khz low input bias current: 470 na typical low power dissipation 21 mw typical at 1 msps ( adaq7980 ) 16.5 mw typical at 500 ksps ( adaq7988 ) flexible power-down modes dynamic power scaling small, 24-lead, 5 mm 4 mm lga package wide operating temperature range: ?55c to +125c applications automated test equipment (ate) battery powered instrumentation communications data acquisition process control medical instruments general description the adaq7980 / adaq7988 are 16-bit analog-to-digital converter (adc) subsystems that integrate four common signal processing and conditioning blocks into a system in package (sip) design that supports a variety of applications. these devices contain the most critical passive components, eliminating many of the design challenges associated with traditional signal chains that use successive approximation register (sar) adcs. these passive components are crucial to achieving the specified device performance. functional block diagram ref gnd v dd vio sdi sck sdo cnv 20 ? v + v? 1.8nf ldo 2.2f 10f ref_out ldo_out pd_ref amp_out pd_amp pd_ldo adc adcn in+ in? adaq7980/ adaq7988 15060-001 figure 1. the adaq7980 / adaq7988 contain a high accuracy, low power, 16-bit sar adc, a low power, high bandwidth, high input impedance adc driver, a low power, stable reference buffer, and an efficient power management block. housed within a tiny, 5 mm 4 mm lga package, these systems simplify the design process for data acquisition systems. the level of system integration of the adaq7980 / adaq7988 solves many design challenges, while the devices still provide the flexibility of a configurable adc driver feedback loop to allow gain and/or common-mode adjustments. a set of four device supplies provides optimal system performance; however, single-supply operation is possible with minimal impact on device operating specifications. using the sdi input, the serial peripheral interface (spi)- compatible serial interface features the ability to daisy-chain multiple devices on a single, 3-wire bus and provides an optional busy indicator. the user interface is compatible with 1.8 v, 2.5 v, 3 v, or 5 v logic. specified operation of these devices is from ?55c to +125c. table 1. integrated sar adc subsystems type 500 ksps 1000 ksps 16-bit adaq7988 adaq7980
adaq7980/adaq7988 data sheet rev. 0 | page 2 of 49 table of contents features .............................................................................................. 1 ? applications ....................................................................................... 1 ? general description ......................................................................... 1 ? functional block diagram .............................................................. 1 ? revision history ............................................................................... 2 ? specifications ..................................................................................... 3 ? dual-supply configuration ........................................................ 3 ? single-supply configuration ...................................................... 7 ? timing specifications ................................................................ 11 ? absolute maximum ratings .......................................................... 13 ? thermal data .............................................................................. 13 ? thermal resistance .................................................................... 13 ? esd caution ................................................................................ 13 ? pin configuration and function descriptions ........................... 15 ? typical performance characteristics ........................................... 17 ? terminology .................................................................................... 25 ? theory of operation ...................................................................... 26 ? circuit information .................................................................... 26 ? converter operation .................................................................. 26 ? typical connecti on diagram ................................................... 27 ? adc driver input ...................................................................... 28 ? input protection .......................................................................... 28 ? noise considerations and signal settling .............................. 28 ? pd_amp operation .................................................................. 31 ? dynamic power scaling (dps) ................................................. 31 ? slew enhancement ..................................................................... 33 ? effect of feedback resistor on frequency response ............ 33 ? voltage reference input ............................................................ 33 ? power supply ............................................................................... 35 ? ldo regulator current-limit and thermal overload protection .................................................................................... 36 ? ldo regulator thermal considerations ............................... 36 ? digital interface .......................................................................... 37 ? 3-wire cs mode without the busy indicator ........................ 38 ? 3-wire cs mode with the busy indicator ............................... 39 ? 4-wire cs mode without the busy indicator ........................ 40 ? 4-wire cs mode with the busy indicator ............................... 41 ? chain mode without the busy indicator ............................... 42 ? chain mode with the busy indicator ...................................... 43 ? application circuits ....................................................................... 44 ? nonunity gain configurations ................................................ 45 ? inverting configuration with level shift ................................ 46 ? using the adaq7980/adaq7988 with active filters ........ 47 ? applications information .............................................................. 48 ? layout .......................................................................................... 48 ? evaluating the performance of the adaq7980/adaq7988 ... 48 ? outline dimensions ....................................................................... 49 ? ordering guide .......................................................................... 49 ? revision history 3/2017revision 0: initial version
data sheet adaq7980/adaq7988 rev. 0 | page 3 of 49 specifications dual - supply configuration vdd = 3.5 v to 10 v, v+ = 6.3 v to 7.7 v, v? = ?2.5 v to ?0.2 v, vio = 1.7 v to 5.5 v, v ref = 5 v, t a = ? 55 c to +125c, adc driver in a unity - gain buffer configuration, f sample = 1 msps ( adaq7980 ) , and f sample = 500 k sps ( adaq7988 ), unless otherwise noted. table 2. parameter test conditions/comments min typ max unit resolution 16 bits system accuracy no missing codes 16 bits differential nonlinearity error (dnl) ?14 7 + 14 ppm 1 integral nonlinearity error (inl) ?20 8 + 20 ppm 1 transition noise 0.6 lsb 1 rms gain error t a = 25c ?0.01 0.002 + 0.01 %fs gain error temperature drift 0.1 0.4 ppm/c zero error t a = 25c ?0.5 0.0 6 + 0.5 mv zero error temperature drift 0.3 1. 3 v /c common - mode rejection ratio adc driver configured as difference amplifier 103 130 db power supply rejection ratio positive v+ = +6.3 v to +8 v, v? = ?2 v 75 105 db negative v+ = +7 v, v? = ?1.0 v to ?2.5 v 80 110 db system ac performance dynamic range 92 db 2 v ref = 2.5 v 87 db 2 total rms noise 44.4 v rms oversampled dynamic range oversample dynamic range frequency ( f odr ) = 10 ksps 111 db 2 signal -to - noise ratio (snr) input frequency (f in ) = 10 khz 90.5 91.5 db 2 f in = 10 khz, v ref = 2.5 v 84.5 86.5 db 2 spurious - free dynamic range f in = 10 khz 106 db 2 total harmonic distortion (thd) f in = 10 khz ?105 ?100 db 2 signal -to - noise - and - distortion ratio f in = 10 khz 90 91 db 2 f in = 10 khz, v ref = 2.5 v 84 86 db 2 effective number of bits f in = 10 khz 14.65 14.8 bits noise free code resolution 14.1 bits system sampling dynamics conversion rate adaq7980 vio 3.0 v 0 1 msps vio 1.7 v 0 833 ksps adaq7988 vio 1.7 v 0 500 ksps transient response full - scale step 430 500 ns ?3 db input bandwidth adc driver rc filter 4.42 mhz ?1 db frequency adc driver rc filter 2.2 mhz ?0.1 db frequency adc driver rc filter 0.67 mhz system 0.1 hz to 10 hz voltage noise 17 v p-p aperture delay 2.0 ns aperture jitter 2.0 ns 1 lsb means least significant bit. with the 5 v input range, 1 lsb is 76.3 v and 1 lsb = 15.26 ppm. 2 all specifications in db are referred to a full - scale input , fs r . tested with an input signal at 0.5 db below full scale, unless otherwise specified.
adaq7980/adaq7988 data sheet rev. 0 | page 4 of 49 vdd = 3.5 v to 1 0 v, v+ = 6.3 v to 7.7 v, v? = ?2.5 v to ?0.2 v, vio = 1.7 v to 5.5 v, v ref = 5 v, t a = ?55 c to +125c, adc driver in a unity - gain buffer configuration, and f sample = 1 msps ( adaq7980 ) and f sample = 500 ksps ( adaq7988 ) , unless otherwise noted. table 3 . parameter test conditions/comments min typ max unit reference input voltage range voltage at ref pin 2. 4 5. 1 v load current ref out 330 a buffer input resistance ref 50 m ? capacitance ref 1 pf bias current 550 800 na offset voltage t a = 25c 13 125 v offset voltage drift 0.2 1. 3 v / c voltage noise f in = 100 khz 5.2 nv/ hz voltage noise 1/f corner frequency 8 hz current noise f in = 100 khz 0.7 pa/ hz 0.1 hz to 10 hz voltage noise 44 nv rms linear output current refout 40 ma short - circuit current refout sinking/sourcing 85/73 ma adc driver characteristics voltage range in+, in ? , amp_out 0 v ref v absolute input voltage in+, in ? , amp_out ?0.1 +5.1 v adcn ?0.1 +0.1 v ?3 db bandwidth g = +1, v amp_out = 0.02 v p -p 37 mhz g = +1, v amp_out = 2 v p -p 35 mhz bandwidth for 0.1 db flatness g = +1, v amp_out = 0.1 v p -p 4 mhz slew rate g = +1, v amp_out = 2 v step 110 v/s g = +1 , v amp_out = 5 v step 40 v/s input voltage noise f = 100 khz 5.2 nv/hz 1/f corner frequency 8 hz 0.1 hz to 10 hz voltage noise 44 nv rms input current noise f = 100 khz 0.7 pa/hz bias in+, in? 550 800 na offset 2.1 na input offset voltage t a = 25c 13 125 v drift 0.2 1.3 v/c open - loop gain 111 db input resistance in+, in? common mode 50 m? differential mode 260 k? input capacitance in+, in? 1 pf input common - mode voltage range specified performance ?0.1 v+ ? 1.3v v output overdrive recovery time v in+ = 10% overdrive, f in = 10 khz 500 ns linear output current 40 ma short - circuit current sinking/sourcing 85/73 ma digital inputs logic levels input voltage low ( v il ) vio > 3.0 v ?0.3 +0.3 vio v vio 3.0 v ?0.3 +0.1 vio v high ( v ih ) vio > 3.0 v 0.7 vio vio + 0.3 v vio 3.0 v 0.9 vio vio + 0.3 v
data sheet adaq79 80/adaq7988 rev. 0 | page 5 of 49 parameter test conditions/comments min typ max unit input current low ( i il ) ?1 +1 a high ( i ih ) ?1 +1 a digital outputs data format serial 16 bits , straight binary pipeline delay conversion results available immediately after completed conversion v ol i sink = 500 a 0.4 v v oh i source = ?500 a vio ? 0.3 v power - down signaling adc driver/ref buffer pd_amp , pd_ref voltage low powered down <2.2 v high enabled >2.6 v turn - off time 50% of pd_amp , pd_ref to <10% of enabled quiescent current 1.25 2.75 s turn - on time specified performance 2 7.25 s dynamic power scaling period specified performance 10 s low dropout ( ldo ) regulator pd_ldo voltage low powered down 1.06 1.12 1.18 v high enabled 1.15 1.22 1.30 v pd_ldo logic hysteresis 100 mv turn - off time 2.2 f capacitive load 460 650 s turn - on time 370 425 s power requirements vdd 3.5 5 10 v ldo voltage accuracy i ldo_out = 10 ma, t a = 25c ?0.8 +0.8 % 100 a < i ldo_out < 100 ma, vdd = 3.5 v to 10 v ?1.8 +1.8 % ldo line regulation vdd = 3.5 v to 10 v ?0.015 +0.015 %/v ldo load regulation i ldo_out = 100 a to 100 ma 0.002 0.004 %/ma ldo start - up time v ldo_out = 2.5 v 380 s ldo current - limit threshold 250 360 460 ma ldo thermal shutdown threshold t j rising 150 c hysteresis 15 c ldo dropout voltage i ldo_out = 10 ma 30 60 mv i ldo_out = 100 ma 200 420 mv v+ 3.7 7 v? + 10 v v? v+ ? 10 ?2 +0.1 v vio 1.7 5.5 v total standby current 1 , 2 static, all devices enabled 1.2 1.7 ma adc driver, ref buffer disable 56 103 a adc driver, ref buffer, ldo disable 14 23 a adaq7980 current draw 1 msps vio 0.3 0.34 ma v+/v? 1.5 2.0 ma vdd 1.45 1.6 ma
adaq7980/adaq7988 data sheet rev. 0 | page 6 of 49 parameter test conditions/comments min typ max unit adaq7980 power dissipation 1 msps v+/v?/vdd 20 36 mw 1 ksps, dynamic power scaling enabled 3 5.8 9 mw vio 1.0 1.9 mw total 21 37.9 4 mw adaq7988 current draw vio 0.15 0.17 ma v+/v? 1.35 1.85 ma vdd 0.73 0.8 ma adaq7988 power dissipation 500 ksps v+/v?/vdd 16 26.5 mw 1 ksps, dynamic power scaling enabled 3 5.8 9 mw vio 0.5 0.95 mw total 16.5 27.5 4 mw temperature range specified performance t min to t max ?55 +125 c 1 with all digital inputs forced to vio or gnd as required. 2 during the acquisition phase. 3 dynamic power scaling duty cycle is 10%. 4 calculated with the maximum supply differential and not the typical supply values.
data sheet adaq7980/adaq7988 rev. 0 | page 7 of 49 single - supply configuration vdd = v+ = 5 .0 v, v? = 0 v, vio = 1.7 v to 5.5 v , v ref = 3.3 v, t a = ? 55 c to +125c, the adc driver in a unity - gain buffer configuration , and f sample = 1 msps ( adaq7980 ) and f sample = 500 ksps ( adaq7988 ), unless otherwise noted. table 4. parameter test conditions/comments min typ max unit resolution 16 bits system accuracy differential nonl inearity error 1 ?14 7 + 14 ppm 2 integral nonl inearity error 1 ?20 8 + 20 ppm 2 transition noise 0.8 lsb 2 rms gain error t a = 25c ?0.013 0.002 + 0.013 %fs gain error temperature drift 0.1 0.4 ppm/c zero error t a = 25c ?0.5 0.06 + 0.5 mv zero error temperature drift 0.35 1.75 v /c common - mode rejection ratio 103 133 db power supply rejection ratio positive v+ = 4.5 v to 5.5 v, v ? = 0 v 75 92 db system ac performance dynamic range 89 db 3 total rms noise 41.4 v rms oversampled dynamic range f odr = 10 ksps 109 db 3 signal -to - noise ratio input frequency (f in ) = 10 khz 87.3 88.7 db 3 spurious - free dynamic range f in = 10 khz 103 db 3 total harmonic distortion f in = 10 khz ?113 ?100 db 3 signal -to - noise - and - distortion ratio f in = 10 khz 87 88.4 db 3 effective number of bits f in = 10 khz 14.1 14.4 bits noise free code resolution 13.5 bits system sampling dynamics conversion rate adaq7980 vio 3.0 v 0 1 msps vio 1.7 v 0 833 ksps adaq7988 vio 1.7 v 0 500 ksps transient response full - scale step 430 500 ns ?3 db input bandwidth adc driver rc filter 4.42 mhz ?1 db frequency adc driver rc filter 2.2 mhz ?0.1 db frequency adc driver rc filter 0.67 mhz system 0.1 hz to 10 hz voltage noise 17 v p -p aperture delay 2.0 ns aperture jitter 2.0 ns 1 nonlinearity guaranteed over input voltage range. codes below 15 0 mv are not represented with a unipolar supply configuration. 2 lsb means le ast significant bit. with the 3.3 v i nput range, 1 lsb = 50.4 v, and 1 lsb = 15.26 ppm. 3 all specifications in db are referred to a full - scale input , fs r . tested with an input signal at 0.5 db below full scale, unless otherwise specified.
adaq7980/adaq7988 data sheet rev. 0 | page 8 of 49 vdd = v+ = 5.0 v, v? = 0 v, vio = 1.7 v to 5.5 v, v ref = 3.3 v, t a = ?55 c to +125c, the adc driver in a unity - gain buffer configuration , and f sample = 1 msps ( adaq7980 ) and f sample = 500 ksps ( adaq7988 ), unless otherwise noted. table 5 . parameter test con ditions/comments min typ max unit reference input voltage range voltage at ref pin 2.4 v+ ? 1.3 v load current refout 330 a buffer input resistance ref 50 m ? capacitance ref 1 pf bias current 470 720 na offset voltage t a = 25c 9 125 v offset voltage drift 0.2 1.5 v/ c voltage noise f in = 100khz 5. 9 nv/ hz voltage noise 1/f corner frequency 8 hz current noise f in = 100khz 0.6 pa/ hz 0.1 hz to 10 hz voltage noise 54 nv rms linear output current refout 40 ma short - circuit current refout sinking/sourcing 73/63 ma adc driver characteristics specified voltage range in+, in?, amp_out 0.1 5 v ref v absolute input voltage in+, in?, amp_out ? 0.1 v+ ? 1.3 v adcn ?0.1 + 0.1 v ?3 db bandwidth g = +1, v amp_out = 0.02 v p -p 31 mhz g = +1, v amp_out = 2 v p -p 3 0 mhz bandwidth for 0.1 db flatness g = +1, v amp_out = 0.1 v p -p 4 mhz slew rate g = +1, v amp_out = 2 v step 31 v/s g = +1 , v amp_out = 3.15 v step 20 v/s input voltage noise f = 100 khz 5.9 nv/hz 1/f corner frequency 8 hz 0.1 hz to 10 hz voltage noise 54 nv rms input current noise f = 100 khz 0.6 pa/hz bias in+, in? 470 720 na offset 0.4 na input offset voltage t a = 25c 9 125 v open - loop gain 109 db input resistance in+, in? common mode 50 m? differential mode 260 k? input capacitance in+, in? 1 pf input common - mode voltage range specified performance ?0.1 v+ ? 1.3 v output overdrive recovery time v in+ = 10% overdrive, f in = 10 khz 800 ns linear output current 40 ma short - circuit current sinking/sourcing 73/63 ma digital inputs logic levels input voltage low ( v il ) vio > 3.0 v ?0.3 +0.3 vio v vio 3.0 v ?0.3 +0.1 vio v high ( v ih ) vio > 3.0 v 0.7 vio vio + 0.3 v vio 3.0 v 0.9 vio vio + 0.3 v
data sheet adaq79 80/adaq7988 rev. 0 | page 9 of 49 parameter test con ditions/comments min typ max unit input current low ( i il ) ?1 +1 a high ( i ih ) ?1 +1 a digital outputs data format serial 16 bits straight binary pipeline delay conversion results available immediately after completed conversion v ol i sink = 500 a 0.4 v v oh i source = ?500 a vio ? 0.3 v power - down signaling adc driver/ref erence buffer pd_amp , pd_ref voltage low powered down <1.5 v high enabled >1.9 v turn - off time 50% of pd_amp , pd_ref to <10% of enabled quiescent current 0.9 1.25 s turn - on time specified performance 2 7.25 s dynamic power scaling period specified performance 10 s ldo pd_ldo voltage low powered down 1.06 1.12 1.18 v high enabled 1.15 1.22 1.30 v pd_ldo logic hysteresis 100 mv turn - off time 2.2 f capacitive load 460 650 s turn - on time 370 425 s power requirements vdd 3.5 5 10 v ldo voltage accuracy i ldo_out = 10 ma, t a = 25c ?0.8 +0.8 % 100 a < i ldo_out < 100 ma, vdd = 3.5 v to 10 v ?1.8 +1.8 % ldo line regulation vdd = 3.5 v to 10 v ?0.015 +0.015 %/v ldo load regulation i ldo_out = 100 a to 100 ma 0.002 0.004 %/ma ldo start - up time v ldo_out = 2.5 v 380 s ldo current - limit threshold 250 360 460 ma ldo thermal shutdown threshold t j rising 150 c hysteresis 15 c ldo dropout voltage i ldo_out = 10 ma 30 60 mv i ldo_out = 100 ma 200 420 mv v+ 3.7 5 v? + 10 v v? v+ ? 10 0 + 0.1 v vio 1.7 5.5 v total standby current 1 , 2 static, all devices enabled 1.1 1.7 ma adc driver, ref buffer disable d 50 103 a adc driver, ref buffer, ldo disable d 7 23 a adaq7980 current draw 1 msps vio 0.3 0.34 ma v+/v? 1.3 2.0 ma vdd 1.45 1.6 ma
adaq7980/adaq7988 data sheet rev. 0 | page 10 of 49 parameter test con ditions/comments min typ max unit adaq7980 power dissipation 1msps v+/v?/vdd 13.75 36 mw 1 ksps, adc driver dynamic power scaling enabled 3 2.9 9 mw vio 1.0 1.9 mw total 14.75 37.9 4 mw adaq7988 current draw vio 0.15 0.17 ma v+/v? 1.15 1.85 ma vdd 0.73 0.8 ma adaq7988 power dissipation 500 ksps v+/v?/vdd 9.4 26.5 mw 1 ksps, adc driver dynamic power scaling enabled 3 2.9 9 mw vio 0.5 0.95 mw total 9.9 27.5 4 mw temperature range specified performance t min to t max ?55 +125 c 1 with all digital inputs forced to vio or gnd as required. 2 during the acquisition phase. 3 dynamic power scaling duty cycle is 10%. 4 calculated with the maximum supply differential and not the typical supply values.
data sheet adaq79 80/adaq7988 rev. 0 | page 11 of 49 timing specification s vdd = 3.5 v to 1 0 v , vio = 1.7 v to 5.5 v , and t a = ? 55 c to +125 c, unless otherwise noted in addition to figure 2 and figure 3 , see figure 72, figure 74, figure 76, figure 78, figure 80 , and figure 82 for the additional timing diagrams detailed in tabl e 6 . tabl e 6 . parameter sym bol min typ max unit conversion time: cnv rising edge to data available t conv vio above 3.0 v ( adaq7980 ) 500 710 ns vio above 1.7 v ( adaq7980 ) 500 800 ns adaq7988 500 1200 ns acquisition phase 1 t acq ns adaq7980 290 ns adaq7988 800 ns time between conversions t cyc vio above 3.0 v ( adaq7980 ) 1000 ns vio above 1.7 v ( adaq7980 ) 1200 ns vio above 1.7 v ( adaq7988 ) 2000 ns cs mode cnv pulse width t cnvh 10 ns sck period t sck vio above 4.5 v 10.5 ns vio above 3 .0 v 12 ns vio above 1.7 v 22 ns cnv or sdi low to sdo d15 msb valid t en vio above 3 .0 v 10 ns vio above 1.7 v 40 ns cnv or sdi high or last sck falling edge to sdo high impedance t dis 20 ns sdi valid hold time from cnv rising edge t hsdicnv vio above 3.0 v 2 ns vio above 1.7 v 10 ns chain mode sck period t sck vio above 4.5 v 11.5 ns vio above 3 .0 v 13 ns vio above 1.7 v 23 ns sdi valid hold time from cnv rising edge t hsdicnv 0 ns sck valid setup time from cnv rising edge t ssckcnv 5 ns sck valid hold time from cnv rising edge t hsckcnv 5 ns sdi valid setup time from sck falling edge t ssdisck 2 ns sdi valid hold time from sck falling edge t hsdisck 3 ns sdi high to sdo high (with busy indicator) t dsdosdi vio above 3.0 v 15 ns vio above 1.7 v 22 ns sck low time t sckl vio above 3.0 v 4.5 ns vio above 1.7 v 6 ns high time t sckh vio above 3.0 v 4.5 ns vio above 1.7 v 6 ns
adaq7980/adaq7988 data sheet rev. 0 | page 12 of 49 parameter symbol min typ max unit falling edge to data remains valid t hsdo 3 ns falling edge to data valid delay t dsdo vio above 4.5 v 9.5 ns vio above 3.0 v 11 ns vio above 1.7 v 21 ns sdi valid setup time from cnv rising edge t ssdicnv 5 ns 1 the acquisition phase is the time available for the adc sampling capacitors to acqu ire a new input with the adc running at a t hroughput rate of 1 msps. 500a i ol 500a i oh 1.4v to sdo c l 20pf 15060-002 figure 2. load circuit fo r digital interface timing x% vio 1 y% vio 1 v ih 2 v il 2 v il 2 v ih 2 t delay t delay 1 for vio 3.0v, x = 90, and y = 10; for vio > 3.0v, x = 70, and y = 30. 2 minimum v ih and maximum v il used. see digital inputs specifications in table 3 or table 5. 15060-003 figure 3. voltage levels for timing
data sheet adaq79 80/adaq7988 rev. 0 | page 13 of 49 absolute maximum rat ings t a = 25c, unless otherwise noted. table 7 . parameter rating v+ to v? 11 v v+ to gnd ?0.3 v to + 11 v v? to gnd ? 11 v to +0.3 v vdd to gnd ? 0.3 v to +24 v ref_out/ vio to gnd ? 0.3 v to +6 v in+ / in? /ref to gnd v? ? 0.7 v to v + + 0.7 v amp_out /adcn to gnd ? 0.3 v to v ref + 0.3 v or 130 ma differential analog input voltage ( in+ ? in ? ) 1 v digital input 1 voltage to gnd ? 0.3 v to vio + 0.3 v digital output 2 voltage to gnd ? 0.3 v to vio + 0.3 v input current to any pin except supplies 3 , 4 10 ma operating temperature range ? 55 c to +125c storage temperature range ? 65c to +150c junction temperature 150c esd human body model (hbm) 4000 v field induced charged device model (ficdm) 1250 v 1 th e digital input pins include the following: cnv, sdi , and sc k. 2 the digital output pin is sdo . 3 transient currents of up to 100 ma do not cause scr latch - up. 4 condition applies when power is provided to subsystem. stresses at or above those listed under absolute maximum ratings may cause permanent damage to the product. this is a stress rating only; functional operation of the product at these or any other conditions above those indicated in the operational section of this specification is not implied. operation beyond the maxim um operating conditions for extended periods may affect product reliability. thermal data absolute maximum ratings apply individually only, not in combination. the adaq7980 / adaq7988 can be damaged when the junction temperature (t j ) limits are exceeded. monitoring ambient temperature does not guarantee that t j is within the specified temperature limits. in applications wit h high power dissipation and poor thermal resistance, the maximum ambient temperature (t a ) may have to be derated. in applications with moderate power dissipation and low printed circuit board (pcb) thermal resistance, the maximum t a can exceed the maximu m limit as long as the junction temperature is within specification limits . the ja of the package is based on modeling and calculation using a 4 - layer board. the ja is highly dependent on the application and board layout. in applications where high maximum power dissipation exists, close attention to thermal board design is required. the ja value may vary depending on pcb material, layout, and environmental conditions. therma l resistance thermal resistance values specified in table 8 were calculated based on jedec spec ification s and must be used in compliance with jesd51 - 12 . because the product contains more than one silicon device , only the worst case junction temperature is reported. table 8 . thermal resistance package type 1, 2 ja jc top 2 jt unit cc -24-2 65 103 12.6 ?c/w 1 these values represent the worst case die junction in the package . 2 table 8 v alues we re calculated based on the s tandard jedec test conditions defined in table 9 , unless otherwise specified. 3 for jc test, 100 m thermal interface material (tim) was used. tim is assumed to be 3.6 w/mk. only use ja and jc top to comp are thermal performance of th e package of the device with other semiconductor packages when all test conditions listed are similar. one common mistake is to use ja and jc to estimate the junction temperature in the system environment. instead, using jt is a more appropriate way to estimate the worst cas e junction temperature of the device in the system en vironment. f irst , take an accurate thermal measurement of the top center of the device (on the mold compound in this case) while the device operat es in the system environment. this measurement is known in the following equation as t top . this equation can then be used to solve for the worst case t j in that given environment as follows : t j = jt p + t t op where: jt is the j unction to t op thermal characterization number as specified in data sheet. p r efers to total power dissipation in the chip (w). t top refers to the package top temperature (c) and is measured at the top center of the package in the environment of the user. esd caution
adaq7980/adaq7988 data sheet rev. 0 | page 14 of 49 table 9 . standard jedec test conditions test conditions ja jc jb main heat transfer mode convection conduction conduction board type 2s2p 1s0p 2s2p board thickness 1.6 mm 1.6 mm 1.6 mm board dimension if package length is <27 mm , 76.2 mm 114.3 mm; otherwise, 101.6 mm 114.3 mm if package length is <27 mm , 76.2 mm 114.3 mm; otherwise, 101.6 mm 114.3 mm if package length is <27 mm , 76.2 mm 114.3 mm; otherwise, 101.6 mm 114.3 mm signal traces thickness 0.07 mm 0.07 mm 0.07 mm pwr/gnd traces thickness 0.035 mm not applicable 0.035 mm thermal vias use thermal vias with 0.3 mm diameter, 0.025 mm plating, and 1.2 mm pitch whenever a package has an exposed thermal pad; vias numbers are maximized to cover the area of the exposed paddle use thermal vias with 0.3 mm diameter, 0.025 mm plating, and 1.2 mm pitch whenever a package has an exposed thermal pad; vias nu mbers are maximized to cover the area of the exposed paddle use thermal vias with 0.3 mm diameter, 0.025 mm plating, and 1.2 mm pitch whenever a package has an exposed thermal pad; vias numbers are maximized to cover the area of the exposed paddle cold pl ate not applicable cold plate attaches to either package top or bottom depending on the path of least thermal resistance fluid cooled, ring style cold plate that clamps both sides of the test board such that heat flows from package radially in the plane of the test board
data sheet adaq7980/adaq7988 rev. 0 | page 15 of 49 pin configuration and fu nction descriptions in+ cnv 1 in? 2 a mp_out 3 adcn 4 gnd 5 g n d g n d v ? g n d p d _ r e f p d _ a m p l d o _ o u t 13 sdo 14 sck 15 sdi 16 vio 17 adaq7980/ adaq7988 top view (not to scale) g n d 18 p d _ l d o 19 v d d 20 v + 21 g n d 22 r e f 23 r e f _ o u t 24 6 7 8 9 101112 15060-004 figure 4. pin configuration table 10. pin function descriptions pin no. mnemonic type 1 description 1 in+ ai adc driver noninverting input. 2 in? ai adc driver inverting input. 3 amp_out ai, ao adc driver output and adc input before low-pass filter (lpf). 4 adcn ai analog input ground sense. connect this pin to the analog ground plane or to a remote sense ground. 5 to 7, 9, 18, 22 gnd p ground. 8 v? p negative power supply line for the adc driver. this pin requires a 100 nf capacitor to gnd for best operation. connect this pin to ground for single-supply operation. 10 pd_ref di active low power-down signal for reference buffer. when powered down, the reference buffer output enters a high impedance (high-z) state. 11 pd_amp di active low power-down signal for adc driver. when powered down, the reference buffer output enters a high-z state. 12 ldo_out p regulated 2.5 output voltage from on-board ldo. an internal 2.2 f bypass capacitor to gnd is provided. 13 cnv di convert input. this input has multiple functions. on its leading edge, it initiates the conversions and selects the interface mode of the device, chain, or cs mode. in cs mode, it enables the sdo pin when low. in chain mode, read the data when cnv is high. 14 sdo do serial data output. the conversion result is output on this pin. sdo synchronizes with sck. 15 sck di serial data clock input. when the device is selected, the conversion result is shifted out onto sdo by this clock. 16 sdi di serial data input. this input provides multiple fe atures. it selects the interface mode of the adc as follows. when sdi is low during the cnv rising edge, chain mo de is selected. in this mode, sdi is used as a data input to daisy-chain the conversion results of two or more adcs onto a single sdo line. the digital data level on sdi is output on sdo with a delay of 16 sck cycles. when sdi is high during the cnv rising edge, cs mode is selected. in this mode, either sdi or cnv can enable the serial output signals when low; if sdi or cnv is low when the conversion is complete, the busy indicator feature is enabled. 17 vio p input/output interface digital power. vio is nominally at the same supply as the host interface (1.8 v, 2.5 v, 3 v, or 5 v). 19 pd_ldo di active low power-down signal for ldo. when powe red down, the ldo output enters a high-z state. for a continuously enabled state or for automatic startup, tie pd_ldo to the vdd pin (pin 20). 20 vdd p regulator input supply. bypass vdd to gnd with a 2.2 f capacitor.
adaq7980/adaq7988 data sheet rev. 0 | page 16 of 49 pin no. mnemonic type 1 description 21 v+ p positive power supply li ne for the adc d river and reference buff er. this pin c an be tied to vdd as long as headroom for the reference buffer is maintained. this pin r equires a 100 nf capacitor t o gnd for best operation. 23 ref ai external reference s ignal . ref is the n oninverting input of on - board reference buffer. connect an external reference source to this pin. a low - pass filter may be required between the reference source and this pin to band limit noise generated by the reference source. 24 ref_out ao reference buffer output. this pin p rovides access to the buffered reference signal presented to the adc. 1 ai is analog input, ao is analog output, p is power , di is digital input, and do is digital output.
data sheet adaq7980/adaq7988 rev. 0 | page 17 of 49 typical performance characteristics vdd = 3.5 v to 10 v, v+ = 6.3 v to 7.7 v, v? = ?1.0 v to ?2.5 v, vio = 1.7 v to 5.5 v, v ref = 5 v, t a = 25c, adc driver in a unity-gain buffer configuration, f sample = 1 msps ( adaq7980 ), f sample = 500 ksps ( adaq7988 ), and f in = 10 khz, unless otherwise noted. 20 15 10 5 0 ?5 ?10 ?15 ?20 060k 50k 40k 30k 20k 10k integral nonlinearity (ppm) code positive inl = +4.3ppm negative inl = ?5.8ppm 15060-105 figure 5. integral nonlinearity vs. code, ref = 5 v 20 15 10 5 0 ?5 ?10 ?15 ?20 2048 62048 52048 42048 32048 22048 12048 integral nonlinearity (ppm) code positive inl = +7.8ppm negative inl = ?4.0ppm 15060-106 figure 6. integral nonlinearity vs. code, v+ = vdd = 5 v, v? = 0 v, ref = 3.3 v 0 ?100 ?120 ?140 ?40 ?20 ?60 ?80 ?160 0500k 400k 300k 200k 100k 450k 350k 250k 150k 50k amplitude (db of full scale) frequency (hz) snr = 91.77db sinad = 91.56db thd = ?104.32db sfdr = 105.08dbc 15060-107 figure 7. fft, ref = 5 v 20 15 10 5 0 ?5 ?10 ?15 ?20 060k 50k 40k 30k 20k 10k differential nonlinearity (ppm) code positive dnl = 6.0ppm negative dnl = ?6.4ppm 15060-108 figure 8. differential nonlinearity vs. code, ref = 5 v 20 15 10 5 0 ?5 ?10 ?15 ?20 2048 62048 52048 42048 32048 22048 12048 code positive dnl = 6.9ppm negative dnl = ?6.1ppm differential nonlinearity (ppm) 15060-109 figure 9. differential nonlinearity vs. code, v+ = vdd = 5 v, v? = 0 v, ref = 3.3 v 0 ?100 ?120 ?140 ?40 ?20 ?60 ?80 ?160 0500k 400k 300k 200k 100k 450k 350k 250k 150k 50k amplitude (db of full scale) frequency (hz) snr = 86.87db sinad = 86.85db thd = ?110.10db sfdr = 103.70dbc 15060-110 figure 10. fft, ref = 2.5 v
adaq7980/adaq7988 data sheet rev. 0 | page 18 of 49 180000 160000 140000 120000 100000 80000 60000 40000 20000 0 counts adc code 32785 32786 32787 32788 32789 32790 32791 32792 32793 15060-111 03 979 52669 170828 37210 454 10 total count = 262144 figure 11. histogram of a dc input at the code center, ref = 5 v 140000 120000 100000 80000 60000 40000 20000 0 counts adc code 15060-112 total count = 262144 0 64 8973 118166 124157 10708 76 0 32790 32791 32792 32793 32794 32795 32796 32797 figure 12. histogram of a dc input at the code transition, ref = 5 v 93 88 87 91 92 90 89 86 15.0 14.3 14.1 14.8 14.9 14.7 14.5 14.4 14.2 14.6 14.0 2.4 4.9 4.4 3.4 3.9 2.9 snr, sinad (db) enob (bits) reference (v) snr sinad enob 15060-113 figure 13. snr, sinad, and enob vs. reference voltage 100000 90000 70000 50000 30000 80000 60000 40000 20000 10000 0 counts adc code 15060-114 total count = 262144 01 23 349 3857 21893 62761 88057 60351 20580 3829 426 17 0 26229 26230 26231 26232 26233 26234 26235 26236 26237 26238 26239 26240 26241 26242 figure 14. histogram of a dc input at the code center, ref = 2.5 v ? 120 ?130 ?132 ?136 ?134 ?138 ?124 ?122 ?126 ?128 ?140 ?10 0 ?2 ?4 ?6 ?8 ?1 ?3 ?5 ?7 ?9 noise floor (db) input level (dbfs) fft size = 65536 15060-115 figure 15. noise fl oor vs. input level ? 95 ?120 ?105 ?100 ?110 ?115 ?125 115 90 105 110 100 95 85 2.25 5.25 4.25 3.25 4.75 3.75 2.75 thd (db) sfdr (db) reference voltage (v) thd sfdr 15060-116 figure 16. thd and sfdr vs. reference voltage
data sheet adaq7980/adaq7988 rev. 0 | page 19 of 49 100 75 90 95 85 80 70 ? 60 ?110 ?80 ?70 ?90 ?100 ?120 1100 10 sinad (db) thd (db) frequency (khz) sinad thd 15060-117 figure 17. sinad and thd vs. frequency 93.0 90.5 92.0 92.5 91.5 91.0 90.0 ?55 125 45 ?15 85 565105 25 ?35 snr, sinad (db) temperature (c) snr sinad 15060-118 figure 18. snr and sinad vs. temperature ? 104.0 ?105.0 ?105.2 ?105.6 ?105.4 ?105.8 ?104.4 ?104.2 ?104.6 ?104.8 ?106.0 ?55 125105 65 25 ?15 85 45 5 ?35 thd (db) temperature (c) 15060-120 figure 19. thd vs. temperature 9 ?6 ?9 3 6 0 ?3 ?12 100 10 1 closed-loop gain (db) frequency (mhz) v+ = +7v v ? = ? 2v v out = 20mv p-p g = +2 g = +1, r f = 0 g = ?1 0.1 15060-122 figure 20. adc driver small signal frequency response for various gains 9 ?6 ?9 3 6 0 ?3 ?12 100 10 1 closed-loop gain (db) frequency (mhz) 20mv p-p g = +1 v+ = +7v, v? = ?2v v+ = +5v, v? = 0v 0.1 15060-123 figure 21. adc driver small signal frequency response for various supply voltages 3 ?6 0 ?3 ?9 100 10 1 closed-loop gain (db) frequency (mhz) 2v p-p g = +1 v+ = +7v v ? = ? 2v +125c +25c ?40c ?55c 0.1 15060-124 figure 22. large signal frequency response for various temperatures
adaq7980/adaq7988 data sheet rev. 0 | page 20 of 49 9 ?6 ?9 3 6 0 ?3 ?12 100 10 1 closed-loop gain (db) frequency (mhz) 20mv p-p g = +1 v+ = +7v v? = ?2v +125c +85c +25c ?40c ?55c 0.1 15060-125 figure 23. adc driver small signal frequency response for various temperatures 6 ?6 ?9 3 0 ?3 ?12 100 10 1 closed-loop gain (db) frequency (mhz) v+ = +7v v? = ?2v v out = 2v p-p g = +2 g = +1,r f = 0 g = ?1 0.1 15060-126 figure 24. adc driver large signal frequency response for various gains 9 ?6 ?9 3 6 0 ?3 ?12 0.1 100 10 1 closed-loop gain (db) frequency (mhz) g = +1 v+ = +7v v? = ?2v 20mv p-p 200mv p-p 500mv p-p 2v p-p 15060-127 figure 25. frequency response for various output voltages 5 1 0 4 3 2 ?1 100 10 1 closed-loop gain (db) frequency (mhz) 100mv p-p g = +1 v+ = 7v v? = ?2v 0.1 15060-128 figure 26. adc driver small signal 0.1 db bandwidth 20 ?5 ?15 ?10 10 15 5 0 ?20 01 0 987654321 voltage noise (v) time (seconds) 15060-227 figure 27. subsystem 0.1 hz to 10 hz voltage noise 300 350 400 450 500 550 600 650 700 750 800 ?40 ?25 ?10 5 20 35 50 65 80 95 110 125 quiescent supply current ( a) temperature (c) v+ = +5v, v? = 0v v+ = +10v, v? = 0v 15060-130 figure 28. adc driver and reference buffer quiescent supply current vs. temperature for various supplies
data sheet adaq7980/adaq7988 rev. 0 | page 21 of 49 0.6 0.5 0.4 0.3 0.2 0.1 0 010 987654321 recovery time (s) overload duration (s) 15060-229 g = +1 v+ = +7v v? = ?2v v in = 10% overdrive figure 29. recovery time vs. overload duration ?180 ?160 ?140 ?120 ?100 ?80 ?60 ?40 ?20 0 ?20 0 20 40 60 80 100 120 10 100 1k 10k 100k 1m 10m 100m open-loop phase (degrees) open-loop gain (db) frequency (hz) gain phase 15060-132 figure 30. adc driver open-loop gain and phase vs. frequency 0 200 400 600 800 100 1200 1400 ?55 ?35 ?15 5 25 45 65 85 105 125 adc driver dynamic power scaling turn-on time (ns) temperature (c) 15060-231 f s = 100ksps v? = 0v v ref = 3.3v v+ = 10v v+ = 7v v+ = 5v figure 31. adc driver dynamic power scaling turn-on time vs. temperature for various supply voltages 1200 1000 800 600 400 200 0 410 9 8 7 6 5 adc driver dynamic power scalin g turn on time (ns) supply (v) 15060-233 f s = 100ksps v? = 0v v ref = 3.3v figure 32. adc driver dynamic power scaling turn on time vs. supply voltage 0 100 200 300 400 500 600 800 700 0123456 supply current (a) time (s) 15060-242 v? = 0v v+ = 10v v+ = 5v v+ = 4v figure 33. supply current vs. adc driver and reference buffer turn-off response time for various supplies 0 100 200 300 400 500 600 700 800 0123456 supply current (a) time (s) 15060-235 v+ = 5v v? = 0v +125c +25c ?40c figure 34. supply current vs. adc driver and reference buffer turn-off response time for various temperatures
adaq7980/adaq7988 data sheet rev. 0 | page 22 of 49 ?140 ?120 ?100 ?80 ?60 ?40 ?20 0 20 10 100 1k 10k 100k 1m 10m 100m frequency (hz) +psrr cmrr cmrr, psrr (db) v+ = 5v, v? = 0v ? v+, ? v cm = 100mv p-p 15060-138 figure 35. cmrr and psrr vs. frequency 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0 power-down threshold (v) supply voltage from ground (v) device enabled device disabled t a = ?40c t a = +25c t a = +125c 15060-139 figure 36. adc driver and reference buffer power-down threshold vs. supply voltage from ground for various temperatures 0.0025 0.0005 0.0020 0.0015 0.0010 0 ?55 125 45 ?15 85 5 65 105 25 ?35 gain error (% fs) temperature (c) 15060-140 figure 37. gain error vs. temperature 90 ?30 70 30 50 10 ?10 ?50 ?55 125 45 ?15 85 565105 25 ?35 reference buffer offset voltage (v) temperature (c) 15060-141 figure 38. reference buffer input offset voltage vs. temperature 1.5 0.7 1.4 1.2 1.3 1.1 0.9 0.6 1.0 0.8 0.5 ?55 125 45 ?15 85 565105 25 ?35 supply current (ma) temperature (c) f sample = 0hz v+ = 10v v+ = 5v v+ = 3.8v 15060-142 figure 39. adc driver and reference buffer static supply current vs. temperature for various supplies 1.9 1.7 1.3 1.5 1.1 0.7 0.9 0.5 ?55 125 45 ?15 85 565105 25 ?35 supply current (ma) temperature (c) v+ = 10v v+ = 5v v+ = 3.8v 15060-143 figure 40. adc driver and reference buffer dynamic supply current vs. temperature for various supplies
data sheet adaq7980/adaq7988 rev. 0 | page 23 of 49 0.020 0.015 0.005 0.010 0 ?55 125 45 ?15 85 565105 25 ?35 pd current (ma) temperature (c) pd current 10v supply delta pd current 5v supply delta pd current 3.8v supply delta f sample = 0hz 15060-144 figure 41. total adc driver and reference buffer power-down (pd) current vs. temperature 0.05 0.03 ?0.01 ?0.03 0.01 ?0.05 ?55 125 45 ?15 85 565105 25 ?35 offset error (mv) temperature (c) offset error v+ = +7v, v? = ?2v offset error v+ = +5v, v? = 0v 15060-145 figure 42. offset error vs. temperature 1.6 1.4 0.8 0.4 1.2 0.6 0.2 1.0 0 ?55 125 45 ?15 85 5 65 105 25 ?35 ldo current (ma) temperature (c) ldo dynamic current 10v input ldo static current 10v input 15060-146 figure 43. ldo current vs. temperature for various supplies 0 0.5 1.0 1.5 2.0 2.5 3.5 3.0 0 100k 200k 300k 400k 500k 600k 700k 800k 900k 1000k total operating current (ma) sample rate (sps) 15060-245 v+ = 3.8v, v? = 0v v+ = 5v, v? = 0v v+ = 7.7v, v? = 0v v+ = 10v, v? = 0v figure 44. total operating current vs. sample rate for various supplies 2.60 2.55 2.45 2.50 2.40 ?55 125 45 ?15 85 565105 25 ?35 ldo_out (v) temperature (c) 15060-148 figure 45. output voltage (ldo_out) vs. temperature 2.55 2.53 2.49 2.47 2.51 2.45 0100 50 20 70 30 60 90 80 40 10 ldo_out (v) load current (ma) 15060-149 figure 46. output voltage (l do_out) vs. load current (i load )
adaq7980/adaq7988 data sheet rev. 0 | page 24 of 49 2.45 2.55 2.53 2.51 2.49 2.47 45678910 ldo_out (v) v dd (v) 15060-248 i load = 1ma i load = 10ma i load = 100ma figure 47. output voltage (ldo_out) vs. v dd ?55 125 45 ?15 85 5 65 105 25 ?35 temperature (c) 0.0020 0.0015 0.0005 0.0010 0 ldo pd current (ma) f sample = 0hz v dd = 5v 15060-151 figure 48. ldo pd current vs. temperature 0 0.20 0.16 0.12 0.08 0.04 0.02 0.18 0.14 0.10 0.06 0.0001 0.001 0.01 0.1 ldo dropout voltage (v) i load (ma) 15060-250 figure 49. ldo dropout voltage vs. load current (i load ), ldo_out = 2.5 v 2.30 2.60 2.55 2.50 2.45 2.40 2.35 2.50 3.002.952.902.852.802.752.702.652.602.55 ldo_out (v) v dd (v) 15060-251 i load = 1ma i load = 10ma i load = 100ma figure 50. ldo_out vs. v dd in dropout, ldo_out = 2.5 v ?90 ?80 ?70 ?60 ?50 ?40 ?30 ?20 ? 10 0.01 0.1 1 10 100 isolation (db) frequency (mhz) v+ = 5v v? = 0v v in = 0.5 v p-p 15060-154 figure 51. forward/off isolation vs. frequency
data sheet adaq7980/adaq7988 rev. 0 | page 25 of 49 terminology 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 a level 1? lsb beyond th e last code transition. the deviation is measured from the middle of each code to the true straight 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. zero error the first transition occurs at a level ? lsb above analog ground (38.1 v for the 0 v to 5 v range). the offset error is the deviation of the actual transition from that point. gain error the last transition (from 111 10 to 111 11) occurs for an analog voltage 1? lsb below the nominal full scale (4.999886 v for the 0 v to 5 v range). the gain error is the deviation of the actual level of the last transition from the i deal level after the offset is adjusted out. 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. it is related to sinad by the following formula: enob = ( sinad db ? 1.76)/6.02 enob is expressed in bits. noise free code resolution noise free code resolution is the number of bits beyond which it is impossible t o distinctly resolve individual codes. c alculate it as follows: noise free code resolution = log 2 (2 n / peak to peak noise ) noise free code resolution is expressed in bits. total harmonic distortion (thd) thd is the ratio of the rms sum of the first five harm onic components to the rms value of a full - scale input signal and is expressed in decibels (db). 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 dyn amic range is expressed in decibels (db). it is measured with a signal at ?60 dbfs to include all noise sources and dnl artifacts. 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 (db). signal -to - noise - and - distortion (sinad) ratio sinad is the ratio of the rms value of the actual input signal to the rms sum of all other s pectral components below the nyquist frequency, including harmonics but excluding dc. the value for sinad is expressed in decibels (db). aperture delay aperture delay is the measure of the acquisition performance. it is the time between the rising edge of the cnv input and when the input signal is held for a conversion. transient response transient response is the time required for the adc to accurately acquire its input after a full - scale step function is applied.
adaq7980/adaq7988 data sheet rev. 0 | page 26 of 49 theory of operation comp switches control output code cnv control logic sw+ lsb sw? lsb in+ ref gnd in? msb msb c c 4c 2c 16,384c 32,768c c c 4c 2c 16,384c 32,768c 15060-055 figure 52. adc simplified schematic circuit information the adaq7980 / adaq7988 system in package (sip) is a fast, low power, precise data acquisition (daq) signal chain that uses a sar architecture. the daq subsystem contains a high bandwidth, analog-to-digital converter (adc) driver, a low noise reference buffer, a low dropout regulator (ldo), and a 16-bit sar adc, along with critical passive components required to achieve optimal performance. all active components in the circuit are designed by analog devices, inc. the adaq7980 / adaq7988 are capable of converting 1,000,000 samples per second (1 msps) and 500,000 samples per second (500 ksps), respectively. the adc powers down between conversions; therefore, power consumption scales with sample rate. the adc driver and reference buffer are capable of dynamic power scaling, where the power consumption of these components scales with sample rate. when operating at 1 ksps, for example, the adaq7980 / adaq7988 consume 2.9 mw typically, ideal for battery-powered applications. the adaq7980 / adaq7988 offer a significant form factor reduction compared to traditional signal chains while still providing flexibility to adapt to a wide array of applications. all three signal pins of the adc driver are available to the user, allowing various amplifier configurations. the devices house the lpf between the driver and the adc, controlling the signal chain bandwidth and providing a bill of materials reduction. the adaq7980 / adaq7988 do not exhibit any pipeline delay or latency, making them ideal for multiplexed applications. the adaq7980 / adaq7988 house a reference buffer and the corresponding decoupling capacitor. the placement of this decoupling capacitor is vital to achieving peak conversion performance. inclusion of this capacitor in the subsystem eliminates this performance hurdle. the reference buffer is configured for unity gain. by only including the reference buffer, the user has the flexibility to choose the reference buffer input voltage that matches the desired analog input range. the adaq7980 / adaq7988 interface to any 1.8 v to 5 v digital logic family. they are housed in a tiny 24-lead lga that provides significant space savings and allows flexible configurations. converter operation the adaq7980 / adaq7988 contain a successive approximation adc based on a charge redistribution digital-to-analog converter (dac). figure 52 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 input of the comparator are connected to gnd via the internal switches (sw+ and sw?). all independent switches are connected to the analog inputs. therefore, the capacitor arrays are used as sampling capacitors and acquire the analog signal on the adc inputs. when the acquisition phase is completed and the cnv input goes high, a conversion phase initiates. when the conversion phase begins, sw+ and sw? open first. the two capacitor arrays are then disconnected from the adc input and connected to the gnd input. therefore, the differential voltage between the adc input pins captured at the end of the acquisition phase are applied to the comparator inputs, causing the comparator to become unbalanced. by switching each element of the capacitor array between gnd and ref, the comparator input varies by binary weighted voltage steps (v ref /2, v ref /4 v ref /65,536). 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 devices return to the acquisition phase, and the control logic generates the adc output code and a busy signal indicator signaling the user that the conversion is complete. because the adaq7980 / adaq7988 have an on-board conversion clock, the serial clock (sck) is not required for the conversion process.
data sheet adaq7980/adaq7988 rev. 0 | page 27 of 49 transfer functions the ideal transfer characteristics for the adaq7980 / adaq7988 are shown in figure 53 and table 11. 000 ... 000 000 ... 001 000 ... 010 111 ... 101 111 ... 110 111 ... 111 ?fsr ?fsr + 1lsb ?fsr + 0.5lsb +fsr ? 1 lsb +fsr ? 1.5 lsb analog input adc code (straight binary) 15060-056 figure 53. adc ideal transfer function table 11. output codes and ideal input voltages analog input 1 description v ref = 5 v digital output code (hex) fsr C 1 lsb 4.999924 v 0xffff 2 midscale + 1 lsb 2.500076 v 0x8001 midscale 2.5 v 0x8000 midscale C 1 lsb 2.499924 v 0x7fff Cfsr + 1 lsb 76.3 v 0x0001 Cfsr 0 v 0x0000 3 1 the adaq7980/ adaq7988 adc driver in the unity-gain buffer configuration. 2 this is also the code for an overranged analog input (in+ ? in? above v ref ? v gnd ). 3 this is also the code for an underra nged analog input (in+ ? in? below v gnd ). typical connection diagram figure 54 shows an example of the recommended connection diagram for the adaq7980 / adaq7988 when multiple supplies are available. ref gnd vdd vio sdi sck sdo cnv 20? v+ v? 1.8nf 10f ldo 2.2f ref_out ldo_out pd_ref 2 amp_out pd_amp 2 negative supply positive supply ref 1 pd_ldo 2 adc adcn in+ in? 1.8v to 5v 100nf 100nf 100nf 2.2f 0 v t o vref 1 see the voltage reference input section for reference selection. 2 power down pins connected to either digital host or positive supply. 15060-057 figure 54. typical application diagram with multiple supplies
adaq7980/adaq7988 data sheet rev. 0 | page 28 of 49 adc driver input the adc driver of the adaq7980 / adaq7988 features a ?3 db bandwidth of 35 mhz and a slew rate of 110 v/s at g = +1 and v amp_out = 2 v step. it features an input voltage noise of 5.9 nv/hz .the driver can operate over a supply voltage range of 3.8 v to 10 v and consumes only 500 a of supply current at a supply difference of 5 v. the low end of the supply range allows ?5% variation of a 4 v supply. the amplifier is unity-gain stable, and the input structure results in an extremely low input voltage noise 1/f corner. the adc driver uses a slew enhancement architecture, as shown in figure 55. the slew enhancement circuit detects the absolute difference between the two inputs. it then modulates the tail current, i tail , of the input stage to boost the slew rate. the architecture allows a higher slew rate and a faster settling time with a low quiescent current while maintaining low noise. the user has access to all three amplifier signal pins, providing flexibility to adapt to the desired application or configuration. in+ v in+ v in? v+ input stage to detect absolute value slew enhancement circuit i tail in? 15060-058 figure 55. adc driver slew enhancement circuit input protection the amplifier is fully protected from esd events, withstanding human body model esd events of 4000 v and field induced charged device model events of 1250 v with no measured performance degradation. the precision input is protected with an esd network between the power supplies and diode clamps across the input device pair, as shown in figure 56. in+ esd esd v? v + bias to the rest of the amplifier in? esd esd 15060-059 figure 56. adc driver input stage and protection diodes for differential voltages more than approximately 1.2 v at room temperature and 0.8 v at 125c, the diode clamps begin to conduct. if large differential voltages must be sustained across the input terminals, the current through the input clamps must be limited to less than 10 ma. external series input resistors that are sized appropriately for the expected differential overvoltage can provide the needed protection. the esd clamps begin to conduct for input voltages that are more than 0.7 v above the positive supply and input voltages more than 0.7 v below the negative supply. if an overvoltage condition is expected, the input current must be limited to less than 10 ma. along with the adc driver inputs, protection is also provided on the adc input. as shown in figure 1, the adaq7980 / adaq7988 house an rc filter between the adc driver and the adc. the series resistor in this low-pass filter acts to limit current in an overvoltage condition. the current sink capability of the reference buffer works to hold the reference node at its desired value when the adc input protection diodes conduct due to an overvoltage event. figure 57 shows an equivalent adc analog input circuit of the adaq7980 / adaq7988 . the two diodes, d1 and d2, provide esd protection for the adc inputs. take care to ensure that the adc analog input signal never exceeds the reference value by more than 0.3 v or drops below ground by more than 0.3 v because this causes diodes to become forward-biased and start conducting current. these diodes can handle a forward-biased current greater than or equal to the short-circuit current of the adc driver. for instance, these conditions can occur when the adc driver positive supply is greater than the reference value. in such a case (for example, an input buffer with a short circuit), use the current limitation to protect the devices. ref r in c in in+ or in? gnd d2 c pin d1 15060-060 figure 57. equivalent adc analog input circuit the analog input structure allows the sampling of the true differential signal between the adc input pins. by using these differential inputs, signals common to both inputs are rejected. noise considerations and signal settling the adc driver of the adaq7980 / adaq7988 is ideal for driving the on-board high resolution sar adc. the low input voltage noise and rail-to-rail output stage of the driver helps to minimize distortion at large output levels. with its low power of 500 a, the amplifier consumes power that is compatible with the low power sar adc. furthermore, the adc driver supports a single-supply configuration; the input common-mode range extends to the negative supply, and 1.3 v below the positive supply.
data sheet adaq7980/adaq7988 rev. 0 | page 29 of 49 figure 58 illustrates the primary noise contributors for the typical gain configurations. the total output noise (v n_out ) is the root sum square of all the noise contributions. r g r s i n? r f v n 4ktr s v n _ rs = 4ktr g v n _ rg = v n _ rf = + v n _ out ? 4ktr f i n+ 15060-061 figure 58. noise sources in typical connection calculate the output noise spectral density of the adc driver by ?? 22 2 222 2 _ 4 414 f n g g f n s n g f f outn riktr r r vriktrs r r ktr v ? ? ? ? ? ? ? ? ? ? ? ??? ? ? ? ? ? ? ? ? ?? ? where: k is the boltzmann constant. t is the absolute temperature in degrees kelvin. r f and r g are the feedback network resistances, as shown in figure 58. r s is the source resistance, as shown in figure 58. i n + and i n ? represent the amplifier input current noise spectral density in pa/hz. v n is the amplifier input voltage noise spectral density in nv/hz. for more information on these calculations, see mt-049 and mt-050 . source resistance noise, amplifier input voltage noise (v n ), and the voltage noise from the amplifier input current noise (i n+ r s ) are all subject to the noise gain term (1 + r f /r g ). figure 59 shows the total referred to input (rti) noise due to the amplifier vs. the source resistance. note that with a 5.9 nv/hz input voltage noise and 0.6 pa/hz input current noise, the noise contributions of the amplifier are relatively small for source resistances from approximately 2.6 k to 47 k. the analog devices, inc., silicon germanium (sige) bipolar process makes it possible to achieve a low voltage noise. this noise is much improved compared to similar low power amplifiers with a supply current in the range of hundreds of microamperes. 1 10 100 1k 100 1k 10k 100k 1m rti noise (nv/ hz) source resistance ( ? ) total noise source resistance noise amplifier noise source resistance = 2.6k ? source resistance = 47k ? 15060-062 figure 59. rti noise vs. source resistance keep the noise generated by the driver amplifier, and its associated passive components, as low as possible to preserve the snr and transition noise performance of the adaq7980 / adaq7988 . the analog input circuit of the adaq7980 / adaq7988 features a one-pole, low-pass filter to band limit the noise coming from the adc driver. because the typical noise of the adaq7980 / adaq7988 is 44.4 v rms in the dual-supply typical configuration, the snr degradation due to the amplifier is ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? 2 db3 2 )( 2 44.4 44.4 log20 n loss nef snr where: f C3 db is the cutoff frequency of the input filter (4.4 mhz). n is the noise gain of the amplifier (for example, 1 in a buffer configuration). e n is the equivalent input noise voltage of the op amp, in nv/hz. for multichannel multiplexed applications, the analog input circuit of the adaq7980 / adaq7988 must settle a full-scale step onto the capacitor array at a 16-bit level (0.0015%, 15 ppm) within one conversion period. as shown in figure 20, the bandwidth of the adc driver changes with the gain setting implemented. the adc driver must maintain a sufficient bandwidth to allow the adc input to settle properly. the rc time constant of the low-pass filter of the adaq7980 / adaq7988 has been set to settle the anticipated sar adc charge redistribution voltage step from a full-scale adc input voltage transition within the minimum acquisition phase of the adc. the maximum full-scale step is based upon the maximum reference input voltage of 5.1 v. the reference sets the maximum analog input range and subsequently the range of voltages that the adc can quantize.
adaq7980/adaq7988 data sheet rev. 0 | page 30 of 49 during the conversion process, the capacitive dac of the sar adc disconnects from the adc input. in a multiplexed application, the multiplexer input channel switches during the conversion time to provide the maximum settling time. at the end of the conversion time, the capacitive dac then connects back to the input. during this time, the dac is disconnected from the adc input, and a voltage change occurs at the adc input node. the voltage step observed at the adc analog input resulting from capacitive charge redistribution attenuates due to the voltage divider created by the parallel combination of the capacitive dac and the capacitor in the external low-pass filter. calculate the voltage step by v step = ( v ref 30 pf)/(30 pf + 1800 pf) = v ref 0.016 for a 5.0 v reference, this results in a maximum step size of 82 mv. to calculate the required filter and adc driver bandwidth, determine the number of time constants required to settle this voltage step within the adc acquisition phase as follows: ? ? ? ? ? ? ? ? ? ? ? ? 116 2 ln ref step tc v v n with the number of time constants known, determine the rc time constant () by = 290 ns/n tc . the minimum acquisition phase of the adc is 290 ns. signals must be fully settled within this acquisition period. calculate the filter bandwidth (bw) by bw = 1/(2 ). the adc driver small signal bandwidth must always remain greater than or equal to the bandwidth previously calculated. when the small signal bandwidth reduces, for example in the presence of a large voltage gain, increase the acquisition phase to increase the required system . an increase in acquisition phase results in a reduction of the maximum sample rate. the method previously described assumes the multiplexer switches shortly after the conversion begins and that the amplifier and rc have a large enough bandwidth to sufficiently settle the low- pass filter capacitor before acquisition begins. during forward settling, approximately 11 time constants are required to settle a full-scale step to 16 bits. for the low-pass rc filter housed in the adaq7980 / adaq7988 , the forward settling time of the filter is 11 36 ns 400 ns, which is much less than the conversion time of 710 ns/1200 ns, respectively. to achieve an adc driver forward settling time of less than 710 ns, maintain an adc driver large signal bandwidth of 2.49 mhz. calculate this as follows: adc driver forward settling time constant = 710 ns/ln(2 16 ) = 64 ns minimum adc driver large signal bandwidth = 1/(2 64 ns) = 2.49 mhz the forward settling does not necessarily have to occur during the conversion time (before the capacitive dac gets switched to the input), but the combined forward and reverse settling time must not exceed the required throughput rate. forward settling is less important for low frequency inputs because the rate of change of the signal is much lower. the importance of which bandwidth specification of the adc driver is used is dependent upon the type of input. focus high frequency (>100 khz) or multiplexed applications on the large signal bandwidth, and concentrate lower input frequency applications on the adc driver small signal bandwidth when performing the previous calculations. conversion mux channel switch cnv adc throughput t cyc t conv t acq acquisition acquisition adc input negative fs positive fs reverse settling capacitive dac switch to acquire forward settling 15060-063 figure 60. multiplexed application timing
data sheet adaq7980/adaq7988 rev. 0 | page 31 of 49 pd_amp operation figure 61 shows the adc driver and reference buffer shutdown circuitry. to maintain a low supply current in shutdown mode, no internal pull-up circuitry exists; therefore, drive the pd_amp pin high or low externally and do not leave it floating. pulling the pd_amp pin to 1 v below midsupply turns the device off, reducing the supply current to 2.9 a for a 5 v supply. when the amplifier powers down, its output enters a high impedance state. the output impedance decreases as frequency increases. in shutdown mode, a forward isolation of ?62 db can be achieved at 100 khz (see figure 51). v + v? pd_amp esd esd 2.2 ? 1.8 ? 1.1v to enable amplifier 15060-073 figure 61. shutdown circuit esd clamps protect the pd_amp pin, as shown in figure 61. voltages beyond the power supplies cause these diodes to conduct. to prote c t t he pd_amp pin, ensure that the voltage to this pin does not exceed 0.7 v above the positive supply or 0.7 v below the negative supply. if expecting an overvoltage condition, limit the input current to less than 10 ma with a series resistor. table 12 summarizes the threshold voltages for the powered down and enabled modes for various supplies. for any supply voltage, pulling the pd_amp pin to 1 v below midsupply turns the device off. table 12. threshold voltages for powered down and enabled modes mode v+/v? +4 v/0 v +5 v/0 v +7 v/?2 v enabled >+1.4 v >+1.9 v >+1.9 v powered down <+1.0 v <+1.5 v <+1.5 v dynamic power scaling (dps) one of the merits of a sar adc is that its power scales with the sampling rate. this power scaling makes sar adcs very power efficient, especially when running at lower sampling frequencies. traditionally, the adc driver associated with the sar adc consumes constant power, regardless of the sampling frequency. the adc driver allows dynamic power scaling. this feature allows the user to provide a periodic signal to the power- down pin of the adc driver that is synchronized to the convert start signal, thus scaling the system power consumption with the sample rate. figure 62 illustrates the method by which the sampling rate of the system dynamically scales the quiescent power of the adc driver. by providing properly timed signals to the convert start (cnv) pin of the adc and the pd_amp pins of the adc driver, both devices run at optimum efficiency. ref gnd v dd vio sdi sck sdo cnv 20? v + v? 1.8nf 10f ldo 2.2f ref_out ldo_out pd_ref amp_out pd_amp pd_ldo adc adcn in+ in? adaq7980/ adaq7988 timing generator 15060-065 figure 62. power management circuitry figure 63 illustrates the relative signal timing for power scaling the adc driver and the adc. to prevent degradation in the performance of the adc, the adc driver must have a fully settled output into the adc before the activation of the cnv pin. in this example, the amplifier is switched to full power mode 3 s prior to the rising edge of the cnv signal. the pd_amp pin of the adc driver is pulled low when the adc input is inactive in between samples. the quiescent current of the amplifier typically falls to 10% of the normal operating value within 0.9 s at a supply difference of 5 v. while in shutdown mode, the adc driver output impedance is high.
adaq7980/adaq7988 data sheet rev. 0 | page 32 of 49 sampling frequency = 100khz t s = 10s acquisition acquisition acquisition shutdown 3s 3s powered on powered on powered on shutdown shutdown v f 1 t f 1 t turnoff1 t f 2 v f 2 t turnoff2 t amp, on v f3 t f3 t turnoff3 adc mode cnv pd_amp conversion conversion conversion minimum powered on time = 3s adc driver output 15060-066 figure 63. timi ng waveforms figure 64 shows the quiescent power of the adc driver with and without the power scaling. without power scaling, the amplifier constantly consumes power regardless of the sampling frequency, as shown in the following equation. p q = i q v s with power scaling, the quiescent power becomes proportional to the ratio of the amplifier on time (t amp, on ) and the sampling time (t s ). p q = i q v s ( t amp, on / t s ) thus, by dynamically switching the driver between shutdown and full power modes during the sample period, the quiescent power of the driver scales with the sampling rate. 1.2 1.0 0.4 0.8 0.2 0.6 0 11m 100k 100 1k 10k 10 quiescent current (ma) sampling frequency ( f s ) amp quiescent current no dps amp quiescent current with dps adc current draw pd_amp on time = 3s v+ = 5v v? = 0v 15060-067 figure 64. quiescent current of the adc driver vs. adc sampling frequency
data sheet adaq7980/adaq7988 rev. 0 | page 33 of 49 slew enhancement the adc driver has an internal slew enhancement circuit that increases the slew rate as the feedback error voltage increases. this circuit improves the amplifier settling response for a large step, as shown in figure 65. this improvement in settling response is useful in applications where the multiplexing of multiple input signals occurs. ?0.5 2.5 2.0 1.5 1.0 0.5 0 0 20 40 60 80 100 120 140 output voltage (v) time (ns) 15060-266 500mv p-p 1v p-p 2v p-p figure 65. step response with selected output steps effect of feedback resistor on frequency response the amplifiers input capacitance and feedback resistor form a pole that, for larger value feedback resistors, can reduce phase margin and contribute to peaking in the frequency response. figure 66 shows the peaking for 500 feedback resistors (r f ) when the amplifier is configured in a gain of +2. figure 66 also shows how peaking can mitigate with the addition of a small value capacitor placed across the feedback resistor of the amplifier. ?6 9 6 3 0 ?3 0.1 1 10 100 closed-loop gain (db) frequency (mhz) 15060-267 8pf 0pf g = +2 r f , r g = 500 ? v out = 200mv p-p figure 66. peaking mitigation in small signal frequency response voltage reference input the adaq7980 / adaq7988 voltage reference input (ref) is the noninverting node of the on-board low noise reference buffer. the reference buffer is included to optimally drive the dynamic input impedance of the sar adc reference node. also housed in the adaq7980 / adaq7988 is a 10 f decoupling capacitor that is ideally laid out within the devices. this decoupling capacitor is a required piece of the sar architecture. the ref_out capacitor is not just a bypass capacitor. this capacitor is part of the sar adc that simply cannot fit on the silicon. during the bit decision process, because the bits are settled in a few 10s of nanoseconds or faster, the storage capacitor replenishes the charge of the internal capacitive dac. as the binary bit weighted conversion is processed, small chunks of charge are taken from the 10 f capacitor. the internal capacitor array is a fraction of the size of the decoupling capacitor, but this large value storage capacitor is required to meet the sar bit decision settling time. there is no need for an addition al lower value ceramic decoupling capacitor (for example, 100 nf) between the ref_out and gnd pins. the reference value sets the maximum adc input voltage that the sar capacitor array can quantize. the reference buffer is set in the unity-gain configuration; therefore, the user sets the reference voltage value with the ref pin and observes this value at the ref_out pin. the user is responsible for selecting a reference voltage value that is appropriate for the system under design. allowable reference values range from 2.4 v to 5.1 v; however, do not violate the input common-mode voltage range specification of the reference buffer. with the inclusion of the reference buffer, the user can implement a much lower power reference source than many traditional sar adc signal chains because the reference source drives a high impedance node instead of the dynamic load of the sar capacitor array. root sum square the reference buffer noise with the reference source noise to arrive at a total noise estimate. generally, the reference buffer has a noise density much less than that of the reference source. adc r g buffer voltage reference optional filter adaq7980 15060-072 figure 67. voltage reference with rc filtering as shown in figure 67, place a passive, rc low-pass filter with a very low cutoff frequency between the reference source and the ref pin of the adaq7980 / adaq7988 to band limit noise from the reference source. this filtering can be useful, considering the voltage reference source is usually the dominant contributor
adaq7980/adaq7988 data sheet rev. 0 | page 34 of 49 to the noise of the reference input circuit. filters with extremely low bandwidths can be used since the reference signal is a dc type signal. however, because with such low frequency cutoffs, the settling time at power on is quite large. for example, a single pole, low-pass filter with a ?3 db bandwidth of 20 hz has a time constant of approximately 8 ms. just like the adc driver, the reference buffer features a pd_ref pin that allows the user to control the power consumption of the adaq7980 / adaq7988 . a timing scheme similar to figure 63 can be implemented for the pd_ref pin. also, use the pd_ref feature during long idle periods where extremely low power consumption is desired. figure 68 shows the reference buffer shutdown circuitry. to maintain very low supply current in shutdown mode, do not supply the internal pull-up resistor; therefore, the drive pd_ref pin high or low externally and do not leave it floating. pulling the pd_ref pin to 1 v below midsupply turns the device off, reducing the supply current to 2.9 a for a 5 v supply. when the amplifier powers down, its output enters a high impedance state. the output impedance decreases as frequency increases. in shutdown mode, a forward isolation of ?80 db can be achieved at frequencies below 10 khz (see figure 51). v + v? pd_ref esd esd 2.2 ? 1.8 ? 1.1v to enable amplifier 15060-064 figure 68. reference buffer shutdown circuit esd clamps protect the pd_ref pin, as shown in figure 68. voltages beyond the power supplies cause these diodes to conduct. to prote c t t h e pd_ref pin, ensure that the voltage to this pin does not exceed 0.7 v above the positive supply or 0.7 v below the negative supply. when expecting an overvoltage condition, limit the input current to less than 10 ma with a series resistor. table 13 summarizes the threshold voltages for the powered down and enabled modes for various supplies. for any supply voltage, pulling the pd_ref pin to 1 v below midsupply turns the device off. table 13. threshold voltages for powered down and enabled modes mode v+/v? +4 v/0 v +5 v/0 v +7 v/?2 v enabled >+1.4 v >+1.9 v >+1.9 v powered down <+1.0 v <+1.5 v <+1.5 v if more than one adaq7980 / adaq7988 is used in a system, for example, in a daisy-chain configuration, it is possible to use the reference buffer of one adaq7980 / adaq7988 to provide the ref_out signal for multiple adaq7980 / adaq7988 devices. enabling the pd_ref pin of the reference buffer places the reference buffer output in a high impedance state. the active reference buffer can drive the subsequent ref_out nodes. see figure 69 for connection details. the sample rate of each individual converter determines the number of adaq7980 / adaq7988 references that can be chained together. each adaq7980 / adaq7988 sar adc reference consumes 330 a of load current at a reference input of 5 v and with the converter running at 1 msps. this current consumption scales linearly with sample rate. for example, reducing the sample rate to 100 ksps reduces the reference current draw to 33 a. the active reference buffer must regulate the cumulative current draw well enough so that the reference voltage does not change by more than ? of an lsb. an unperceived change in reference value manifests as a gain error. adc r g buffer voltage reference optional filter adaq7980/ adaq7988 device 1 adaq7980/ adaq7988 device 2 adc r g buffer adaq7980/ adaq7988 device 3 adc r g buffer pd_ref 1 pd_ref 2 pd_ref 3 powered off powered off powered on 15060-074 figure 69. reference configuration for multiple adaq7980 / adaq7988 devices
data sheet adaq7980/adaq7988 rev. 0 | page 35 of 49 power supply power supply bypassing is a critical aspect in the performance of the adc driver. a parallel connection of capacitors from each amplifier power supply pin (v+ and v?) to ground works best. smaller value ceramic capacitors offer improved high frequency response, whereas larger value ceramic capacitors offer improved low frequency performance. paralleling different values and sizes of capacitors helps to ensure that the power supply pins are provided with a low ac impedance across a wide band of frequencies. parralleling is important for minimizing the coupling of noise into the amplifierespecially when the amplifier psrr begins to roll offbecause the bypass capacitors can help lessen the degradation in psrr performance. place the smallest value capacitor on the same side of the board as the adaq7980 / adaq7988 and as close as possible to the amplifier power supply pins. connect the ground end of the capacitor directly to the ground plane. the adaq7980 / adaq7988 feature two other power supply pins: the input to the ldo regulator that supplies the adc (vdd) and a digital input/output interface supply (vio). vio allows direct interface with any logic between 1.8 v and 5.0 v. the adaq7980 / adaq7988 are independent of power supply sequencing between vio and vdd. it is recommended to provide power to vio and vdd prior to v+ and v?. in addition, while not required, it is recommended to place the adc driver and reference buffer in power-down by applying a logic low to the pd_amp and pd_ref pins during the power-on sequence of the adaq7980 / adaq7988 . the following are the recommended sequences for applying and removing power to the subsystems. the recommended dual-supply, power-on sequence follows: 1. apply a logic low to pd_amp , pd_ref , and pd_ldo . 2. apply a voltage to vio. 3. apply a voltage to vdd. 4. apply a logic high to pd_ldo . 5. apply a voltage to v+ and v?. 6. apply a logic high to pd_amp and pd_ref . the recommended single-supply, power-on sequence follows: 1. apply a logic low to pd_amp , pd_ref , and pd_ldo . 2. apply a voltage to vio. 3. apply a voltage to vdd and v+. 4. apply a logic high to pd_ldo . 5. apply a logic high to pd_amp and pd_ref . the recommended dual-supply, power-down sequence follows: 1. apply a logic low to pd_amp and pd_ref . 2. remove the voltage from v+ and v?. 3. apply a logic low to pd_ldo . 4. remove the voltage from vdd. 5. remove the voltage from vio. the recommended single-supply, power-down sequence follows: 1. apply a logic low to pd_amp and pd_ref . 2. apply a logic low to pd_ldo . 3. remove the voltage from v+ and vdd. 4. remove the voltage from vio. additionally, the adaq7980 / adaq7988 are insensitive to power supply variations over a wide frequency range, as shown in figure 70. 80 55 11000 frequency (khz) psrr (db) 10 100 75 70 65 60 15060-075 figure 70. psrr vs. frequency the vdd input is the input of an on-board ldo regulator that supplies 2.5 v to the sar adc. by housing an ldo regulator, the adaq7980 / adaq7988 provide a wide supply range to the user. when operating these devices in a single-supply configuration, tie the v+ and vdd pins together and connect the v? pin to ground. refer to table 4 for the full list of operating requirements associated with a single-supply system. the ldo regulator of the adaq7980 / adaq7988 is a 2.5 v, low quiescent current, linear regulator that operates from 3.5 v to 10 v and provides up to 100 ma of output current. the ldo regulator draws a low 180 a of quiescent current (typical) at full load. the typical shutdown current consumption is less than 3 a at room temperature. typical start-up time for the ldo regulator is 380 s. the adaq7980 / adaq7988 require a small 2.2 f ceramic capacitor connected between the vdd pin and ground. any quality ceramic capacitors can be used as long as they meet the minimum capacitance and maximum equivalent series resistance (esr) requirements. ceramic capacitors are manufactured with a variety of dielectrics, each with different behavior over temperature and applied voltage. capacitors must have a dielectric adequate to ensure the minimum capacitance over the necessary temperature range and dc bias conditions. x5r or x7r dielectrics with a voltage rating of 6.3 v to 100 v are recommended. y5v and z5u dielectrics are not recommended due to their poor temperature and dc bias characteristics.
adaq7980/adaq7988 data sheet rev. 0 | page 36 of 49 internally, the ldo regulator consists of a reference, an error amplifier, a feedback voltage divider, and a positive metal - oxide semiconductor ( pmos ) pass transistor. the pmos pass device, which is controlled by the error amplifier, delivers the o utput current. the error amplifier compares the reference voltage with the feedback voltage from the output and amplifies the difference. if the feedback voltage is lower than the reference voltage, the gate of the pmos device pull s lower, allowing more current to pass and increasing the output voltage. if the feedback voltage is higher than the reference voltage, the gate of the pmos device pull s higher, allowing less current to pass and decreasing the output voltage. the ldo regulator uses the pd_ldo pin to enable and disable the ldo_out pin under n ormal operating conditions. when pd_ldo is high, ldo_out turns on, and when pd_ldo is low, ldo_out turns off. for automatic startup, tie pd_ldo to vdd. only apply a logic low to pd_ldo if a logic low is applied to pd_amp and pd_ref as well. ldo regulator current - limit and thermal overload protection the current and thermal overload protection circuits protect the ldo regulator of the adaq7980 / adaq7988 against damage due to excessive power dissipation. the ldo regulator current limit s when the output load reaches 360 ma (typical). when the output load exceeds the current limit threshold , the output voltage reduce s to maintain a constant current limit. thermal overload protection is included, which limits th e ldo regulator junction temperature to a maximum of 150c (typical). under extreme conditions (that is, high ambient temperature and/or high power dissipation) , when the junction temperature starts to rise above 150c, the output turn s off, reducing the o utput current to zero. when the junction temperature drops below 135c, the output turn s on again, and the output current restore s to its operating value. consider the case where a hard short circuit from ldo_out to ground occurs. at first, the ldo regulator limits the current threshold that can be conducted into the short circuit . if self heating of the junc tion is enough to cause its temperature to rise above 150c, thermal s hutdown ac tivates, turning off the output and reducing the output current to zero. as the junction temperature cools and drops below 135c, the output turns on and conducts the current limit into the short, again causing the junction temperature to rise above 150c. this thermal oscillation between 135c and 150c causes a current oscillation between the maximum current and 0 ma that continues as long as the short circuit remains at the outpu t. current - limit and thermal limit protections protect the device against a ccidental overload conditions. for reliable operation, externally limit the power dissipation of the devices so that the junction temperature does not exceed 125c. ldo regulator thermal consideratio ns in applications with a low , input to output voltage differential, the ldo regulator does not dissipate much heat. however, in applications with high ambient temperature and/or high input voltage , the heat dissipated in the package may become large enough to cause the junction temperature of the die to exceed the specified junction temperature of 125c . when the junction temperature exceeds 150c, the ldo regulator enters thermal shutdown. it recovers only after the junction temper a ture decrease s below 135c to prevent any permanent damage. therefore, thermal analysis for the chosen application is important to guarantee reliable performance over all conditions. to guarantee specified operation, the junction temperature of the ldo regulator must not exceed 125c. to ensure that the junction temperature stays below this value, the user must be aware of the parameters that contribute to junction temperature changes. these para meters include ambient temperature , power dissipation in the power device, and thermal resistances between the junction and ambient air ( ja ). the ja number is dependent on the package assembly compounds used and the amount of material used to solder the package gnd pins to the pcb .
data sheet adaq7980/adaq7988 rev. 0 | page 37 of 49 digital interface though the adaq7980 / adaq7988 ha ve a reduced number of pins, they offer flexibility in their serial interface modes. the adaq7980 / adaq7988 , when in cs mode, are compatible with spi, qspi ? , and digital hosts. this interface can use either a 3 - wire or 4 - wire interface. a 3 - wire interface using the cnv, sck, and sdo signals minimizes wiring connections useful, for instance, in isolated applications. a 4 - wire in terface using the sdi, cnv, sck, and sdo signals allows cnv, which initiates the conversions, to be independent of the readback timing (sdi). this independence is useful in low jitter sampling or simultaneous sampling applications. the adaq7980 / adaq7988 , when in chain mode, provide a daisy - chain feature using the sdi input for cascading multiple adcs on a single data line similar to a shift register. the mode in which the se devices operate depends on the sdi level when the cnv rising edge occurs. to select cs mode , set sdi high, and to select chain mode , set sdi low. the sdi hold time is such that when sdi and cnv are connected together, chain mode is selected. in either mode, the adaq7980 / adaq7988 offer the flexi bility to optionally force a start bit in front of the data bits. this start bit can be used as a busy signal indicator to interrupt the digital host and trigger the data reading. otherwise, without a busy indicator, the user must time out the maximum conv ersion time prior to readback. t he busy indicator enables ? in cs mode if cnv or sdi is low when the adc conversion ends (see figure 74 and figure 78). ? in chain mode if sck is high during the cnv rising edge (see figure 82).
adaq7980/adaq7988 data sheet rev. 0 | page 38 of 49 3-wire cs mode without the busy indicator to c on ne c t a s i ng l e adaq7980 / adaq7988 to an spi-compatible digital host, use 3-wire cs mode without the busy indicator. figure 71 shows the connection diagram, and figure 72 shows the corresponding timing. with sdi tied to vio, a rising edge on cnv initiates a conversion, selects cs mode, and forces sdo to high impedance. after a conversion initiates, it continues until completion irrespective of the state of cnv, which is useful, for instance, to bring cnv low to select other spi devices, such as analog multiplexers. however, before the minimum conversion time elapses, return cnv high and then hold it high for the maximum conversion time to avoid the generation of a busy signal indicator. when the conversion completes, the adaq7980 / adaq7988 enter the acquisition phase and power down. when cnv goes low, the msb is output onto sdo. then, the remaining data bits clock out by subsequent sck falling edges. the data is valid on both sck edges. although the rising edge can capture data, a digital host using the sck falling edge allows a faster reading rate if it has an acceptable hold time. after the 16th sck falling edge, or when cnv goes high, whichever is earlier, sdo returns to high impedance. adaq7980/ adaq7988 sdo sdi data in digital host convert clk v i o cnv sck 15060-076 figure 71. 3-wire cs mode without the busy indicator connection diagram (sdi = 1, high) sdi = 1 t cnvh t conv t cyc cnv a cquisition acquisition t acq t sck t sckl conversion sck sdo d15 d14 d13 d1 d0 t en t hsdo 123 14 1516 t dsdo t dis t sckh 15060-077 figure 72. 3-wire cs mode without the busy indicator seri al interface timing (sdi = 1, high)
data sheet adaq7980/adaq7988 rev. 0 | page 39 of 49 3-wire cs mode with the busy indicator to c on ne c t a s i ng l e adaq7980 / adaq7988 to an spi-compatible digital host that has an interrupt input, use 3-wire cs mode with the busy indicator. figure 73 shows the connection diagram, and figure 74 shows the corresponding timing. with sdi tied to vio, a rising edge on cnv initiates a conversion, selects cs mode, and forces sdo to high impedance. sdo stays in high impedance until the completion of the conversion irrespective of the state of cnv. prior to the minimum conversion time, use cnv to select other spi devices, such as analog multiplexers; however, return cnv to low before the minimum conversion time elapses and then hold it low for the maximum conversion time to guarantee the generation of the busy signal indicator. when the conversion completes, sdo goes from high impedance to low impedance. with a pull-up on the sdo line, use this transition as an interrupt signal to initiate the data reading controlled by the digital host. the adaq7980 / adaq7988 then enter the acquisition phase and power down. the data bits clock out, msb first, by subsequent sck falling edges. the data is valid on both sck edges. although the rising edge can capture the data, a digital host using the sck falling edge allows a faster reading rate if it has an acceptable hold time. after the optional 17th sck falling edge, or when cnv goes high, whichever is earlier, sdo returns to high impedance. if selecting multiple adaq7980 / adaq7988 devices at the same time, the sdo output pin handles this contention without damage or induced latch-up. meanwhile, it is recommended to keep this contention as short as possible to limit extra power dissipation. adaq7980/ adaq7988 sdo sdi data in irq digital host convert clk vio vio 47k ? cnv sck 15060-078 figure 73. 3-wire cs mode with the busy indicator connection diagram (sdi = 1, high) t conv t cnvh t cyc acquisition acquisition t acq t sck t sckh t sckl conversion sck cnv sdi = 1 sdo d15 d14 d1 d0 t hsdo 123 15 1617 t dsdo t dis 15060-079 figure 74. 3-wire cs mode with the busy indicator serial interface timing (sdi = 1, high)
adaq7980/adaq7988 data sheet rev. 0 | page 40 of 49 4-wire cs mode without the busy indicator to c on ne c t i ng mu lt ipl e adaq7980 / adaq7988 devices to an spi-compatible digital host, use 4-wire cs mode without the busy indicator. figure 75 shows a connection diagram example using two adaq7980 / adaq7988 devices, and figure 76 shows the corresponding timing. with sdi high, a rising edge on cnv initiates a conversion, selects cs mode, and forces sdo to high impedance. in this mode, hold cnv high during the conversion phase and the subsequent data readback (if sdi and cnv are low, sdo is driven low). prior to the minimum conversion time, use sdi to select other spi devices, such as analog multiplexers; however, return sdi to high before the minimum conversion time elapses and then hold it high for the maximum conversion time to avoid the generation of the busy signal indicator. when the conversion completes, the adaq7980 / adaq7988 enter the acquisition phase and power down. bringing the sdi input low reads each adc result, which consequently outputs the msb onto sdo. then, the remaining data bits clock out by subsequent sck falling edges. the data is valid on both sck edges. although the rising edge can capture the data, a digital host using the sck falling edge allows a faster reading rate if it has an acceptable hold time. after the 16th sck falling edge, or when sdi goes high, whichever is earlier, sdo returns to high impedance and another adaq7980 / adaq7988 can be read. digital host convert cs2 cs1 clk data in adaq7980/ adaq7988 sdo sdi cnv sck adaq7980/ adaq7988 sdo sdi cnv sck 15060-080 figure 75. 4-wire cs mode without the busy indicator connection diagram t conv t cyc a cquisition acquisition t acq t sck t sckh t sckl conversion sck cnv t ssdicnv t hsdicnv sdo d15 d13 d14 d1 d0 d15 d14 d1 d0 t hsdo t en 1 2 3 14 15 16 17 18 30 31 32 t dsdo t dis sdi(cs1) sdi(cs2) 15060-081 figure 76. 4-wire cs mode without the busy indicator serial interface timing
data sheet adaq7980/adaq7988 rev. 0 | page 41 of 49 4-wire cs mode with the busy indicator to c on ne c t a s i ng l e adaq7980 / adaq7988 to an spi-compatible digital host that has an interrupt input, and when keeping cnv, which samples the analog input, independent of the signal used to select the data reading, use 4-wire cs mode with the busy indicator. this requirement is particularly important in applications where low jitter on cnv is a requirement. figure 77 shows the connection diagram, and figure 78 shows the corresponding timing. with sdi high, a rising edge on cnv initiates a conversion, selects cs mode, and forces sdo to high impedance. in this mode, hold cnv high during the conversion phase and the subsequent data readback (if sdi and cnv are low, sdo is driven low). prior to the minimum conversion time, use sdi to select other spi devices, such as analog multiplexers; however, return sdi low before the minimum conversion time elapses and then hold it low for the maximum conversion time to guarantee the generation of the busy signal indicator. when the conversion completes, sdo goes from high impedance to low impedance. with a pull-up resistor on the sdo line, use this transition as an interrupt signal to initiate the data readback controlled by the digital host. the adaq7980 / adaq7988 then enter the acquisition phase and power down. the data bits clock out, msb first, by subsequent sck falling edges. the data is valid on both sck edges. although the rising edge can capture the data, a digital host using the sck falling edge allows a faster reading rate if it has an acceptable hold time. after the optional 17th sck falling edge, or sdi going high, whichever is earlier, the sdo returns to high impedance. adaq7980/ adaq7988 sdo sdi data in irq digital host convert cs1 clk vio 47k? cnv sck 15060-082 figure 77. 4-wire cs mode with the busy indicator connection diagram t conv t cyc acquisition t ssdicnv acquisition t acq t sck t sckh t sckl conversion sdi t hsdicnv sck cnv s do t en d15 d14 d1 d0 t hsdo 1 2 3 15 16 17 t dsdo t dis 15060-083 figure 78. 4-wire cs mode with the busy indica tor serial interface timing
adaq7980/adaq7988 data sheet rev. 0 | page 42 of 49 chain mode without the busy indicator to daisy-chain multiple adaq7980 / adaq7988 devices on a 3-wire serial interface, use chain mode without the busy indicator. this feature is useful for reducing component count and wiring connections, for example, in isolated multiconverter applications or for systems with a limited interfacing capacity. data readback is analogous to clocking a shift register. figure 79 shows a connection diagram example using two adaq7980 / adaq7988 devices, and figure 80 shows the corresponding timing. when sdi and cnv are low, drive sdo low. with sck low, a rising edge on cnv initiates a conversion, selects chain mode, and disables the busy indicator. in this mode, hold cnv high during the conversion phase and the subsequent data readback. when the conversion completes, the msb is output onto sdo, and the adaq7980 / adaq7988 enter the acquisition phase and power down. the remaining data bits stored in the internal shift register clock out by subsequent sck falling edges. for each adc, sdi feeds the input of the internal shift register and it clocks out by the sck falling edge. each adc in the chain outputs its data msb first, and 16 n clocks are required to read back the n adcs. the data is valid on both sck edges. although the rising edge can capture the data, a digital host using the sck falling edge allows a faster reading rate and, consequently, more adaq7980 / adaq7988 devices in the chain, if the digital host has an acceptable hold time. the total readback time can reduce the maximum conversion rate. digital host convert clk data in adaq7980/ adaq7988 sdo sdi cnv a sck adaq7980/ adaq7988 sdo sdi cnv b sck 15060-084 figure 79. chain mode without the bu sy indicator connection diagram t conv t cyc t ssdisck t sckl t sck t hsdisck t acq acquisition t ssckcnv acquisition t sckh conversion sdo a = sdi b t hsckcnv sck cnv sdi a = 0 sdo b t en d a 15 d a 14 d a 13 d b 15 d b 14 d b 13 d b 1d b 0d a 15 d a 14 d a 0 d a 1 d a 1d a 0 t hsdo 123 151617 14 18 30 31 32 t dsdo 15060-085 figure 80. chain mode without the busy indicator serial interface timing
data sheet adaq7980/adaq7988 rev. 0 | page 43 of 49 chain mode with the busy indicator to daisy-chain multiple adaq7980 / adaq7988 devices on a 3-wire serial interface while providing a busy indicator, use chain mode with the busy indicator. this feature is useful for reducing component count and wiring connections, for example, in isolated multiconverter applications or for systems with a limited interfacing capacity. data readback is analogous to clocking a shift register. figure 81 shows a connection diagram example using three adaq7980 / adaq7988 devices, and figure 82 shows the corresponding timing. when sdi and cnv are low, drive sdo low. with sck high, a rising edge on cnv initiates a conversion, selects chain mode, and enables the busy indicator feature. in this mode, hold cnv high during the conversion phase and the subsequent data readback. when all adcs in the chain complete their conversions, drive the sdo pin of the adc closest to the digital host (see the adaq7980 / adaq7988 adc labeled c in figure 81) high. use this transition on sdo as a busy indicator to trigger the data readback controlled by the digital host. the adaq7980 / adaq7988 then enter the acquisition phase and power down. the data bits stored in the internal shift register clock out, msb first, by subsequent sck falling edges. for each adc, sdi feeds the input of the internal shift register and clocks out by the sck falling edge. each adc in the chain outputs its data msb first, and 16 n + 1 clocks are required to read back the n adcs. although the rising edge can capture the data, a digital host using the sck falling edge allows a faster reading rate and, consequently, more adaq7980 / adaq7988 devices in the chain, if the digital host has an acceptable hold time. adaq7980/ adaq7988 c sdo sdi data in irq digital host convert clk cnv sck adaq7980/ adaq7988 b sdo sdi cnv sck adaq7980/ adaq7988 a sdo sdi cnv sck 15060-086 figure 81. chain mode with the bu sy indicator connection diagram t conv t cyc t ssdisck t sckh t sck t hsdisck t acq t dsdosdi t dsdosdi t dsdodsi acquisition t ssckcnv acquisition t sckl conversion t hsckcnv sck cnv = sdi a sdo a = sdi b sdo b = sdi c sdo c t en d a 15 d a 14 d a 13 d b 15 d b 14 d b 13 d c 15 d c 14 d c 13 d b 1d b 0d a 15 d a 14 d a 1d a 0 d c 1d c 0d b 15 d b 14 d a 0 d a 1 d b 0 d b 1d a 14 d a 15 d a 1d a 0 t hsdo 123 151617 4 1819 3132333435 474849 t dsdo t dsdosdi t dsdosdi 15060-087 figure 82. chain mode with busy indicator serial interface timing
adaq7980/adaq7988 data sheet rev. 0 | page 44 of 49 application circuits table 14 provides recommended component values at various gains and the corresponding slew rate, bandwidth, and noise of a given configuration. as shown in figure 83, the noise gain, g n , of an op amp gain block is equal to its noninverting voltage gain, regardless of whether it is actually used for inverting or noninverting gain. thus, noninverting g n = r f / r g + 1 inverting g n = r f / r g + 1 + ? ? + noninverting 1 r s g = g n = +5 r f 1k? r g 249? r f 1k ? r g 249? g = ?4 g n = +5 inverting 15060-088 figure 83. noise gain of both equals 5 with the adc driver, a variety of trade-offs can be made to fine tune its dynamic performance. as with all high speed amplifiers, parasitic capacitance and inductance around the amplifier can affect its dynamic response. often, the input capacitance (due to the op amp itself, as well as the pcb) has a significant effect. the feedback resistance, together with the input capacitance, can contribute to a loss of phase margin, thereby affecting the high frequency response. a capacitor (c f ) in parallel with the feedback resistor can compensate for this phase loss. additionally, any resistance in series with the source creates a pole with the input capacitance (as well as dampen high frequency resonance due to package and board inductance and capacitance). it must also be noted that increasing resistor values increases the overall noise of the amplifier and that reducing the feedback resistor value increases the load on the output stage, thus increasing distortion. the adc driver, which has no crossover region, has a wide linear input range from 100 mv below ground to 1.3 v below positive rail. the amplifier, when configured as a follower, has a linear signal range from 150 mv above the negative supply voltage (limited by the output stage of the amplifier) to 1.3 v below the positive supply (limited by the amplifier input stage). if the supply differential between v+ and v? is less than 5 v, the linear range of the adc driver is reduced from 150 mv above the negative supply voltage to 200 mv above the minus supply voltage. a 0 v to +4.096 v signal range can be accommodated with a positive supply as low as +5.4 v and a negative power supply of ?0.2 v. if ground is used as the amplifier negative supply, at the low end of the input range close to ground, the adc driver exhibits substantial nonlinearity, as with any rail- to-rail output amplifier. the amplifier drives a one-pole, low-pass filter. this filter limits the already very low noise contribution from the amplifier to the sar adc. table 14. recommended component values noise gain, noninverting gain r s () r f () r g () c f (pf) 1 49.9 49.9 not applicable not applicable 1.25 49.9 249 1 k 8 2 49.9 499 499 8 5 49.9 1 k 249 8 table 15. system performance at selected input frequency with 5 v reference value results input frequency (khz) adc driver gain snr (db) thd (db) sinad (db) enob 1 1 91.9 ?106.1 91.5 14.9 10 1 91.5 ?105.0 91.0 14.8 20 1 90.7 ?103.6 90.1 14.7 50 1 88.3 ?99.7 87.6 14.2 100 1 84.5 ?93.3 83.3 13.5
data sheet adaq7980/adaq7988 rev. 0 | page 45 of 49 nonunity gain configurations figure 84 shows a typical connection diagram and the major dc error sources. the ideal transfer function (all error sources set to 0 and infinite dc gain) can be written as in g f ip g f out v r r v r r v ? ? ? ? ? ? ? ? ? ?? ? ? ? ? ? ? ? ? ?? 1 (1) r g ? v in + r s ? v ip + i b + i b ? + v out ? r f + v os ? 15060-089 figure 84. typical adc driver connection diagram and dc error sources this function reduces to the following familiar forms for noninverting and inverting op amp gain expressions. ip g f out v r r v ? ? ? ? ? ? ? ? ? ?? 1 (2) (noninverting gain, v in = 0 v) in g f out v r r v ? ? ? ? ? ? ? ? ? ? ? (3) (inverting gain, v ip = 0 v) the total output voltage error is the sum of errors due to the amplifier offset voltage and input currents. estimate the output error due to the offset voltage by the following: ? ? ? ? ? ? ? ? ?? ? ? ? ? ? ? ? ? ?? ? g f out pnom p offset out r r a v psrr vv cmrr vcm v v nom error 1 (4) where: nom offset v is the offset voltage at the specified supply voltage, which is measured with the input and output at midsupply. vcm is the common-mode voltage. v p is the power supply voltage. no m p v is the specified power supply voltage. cmrr is the common-mode rejection ratio. psrr is the power supply rejection ratio. a is the dc open-loop gain. estimate the output error due to the input currents by the following: ? ? ? ? ? ? ? ? ? ? ? ??? ? ? ? ? ? ? ? ? ?? ? b g f s b g f g f out i r r ri r r rr v error 1 1 )||( (5) note that setting r s equal to r f ||r g compensates for the voltage error due to the input bias current. figure 85 shows the adc driver noninverting gain connection. the circuit was tested with multiple gain settings and an output voltage of approximately 5 v p-p for optimum resolution and noise performance. ref gnd vdd vio 1.8v to 5v 100nf sdi sck sdo cnv 20? v+ v? 1.8nf 10f ldo 2.2f ref_out ldo_out pd_ref amp_out pd_amp pd_ldo adc adcn in+ in? adaq7980/ adaq7988 r g 499 ? 2.2f 100nf ref 1 positive supply 49.9 ? 49.9 ? 50 ? r f 499 ? optional c f negative supply 100nf 15060-090 figure 85. noninverting adc driver, gain = 2
adaq7980/adaq7988 data sheet rev. 0 | page 46 of 49 table 16. typical ambient temperature performance for the adaq7980 / adaq7988 for various gain configurations (f in = 10 khz) gain (v/v) snr (db) thd (db) sinad (db) sfdr (db) enob (bits) ?1 88.3 ?103.4 88.0 104.5 14.3 ?0.25 90.6 ?96.9 90.2 102.0 14.7 1 91.5 ?105 91.0 106.0 14.8 2 89.7 ?103.9 89.3 102.9 14.5 the typical ambient temperature results are listed table 16. inverting configuration with level shift configuration of the adaq7980 / adaq7988 to acquire bipolar inputs is possible. for example, the device configuration can be made such that 10 v signals can fit the 0 v to v ref volt input range. in this example, because a 20 v p-p signal is fit to a smaller peak-to-peak input range, an inverting configuration must be selected. attenuation of the input signal requires an inverting configuration. this configuration results in an 180 o phase shift due to the inversion. with the sar adc input range being unipolar, a level shift must be performed to fit a bipolar signal into the unipolar input of the adc. this level shift is performed using a difference amplifier configuration. the resistor ratios selected for the difference amplifier depend upon the peak-to-peak voltage of the bipolar input signal and the reference voltage being used for the subsystem that sets the full scale of the adc conversion range. ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?? ? ?? ? ? ? ? ? ? ? ? ? ? ? g f t s t g f in adcp r r rr r ref r r vbipolar v 1 adc 10f r t r g 20? r f r s ref 1.8nf amp_out in? in+ bipolar source 15060-091 figure 86. difference amplifier configuration used to fit bipolar signals to the adaq7980/ adaq7988 for both noninverting and inverting gain configurations, it is often useful to increase the r f value to decrease the load on the output. increasing the r f value improves harmonic distortion at the expense of reducing the bandwidth of the amplifier. note that as the gain increases, the small signal bandwidth decreases, as is expected from the gain bandwidth product relationship. in addition, the phase margin improves with higher gains, and the amplifier becomes more stable. as a result, the peaking in the frequency response is reduced. the pcb layout configuration and bond pads of the chip often contribute to stray capacitance. the stray capacitance at the inverting input forms a pole with the feedback and gain resistors. this additional pole adds phase shift and reduces phase margin in the closed-loop phase response, causing instability in the amplifier and peaking in the frequency response. to obtain the desired bandwidth, adjust the feedback resistor, r f . if r f cannot be adjusted, a small capacitor can be placed in parallel with r f to reduce peaking. the feedback capacitor, c f , forms a zero with the feedback resistor, which cancels out the pole formed by the input stray capacitance and the gain and feed back resistors. for the first pass in determining the c f value, use the following equation: r g c s = r f c f where: r g is the gain resistor. c s is the input stray capacitance. r f is the feedback resistor. c f is the feedback capacitor. using this equation, the original closed-loop frequency response of the amplifier is restored, as if there is no stray input capacitance. most often, however, the value of c f is determined empirically. see table 14 for recommended values.
data sheet adaq7980/adaq7988 rev. 0 | page 47 of 49 using the adaq7980/ adaq7988 with active filters the low noise and high gain bandwidth of the adc driver make it an excellent choice in active filter circuits. most active filter literature provides resistor and capacitor values for various filters but neglects the effect of the finite bandwidth of the op amp on filter performance; ideal filter response with infinite loop gain is implied. unfortunately, real filters do not behave in this manner. instead, they exhibit finite limits of attenuation, depending on the gain bandwidth of the active device. optimal low-pass filter performance requires an op amp with high gain bandwidth for attenuation at high frequencies, and low noise and high dc gain for low frequency, pass-band performance. figure 87 shows the schematic of a second-order, low-pass active filter and lists typical component values for filters having a bessel type response with a gain of 2 and a gain of 5. in+ in? r f r2 r1 c1 r g c2 v in amp_out 15060-092 figure 87. schematic of a second-order, low-pass active filter table 17. typical component values for second-order, low-pass active filter of figure 87 gain r1 () r2 () r f () r g () c1 (nf) c2 (nf) 2 71.5 215 499 499 10 10 5 44.2 365 365 90.9 10 10 figure 88 is a network analyzer plot of the performance of this filter. 1k 10k 100k 1m 10m frequency (hz) 50 40 30 20 10 0 ?10 ?20 ?30 ?40 ?50 gain (db) g = 2 g = 5 15060-093 figure 88. frequency response of the filter circuit of figure 87 for two different gains
adaq7980/adaq7988 data sheet rev. 0 | page 48 of 49 applications information layout heat dissipation from the package can be improved by increasing the amount of copper attached to the pins of the adaq7980 / adaq7988 . however, a point of diminishing returns is eventually reached, beyond which an increase in the copper size does not yield significant heat dissipation benefits. when designing the pcb, separate and confine the analog and digital sections to certain areas of the pcb that houses the adaq7980 / adaq7988 . the adaq7980 / adaq7988 pinouts with all their analog signals on the left side and all their digital signals on the right side eases this task. avoid running digital lines under the devices because these couple noise onto the die, unless using a ground plane under the adaq7980 / adaq7988 as a shield. never run fast switching signals, such as cnv or clocks, near the analog signal paths. avoid crossover of digital and analog signals. use at least one ground plane, and it can be common or split between the digital and analog section. in the latter case, join the planes underneath the adaq7980 / adaq7988 devices. finally, decouple the power supplies (v+, v?, vdd, and vio) of the adaq7980 / adaq7988 with low esr ceramic capacitors that are placed close to the adaq7980 / adaq7988 and that are connected using short and wide traces to provide low impedance paths and reduce the effect of glitches on the power supply lines. place the smallest value capacitor on the same side of the board as the adaq7980 / adaq7988 and as close as possible to the amplifier power supply pins. connect the ground end of the capacitor directly to the ground plane. see figure 89 for an example layout of the adaq7980 / adaq7988 that can save 50% pcb area compared to similar designs using individual components for each section of the subsystem. evaluating the performance of the adaq7980 / adaq7988 the evaluation board ( eval-adaq7980sdz ) user guide for the adaq7980 / adaq7988 outlines the other recommended layouts for the adaq7980 / adaq7988 . the evaluation board package includes a fully assembled and tested evaluation board, documentation, and software for controlling the board from a pc via the separately purchased eval-sdp-cb1z . 15060-290 adaq7980/ adaq7988 pin 1 figure 89. example layout for the adaq7980 / adaq7988
data sheet adaq7980/adaq7988 rev. 0 | page 49 of 49 outline dimensions 0 9-11-2015-a p kg-004990 bottom view top view side view 2.08 1.98 1.88 1.65 ref 0.362 0.332 0.302 0.45 0.40 0.35 0.30 0.25 0.20 5.10 5.00 4.90 4.10 4.00 3.90 pin 1 corner area 1 5 6 12 13 17 18 24 0.50 bsc 0.10 ref 2.00 ref 3.00 ref p i n 1 i n d i c a t o r figure 90. 24-lead land grid array [lga] 5 mm 4 mm body and 1.98 mm package height (cc-24-2) dimensions shown in millimeters ordering guide model 1 temperature range package description package option adaq7980bccz ?55c to +125c 24-lead land grid array [lga] cc-24-2 ADAQ7980BCCZ-RL7 ?55c to +125c 24-lead land grid array [lga] cc-24-2 adaq7988bccz ?55c to +125c 24-lead land grid array [lga] cc-24-2 adaq7988bccz-rl7 ?55c to +125c 24-lead land grid array [lga] cc-24-2 eval-adaq7980sdz evaluation board eval-sdp-cb1z evaluation controller board 1 z = rohs compliant part. ?2017 analog devices, inc. all rights reserved. trademarks and registered trademarks are the prop erty of their respective owners. d15060-0-3/17(0)

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