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micropower, zero drift, true rail - to - rail instrumentation amplifier data sheet ad8237 rev. 0 informati on 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 prope rty of their respective owners. one technology way, p.o. box 9106, norwood, ma 02062 - 9106, u.s.a. tel: 781.329.4700 www.analog.com fax: 781.461.3113 ? 2012 analog dev ices, inc. all rights reserved. features gain set with 2 external resistors can achieve low gain drift at all gains ideal for battery powered instruments supply current: 1 15 a rail - to - rail input and output zero input crossover distortion designed for excellent dc performance minimum cm rr: 106 db m aximum offset voltage drift: 0.3 v/ c maximum gain error: 0.005% (all gains) maximum gain drift: 0.5 ppm/c (all gains) i nput bias current: 1 n a guaranteed to 125c bandwidth mode pin (bw) to adjust compensation 8 kv hbm esd rating rfi fil ter on - chip s ingle -s upply operation : 1.8 v to 5.5 v 8- lead msop package applications bridge a mplifi cation pressure m easurement medical i nstrumentation thermocouple interface portable s ystems current m easurement pin configuration bw 1 +in 2 ?in 3 ?v s 4 v out 8 fb 7 ref 6 +v s 5 ad8237 top view (not to scale) + ? ? + 10289-001 figure 1. table 1 . instrumentation amplifiers by category 1 general purpose zero drift military grade microp ower digital gain ad8421 ad8237 ad620 ad8237 ad8250 ad8221 / ad8222 ad8231 ad621 ad8420 ad8251 ad8220 / ad8224 ad8293 ad524 ad8235 / ad8236 ad8253 ad8228 ad8553 ad526 ad627 ad8231 ad8295 ad8556 ad624 ad8226 ad8557 1 see www.analog.com for the latest instrumentation amplifiers. general description the ad8237 is a micropower, zero drift, rail - to - rail input and output instrumentation amplifier. the relative match of two resistors sets any gain from 1 to 1000. the ad8237 has excellent gain accuracy performance that can be preserved at any gain with two ratio - matched resistors. the ad8237 employs the indirect current feedback architecture to achieve a true rail - to - rail capability. unlike conventional in - amps, the ad8237 can fully amplify signals with common - mode voltage at or even slightly beyond its supplies. this enables applications with high common - mode voltages to use smaller supplies and save power. the ad8237 is an excellent choice for portable systems. with a minimum supply voltage of 1.8 v, a 115 a typical supply current , and wide input range, the ad8237 makes full use of a li mited power budget, yet offer s bandwidth and drift performance suitable for bench - top systems. the ad8237 is available in an 8 - lead msop package. performance is specified over the full temperature range of ?40 c to +125c. 6 5 4 3 2 1 ?1 0 0 1 2 3 4 5 input common-mode voltage (v) output voltage (v) traditional in-amp (rail-to-rail out) ad8237 10289-002 g = 100 v s = 5v v ref = 2.5v figure 2. input common - mode voltage vs. output voltage, +v s = 5 v, g = 100
ad8237 data sheet rev. 0 | page 2 of 28 table of contents features .............................................................................................. 1 applications ....................................................................................... 1 pin configuration ............................................................................. 1 general description ......................................................................... 1 revision history ............................................................................... 2 specifications ..................................................................................... 3 absolute maximum ratings ............................................................ 7 th ermal resistance ...................................................................... 7 esd caution .................................................................................. 7 pin configuration and function descriptions ............................. 8 typical performance characteristics ............................................. 9 theory of operation ...................................................................... 20 architecture ................................................................................. 20 setting the gain .......................................................................... 20 gain accuracy ............................................................................. 21 clock feedthrough ..................................................................... 21 input voltage range ................................................................... 21 input protection ......................................................................... 22 filtering radio frequency interference .................................. 22 using the reference pin ............................................................ 22 layout .......................................................................................... 23 input bias current return path ............................................... 23 applications information .............................................................. 25 battery current monitor ........................................................... 25 programmable gain in - amp .................................................... 25 ad8237 in an electrocardiogram (ecg) front end ............. 26 outline dimensions ....................................................................... 27 orderin g guide .......................................................................... 27 revision history 8 /12 revision 0: initial version
data sheet ad8237 rev. 0 | page 3 of 28 specifications +v s = +5 v, ? v s = 0 v, v ref = 2.5 v, v cm = 2.5 v, t a = 25c, g = 1 to 1000, r l = 1 0 k to ground , specifications referred to input, unless otherwise noted . table 2 . parameter test conditions /comments min typ max unit common - mode rejection ratio (cmrr) v cm = 0 .1 v to 4.9 v cmrr at dc g = 1, g = 10 1 06 120 db g = 100, g = 1000 1 14 140 db over t emperature (g = 1) t a = ?40c to +125c 10 4 db cmrr at 1 khz 80 db noise voltage noise spectral density f = 1 kh z 68 nv/hz peak to peak f = 0.1 hz to 10 hz 1.5 v p -p current noise spectral density f = 1 khz 70 fa/hz peak to peak f = 0.1 hz to 10 hz 3 pa p -p voltage offset offset 30 75 v average temperature coefficient t a = ?40c to + 125c 0. 3 v/c offset rti vs. supply (psr) 10 0 db inputs 1 valid for ref and fb pair, as well as +in and ?in input bias current t a = +25c 250 650 p a over temperature t a = ? 40c to +12 5c 1 na average temperature coefficient 0. 5 pa/c input offset current t a = +25c 250 6 50 pa over temperature t a = ?40c to +12 5c 1 na average temperature coefficient 0.5 pa /c input impedance differential 1 00||5 m||pf common mode 8 00||10 m||pf differential input operating volta ge t a = ? 40c to +125c 3.85 v input operating voltage (+in, ?in, or ref) t a = +25c ?v s ? 0.3 +v s + 0.3 v t a = ? 40c to +125c ?v s ? 0.2 +v s + 0.2 v dynamic response small signal bandwidth ? 3 db low bandwidth mode pin 1 connected to ? v s g = 1 200 khz g = 10 20 khz g = 100 2 khz g = 1000 0.2 khz high bandwidth mode pin 1 connected to +v s g = 10 100 khz g = 100 10 khz g = 1000 1 khz
ad8237 data sheet rev. 0 | page 4 of 28 parameter test conditions /comments min typ max unit settling time 0.01% 4 v output step low bandwidth mode pin 1 co nnected to ?v s g = 1 80 s g = 10 100 s g = 100 440 s g = 1000 4 ms high bandwidth mode pin 1 connected to +v s g = 10 80 s g = 100 100 s g = 1000 820 s slew rate low bandwidth mode 0.05 v/s high bandwi dth mode 0.15 v/s emi filter frequency 6 mhz gain 2 g = 1 + (r2/r1) gain range 3 1 1000 v/v gain error v out = 0. 1 v to 4. 9 v, g = 1 to g = 1000 0.005 % gain error vs. v cm 15 ppm/v gain vs. temperature t a = ? 40c to +125c 0.5 ppm/ c gain nonlinearity v out = 0.2 v to 4.8 v, r l = 10 k t o ground g = 1, g = 10 3 ppm g = 100 6 ppm g = 1000 10 ppm output output swing r l = 10 k to midsupply t a = +25c ?v s + 0.05 +v s ? 0.05 v t a = ?40c to 125c ?v s + 0 .07 +v s ? 0.07 v r l = 100 k to midsupply t a = +25c ?v s + 0.02 +v s ? 0.02 v t a = ?40c to 125c ?v s + 0.03 +v s ? 0.03 v short - circuit current 4 ma power supply operating range 1.8 5.5 v quiescent current t a = +25c 115 130 a t a = ? 40c to +125c 150 a temperature range specified ? 40 +125 c 1 specifications apply to input voltages between 0 v and 5 v. when measuring voltages beyond the supplies, there is additional offset error, bias currents increase , and input impedance decrease s , especially at higher temperatures. 2 for g > 1, errors from the external resistors, r1 and r2, must be added to these specifications, including error from the fb pin bias current. 3 the ad8237 has only been characterized for gains of 1 to 1000; however, higher gains are possible.
data sheet ad8237 rev. 0 | page 5 of 28 +v s = 1.8 v, ?v s = 0 v, v ref = 0 .9 v, v cm = 0.9 v, t a = 25c, g = 1 to 1000 , r l = 1 0 k to ground , specifications referred to input, unless otherwise noted . table 3 . parameter test conditions /comments min typ max unit common - mode rejection ratio (cmrr) v cm = 0.2 v to 1.6 v cmrr at dc g = 1, g = 10 10 0 120 db g = 100, g = 1000 1 14 140 db over t emperature (g = 1) t a = ?40c to +125 c 94 db cmrr at 1 khz 80 db noise voltage noise spectral density f = 1 khz, v diff 100 mv 68 nv/hz peak to peak f = 0.1 hz to 10 hz, v diff 100 mv 1 .5 v p -p current noise spectral density f = 1 khz 70 fa/hz peak to peak f = 0.1 hz to 10 hz 3 pa p -p voltage offset offset 25 75 v average temperature coefficient t a = ? 40c to +125c 0. 3 v/c offset rti vs. supply (psr) 10 0 db inputs 1 valid for ref and fb pair, as well as +in and ? in input bia s current t a = +25c 250 650 pa over temperature t a = ?40c to +125c 1 na average temperature coefficient 0.5 pa/c input offset current t a = +25c 250 6 50 pa over temperature t a = ?40c to +125c 1 na average temperature coefficient 0. 5 pa/c input impedance differential 1 00||5 m||pf common mode 8 00||10 m||pf differential input operating voltage t a = ? 40c to + 125c 0.75 v input operating voltage (+in, ? in, ref , or fb ) t a = +25c ?v s ? 0.3 +v s + 0.3 v t a = ? 40c to +125c ?v s ? 0.2 +v s + 0.2 v dynamic response small signal bandwidth ? 3 db low bandwidth mode pin 1 connected to ?v s g = 1 200 khz g = 10 20 khz g = 100 2 khz g = 1000 0.2 khz high bandwidth mode pin 1 connected to +v s g = 10 100 khz g = 100 10 khz g = 1000 1 khz slew rate low bandwidth mode 0.05 v/s high bandwidth mode 0.15 v/s emi filter frequency 6 mhz
ad8237 data sheet rev. 0 | page 6 of 28 parameter test conditions /comments min typ max unit gain 2 g = 1 + (r2/r1) gain range 3 1 1 000 v/v gain error v out = 0.2 v to 1.6 v, g = 1 to g = 1000 0.005 % gain error vs. v cm 15 ppm/v gain vs. temperature t a = ? 40c to +125c 0.5 ppm/c gain nonlinearity v out = 0.2 v to 1.6 v g = 1, g = 10 3 ppm g = 100 6 ppm g = 1000 10 ppm output output swing r l = 10 k to midsupply t a = +25c ?v s + 0.05 +v s ? 0.05 v t a = ?40c to 125c ?v s + 0.07 +v s ? 0.07 v r l = 100 k to midsupply t a = +25c ? v s + 0.02 +v s ? 0.02 v t a = ?40c to 125c ?v s + 0.03 +v s ? 0.03 v short - circuit current 4 ma power supply operating range 1.8 5.5 v quiescent current t a = +25c 115 130 a t a = ?40c to +125c 150 a temperature range specified ? 40 +125 c 1 specifications apply to input voltages betw een 0 v and 1.8 v. when measuring voltages beyond the supplies, there is additional offset error, bias currents increase, and input impedance decrease s , especially at higher temperatures. 2 for g > 1, errors from the external resistors, r1 and r2, must be a dded to these specifications, including error from the fb pin bias current. 3 the ad8237 has only been characterized for gains of 1 to 1000; however, higher gains are possible.
data sheet ad8237 rev. 0 | page 7 of 28 absolute maximum rat ings table 4. parameter rating supply voltage 6 v output short - circuit current duration indefinite maximum voltage at ? in, +in , f b, or ref 1 +v s + 0. 5 v minimum voltage at ? in, +in, fb, or ref 1 ?v s ? 0. 5 v storage temperature range ? 65c to +150c junction temperature range ? 65c to +150c esd human body model 8 kv charge device model 1.25 kv machine model 0.2 kv 1 if input voltages beyond the specified minimum or maximum voltages are expected, place resistors in series with the inputs to limit the current to 5 ma. stresses above those listed under absolute maximum ratings may cause permanent damage to the device. this is a s tress rating only; functional operation of the device at these or any other conditions above those indicated in the operational section of this specification is not implied. exposure to absolute maximum rating conditions for extended periods may affect dev ice reliability. thermal resistance ja is specified for a device in free air. table 5 . package ja unit 8- lead msop, 4 - layer jedec board 1 45.7 c/w esd caution
ad8237 data sheet rev. 0 | page 8 of 28 pin configuration and function descripti ons bw 1 +in 2 ?in 3 ?v s 4 v out 8 fb 7 ref 6 +v s 5 ad8237 top view (not to scale) + ? ? + 10289-003 figure 3 . pin configuration table 6 . pin function descriptions pin no. mnemonic description 1 bw for high bandwidth mode, c onnect this pin to +v s , or for low bandwidth mode, connect this pin to ?v s . do not leave t his pin floating. 2 +in positive input. 3 ? in negative input . 4 ?v s negative supply. 5 +v s positive supply. 6 ref reference input . 7 fb feedback input . 8 v out output.
data sheet ad8237 rev. 0 | page 9 of 28 typical performance characteristics +v s = +5 v, ?v s = 0 v, v ref = 2.5 v, t a = 25c, r l = 10 k to ground, unless otherwise noted. 16 14 12 8 10 6 4 2 0 ?60 ?20 ?40 0 20 40 60 units offset voltage (v) 10289-004 figure 4 . typical distribution of offset voltage 18 21 15 12 9 6 3 0 ?0.6 ?0.4 ?0.2 0 0.2 0.4 0.6 units positive input bias current (na) 10289-005 figure 5 . typical distribution of input bias current 35 30 20 25 15 10 5 0 ?60 ?20 ?40 0 20 40 60 units gain error (v/v) 10289-006 figure 6. typical distribution of gain error (g = 1) 30 35 40 25 20 15 10 5 0 ?6 ?4 ?2 0 2 4 6 units cmrr (v/v) 10289-007 figure 7 . typical distribution of cmrr 18 15 12 9 6 3 0 ?0.6 ?0.4 ?0.2 0 0.2 0.4 0.6 units input offset current (na) 10289-008 figure 8. typical distribution of input offset current 24 18 21 12 15 9 6 3 0 100 110 105 115 120 125 130 units supply current (a) 10289-009 figure 9. typical distribution of supply current
ad8237 data sheet rev. 0 | page 10 of 28 6 5 ?1 0 1 2 3 4 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 common-mode voltage (v) output voltage (v) v s = 1.8v v s = 5v g = 1 v ref = 0v r l = 10k? 10289-010 figure 10 . input common - mode voltage vs. output voltage, g = 1, v ref = 0 v, v s = 5 v and v s = 1.8 v, r l = 10 k? to g round 6 5 ?1 0 1 2 3 4 0 1 2 3 4 5 6 common-mode voltage (v) output voltage (v) v s = 1.8v v s = 5v g = 100 v ref = 0v r l = 10k? 10289-0 11 figure 11 . input common - mode voltage vs. output voltage, g = 100, v ref = 0 v, v s = 5 v and v s = 1.8 v, r l = 10 k? to g round 4 3 2 1 ?4 ?3 ?2 ?1 0 ?3 3 2 1 0 ?1 ?2 common-mode voltage (v) voltage output (v) g = 1 v ref = 0v r l = 5k? v s = 2.5v v s = 0.9v 10289-012 figure 12 . input common - mode voltage vs. output voltage, g = 1 , v ref = 0 v, v s = 2.5 v and v s = 0.9 v, r l = 5 k? to g round 4 3 2 1 ?4 ?3 ?2 ?1 0 ?3 3 2 1 0 ?1 ?2 common-mode voltage (v) output voltage (v) v s = 2.5v v s = 0.9v g = 100 v ref = 0v r l = 5k? 10289-013 figure 13 . input common - mode voltage vs. output voltage, g = 100 , v ref = 0 v, v s = 2.5 v and v s = 0.9 v, r l = 5 k? to g round 5.0 4.5 4.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 0 1.8 2.3 2.8 3.3 3.8 4.3 4.8 v in (v) supply voltage (v) ?40c +25c +85c +105c +125c 10289-014 figure 14 . maximum differential input vs. supply voltage 5 ?5 ?3.0 3.0 input bias current (na) common-mode voltage (v) ?4 ?3 ?2 ?1 0 1 2 3 4 ?2.5 ?2.0 ?1.5 ?1.0 ?0.5 0 0.5 1.0 1.5 2.0 2.5 +v s ?v s i b + i b ? 10289-015 representative sample figure 15 . input bias c urrent vs. common - mode voltage
data sheet ad8237 rev. 0 | page 11 of 28 15 ?15 ?2.5 2.5 input bias current (na) differential input voltage (v) ?10 ?5 0 5 10 ?2.0 ?1.5 ?1.0 ?0.5 0 0.5 1.0 1.5 2.0 i b + i b ? v s = 2.5v v cm = 0v representative sample 10289-016 figure 16 . input bias current vs. differential input voltage 140 120 100 80 60 40 20 0 0.1 10k 1k 100 10 1 psrr (db) frequency (hz) gain = 1 gain = 10 gain = 100 gain = 1000 bw limit 10289-017 low bandwidth mode v s = 5v figure 17 . positive psrr vs . frequency, rti, low bandwidth mode, v s = 5 v 140 120 100 80 60 40 20 0 ?20 0.1 10k 1k 100 10 1 negative psrr (db) frequency (hz) gain = 1 gain = 10 gain = 100 gain = 1000 10289-018 bw limit low bandwidth mode v s = 5v figure 18 . negative psrr vs. frequency, rti, low bandwidth mode, v s = 5 v 140 120 100 80 60 40 20 0 0.1 10k 1k 100 10 1 positive psrr (db) frequency (hz) gain = 10 gain = 100 gain = 1000 bw limit 10289-019 high bandwidth mode figure 19 . positive psrr vs. frequency, rti, high bandwidth mode 140 120 100 80 60 40 20 0 0.1 10k 1k 100 10 1 negative psrr (db) frequency (hz) gain = 10 gain = 100 gain = 1000 10289-020 bw limit high bandwidth mode figure 20 . negative psrr vs. frequency , rti, high bandwidth mode 80 70 60 50 40 30 20 10 0 ?10 ?20 10 1m 100k 10k 1k 100 gain (db) frequency (hz) low bandwidth mode v s = 5v gain = 1 gain = 10 gain = 100 gain = 1000 10289-021 figure 21 . gain vs. frequency, low bandwidth mode, v s = 5 v
ad8237 data sheet rev. 0 | page 12 of 28 80 70 60 50 40 30 20 10 0 ?10 ?20 10 1m 100k 10k 1k 100 gain (db) frequency (hz) low bandwidth mode v s = 1.8v gain = 1 gain = 10 gain = 100 gain = 1000 10289-022 figure 22 . gain vs. frequency, low bandwidth mode, v s = 1.8 v 80 70 60 50 40 30 20 10 0 ?10 ?20 10 1m 100k 10k 1k 100 gain (db) frequency (hz) high bandwidth mode v s = 5v gain = 10 gain = 100 gain = 1000 10289-023 figure 23 . gain vs. frequency, high bandwidth mode, v s = 5 v 80 70 60 50 40 30 20 10 0 ?10 ?20 10 1m 100k 10k 1k 100 gain (db) frequency (hz) high bandwidth mode v s = 1.8v gain = 10 gain = 100 gain = 1000 10289-024 figure 24 . gain vs. frequency, high bandwidth mode, v s = 1.8 v 5.0 0 10 100 1k 100k 10k output voltage (v p-p) frequency (hz) 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 g = 1 low bandwidth mode differential input +in ?in 10289-025 figure 25 . large signal frequency response, low bandwidth mode , g = 1 5.0 0 10 100 1k 100k 10k output voltage (v p-p) frequency (hz) 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 g = 10 high bandwidth mode +in ?in differential input 10289-026 figure 26 . large signal frequency response, high bandwidth mode , g = 10 6 5 4 3 2 1 0 1 100k 10k 1k 100 10 maximum common-mode voltage (v p-p) frequency (hz) 10289-080 v s = 2.5 v v s = 0.9v figure 27 . maximum common - mode voltage vs. frequency
data sheet ad8237 rev. 0 | page 13 of 28 160 140 120 100 60 80 40 20 0 0.1 100k 10k 1k 100 10 1 cmrr (db) frequency (hz) gain = 1 gain = 10 gain = 100 gain = 1000 both bandwidth modes only bw limit changes gain = 1 low bandwidth mode only bw limit 10289-027 figure 28 . cmrr vs. frequency 160 140 120 100 60 80 40 20 0 0.1 100k 10k 1k 100 10 1 cmrr (db) frequency (hz) gain = 1 gain = 10 gain = 100 gain = 1000 both bandwidth modes only bw limit changes gain = 1 low bandwidth mode only bw limit 10289-028 figure 29 . cmrr vs. frequency, 1 k? source imbalance 10k 1k 100 10 0. 1 100k 10k 1k 100 10 1 noise (nv/hz) frequency (hz) g = 1, low bandwidth mode g = 10, low bandwidth mode g = 10, high bandwidth mode g = 100, low bandwidth mode g = 100, high bandwidth mode 10289-029 figure 30 . voltage noise spectral density vs. frequency 0.4v/div 1s/div 10289-031 figure 31 . 0.1 hz to 10 hz rti voltage noise 1k 100 10 1 100k 10k 1k 100 10 noise (nv/hz) frequency (hz) valid for both bandwidth modes 10289-032 figure 32 . current noise spectral density vs. frequency 1.5pa/div 1s/div 10289-033 figure 33 . 0.1 hz to 10 hz rti current noise
ad8237 data sheet rev. 0 | page 14 of 28 0.010 ?0.010 ?2.5 ?2.0 2.5 gain error (%) common-mode voltage (v) ?0.008 ?0.006 ?0.004 ?0.002 0 0.002 0.004 0.006 0.008 ?1.5 ?1.0 ?0.5 0 0.5 1.0 1.5 2.0 v in = 500mv 10289-034 figure 34 . gain error vs. common -m ode voltage , g = 1 10 8 ?10 ?8 ?6 6 4 2 ?4 ?2 0 0 0.5 1.0 5.04.54.03.53.02.52.01.5 nonlinearity (ppm) output voltage (v) g = 1 10289-037 figure 35 . gain nonlinearity , g = 1, v s = 5 v, r l = 10 k ? t o ground, low bandwidth mode 10 8 ?10 ?8 ?6 6 4 2 ?4 ?2 0 0 0. 5 1.0 5.04.54.03.53.02.52.01.5 nonlinearity (ppm) output voltage (v) g = 10 10289-038 figure 36 . gain nonlinearity , g = 10, v s = 5 v, r l = 10 k ? to ground 20 ?20 ?15 ?10 ?5 0 5 10 15 0 0.5 1.0 5.04.54.03.53.02.52.01.5 nonlinearity (ppm) output voltage (v) g = 100 10289-039 figure 37 . gain nonlinearity , g = 100, v s = 5 v, r l = 10 k ? to g round 50 40 ?50 ?40 ?30 30 20 10 ?20 ?10 0 0 0.5 1.0 5.04.54.03.53.02.52.01.5 nonlinearity (ppm) output voltage (v) g = 1000 10289-040 figure 38 . gain nonlinearity , g = 1000, v s = 5 v, r l = 10 k ? to g round g = 1 low bandwidth mode 1v/div 400s/div 10289-041 figure 39 . large signal pulse response, low bandwidth mode, g = 1, r l = 10 k?, c l = 10 pf
data sheet ad8237 rev. 0 | page 15 of 28 g = 10 low bandwidth mode 1v/div 400s/div 10289-042 figure 40 . large signal pulse response, low bandwidth mode, g = 10, r l = 10 k?, c l = 10 pf g = 100 low bandwidth mode 1v/div 400s/div 10289-043 figure 41 . large signal pulse response, low bandwidth mode, g = 100, r l = 10 k?, c l = 10 pf g = 1000 low bandwidth mode 1v/div 2ms/div 10289-044 figure 42 . large signal pulse response, low bandwidth mode, g = 1000, r l = 10 k?, c l = 10 pf g = 10 high bandwidth mode 1v/div 400s/div 10289-045 figure 43 . large signal pulse response, high bandwidth mode, g = 10, r l = 10 k?, c l = 10 pf g = 100 high bandwidth mode 1v/div 400s/div 10289-046 figure 44 . large signal pulse response, high bandwidth mode, g = 100, r l = 10 k?, c l = 10 pf g = 1000 high bandwidth mode 1v/div 400s/div 10289-047 figure 45 . large signal pulse response, high bandwidth mode, g = 1000, r l = 10 k?, c l = 10 pf
ad8237 data sheet rev. 0 | page 16 of 28 g = 1 low bandwidth mode 20mv/div 10s/div 10289-048 f igure 46 . small signal pulse response , g = 1, r l = 10 k ? , c l = 100 pf , low bandwidth mode g = 10 low bandwidth mode 20mv/div 50s/div f chop 10289-049 f igure 47 . small signal pulse response , g = 10, r l = 10 k ? , c l = 100 pf , low bandwidth mode g = 100 low bandwidth mode 20mv/div 200s/div 10289-050 f igure 48 . small signal pulse response , g = 100, r l = 10 k ? , c l = 100 pf , low bandwidth mode g = 1000 low bandwidth mode 20mv/div 2ms/div 10289-051 f igure 49 . small signal pulse response , g = 1000, r l = 10 k ? , c l = 100 pf , low bandwidth mode g = 1 low bandwidth mode 20mv/div 20s/div 100pf 1nf no load 560pf 10289-052 figure 50 . small signal pulse response with various capacitive loads , g = 1, r l = infinity, low bandwidth mode g = 10 high bandwidth mode 10289-053 20mv/div 10s/div f igure 51 . small signal pulse response , g = 10 , r l = 10 k ? , c l = 100 pf , high bandwidth mode
data sheet ad8237 rev. 0 | page 17 of 28 g = 100 high bandwidth mode 20mv/div 100s/div f chop 10289-054 f igure 52 . small signal pulse response , g = 100, r l = 10 k ? , c l = 100 pf , high bandwidth mode g = 1000 high bandwidth mode 20mv/div 1ms/div 10289-055 f igure 53 . small signal pulse response , g = 1000, r l = 10 k ? , c l = 100 pf , high bandwidth mode g = 10 high bandwidth mode r l = 100k? 50mv/div 40s/div 100pf 2nf no load 560pf 10289-056 figure 54 . small signal pulse response with various capacitive loads , g = 10 , r l = 100 k?, high bandwidth mode 80 60 40 20 0 ?80 ?60 ?40 ?20 ?40 ?25 ?10 5 20 1251109580655035 offset voltage (v) temperature (c) normalized to 25c v s = 2.5v 10289-057 figure 55 . offset voltage vs. temperature 50 40 20 10 30 0 ?50 ?40 ?20 ?10 ?30 ?40 ?25 ?10 5 20 1251109580655035 gain error (v/v) temperature (c) normalized to 25c gain = 1 v s = 2.5v v out = 2v 10289-058 figure 56 . gain vs. temperature 1.0 0.8 0.4 0.2 0.6 0 ?1.0 ?0.8 ?0.4 ?0.2 ?0.6 ?40 ?25 ?10 5 20 1251109580655035 cmrr (v/v) temperature (c) normalized to 25c g = 1 v s = 2.5v v cm = 2v 10289-059 figure 57 . cmrr vs. temperature
ad8237 data sheet rev. 0 | page 18 of 28 500 ?500 ?40 125110958065503520 5 ?10?25 bias current and offset current (pa) temperature (c) input offset current ?in bias current +in bias current ?400 ?300 ?200 ?100 0 100 200 300 400 10289-060 representative sample figure 58 . input bias current and input offset current vs. temperature 500 ?500 ?40 125110958065503520 5 ?10?25 bias current and offset current (pa) temperature (c) offset current ref bias current fb bias current ?400 ?300 ?200 ?100 0 100 200 300 400 10289-061 representative sample figure 59 . ref input bias cur rent, fb input bias current, and offset current vs . temperature +v s ?v s +300 +200 +100 ?300 ?200 ?100 0.9 2.5 2.3 2.1 1.9 1.7 1.5 1.3 1.1 output voltage swing (mv) referred to supply voltages supply voltage (v s ) r l = 5k ?40c +25c +85c +125c 10289-062 figure 60 . output voltage s wing vs. supply voltage +v s ?v s +0.4 +0.2 ?0.4 ?0.2 1k 10k 100k 1m output voltage swing (v) referred to supply voltages load resistance (?) ?40c +25c +85c +125c 10289-063 figure 61 . output voltage swing vs. load resistance, v s = 2.5 v +v s ?v s +0.2 +0.3 +0.4 +0.1 ?0.2 ?0.3 ?0.4 ?0.1 1k 10k 100k 1m output voltage swing (v) referred to supply voltages load resistance (?) ?40c +25c +85c +125c 10289-064 figure 62 . output voltage swing vs. load resistance, v s = 0.9 v +v s ?v s +1.2 +0.8 +0.4 ?1.2 ?0.8 ?0.4 0 3.0 2.5 1.5 2.0 1.0 0.5 output voltage swing (v) referred to supply voltages output current (ma) ?40c +25c +85c +125c 10289-065 figure 63 . o utput voltage swing vs. output current
data sheet ad8237 rev. 0 | page 19 of 28 200 180 0 20 40 60 80 100 120 140 160 ?40 ?25 ?10 5 20 35 12511095806550 supply current (a) temperature (c) v s = 5v v s = 1.8v 10289-066 figure 64 . supply current vs. temperature, v s = 5 v, v s = 1.8 v
ad8237 data sheet rev. 0 | page 20 of 28 theory of operation +in ?in g m1 i2 i1 i1 ? i2 + ? r2 r1 v out fb ref ad8237 g m2 rfi filter tia + ? + ? rfi filter als als + ? internal in-amp v cm = v s 2 v cm = v s 2 ?in fb to g m2 to g m1 +v s ?v s +v s ?v s rfi filter rfi filter + ? + ? +v s ?v s +v s ?v s 10289-067 figure 65 . simplified schematic architecture the ad8237 is based on an indirect current feedback topology consisting of three amplifiers: two matched transconductance amplifiers that convert voltage to current , and one transimpedance amplifier , tia, that converts current to voltage. to understand how the ad8237 works, first consider only the internal in - amp. a ssume a positive dif ferential voltage is applied acro ss the inputs of the transconductance amplifier , g m1 . this input voltage is converted i nto a differential current , i1, by the g m . initially, i2 is zero ; therefore, i1 is fed into the tia, causing the output to increase. if there is feedback from the output of the ti a to the negative terminal of g m2 , and the positive terminal is held constant , the increasing output of the ti a cause s i2, as shown , to increase. when it is as s ume d that the tia has infinite gain, the loop is satisfied when i2 equals i1 . because the gain of g m1 and g m2 are matched, this means that the differe ntial input voltage across g m 1 appear s across the inputs of g m2 . thi s behavioral model is all that is needed for proper operation of the ad8237 , and the rest of the circuit is for performance optimization. the ad8237 employs a novel adaptive level shift ( als ) technique. this switched capacitor method shifts the common - mode level of the input signal to the optimal level for the in - amp while preserving the differential signal. once this is accomplished, additional performance benefits c an be ac hieved by using the internal in - amp to compare +in to fb and ? in to ref. this is only p ractical because the signals emitting from the als blocks are all referred to the same common - mode potential. in traditional instrumentation amplifiers, the inpu t common - mode voltage can limit the available output swing, typically depicted in a hexagon plot of the input common - mode vs. the output voltage . because of this limit, very few instrumentation amplifiers can measure small signals near either supply rail. using the indirect current feedback topology and als, t he ad8237 achieve s a tr uly rail - to - rail characteristic. this increases power efficiency in many applications by allowing for power supply reduction. the ad8237 includes an rfi filter to remove high frequency out - of - band signals without affecting input impedance and cmrr over frequency. additionally, there is a bandwidth mode pin to adjust the compensation. for ga ins greater than or equal to 10, the bandwidth mode pin (bw) can be tied to +v s to change the com pensation and increase the gain bandwidth product of the amplifier to 1 mhz. otherwise , connect bw to ?v s for a 200 khz gain bandwidth product. setting the gain there are several ways to configure the ad8237 . the transfer function of the ad8237 in the configuration in figure 65 is v out = g ( v + in ? v ? in ) + v ref where: r1 r2 1 += g table 7. suggested resistors for various gains ( 1% resistors ) r1 (k) r2 (k) gain n one s hort 1.00 49.9 49.9 2.00 20 80.6 5.0 3 10 90.9 10.09 5 95.3 20.06 2 97.6 49. 8 1 100 101 1 200 201 1 499 50 0 1 100 0 1001 whe reas the ratio of r2 to r1 sets the gain, the designer determines the absolute value of the resistors. larger values reduce power consumption and output loading; smaller values limit the fb input bias current and input impedance error s . if the parallel combination of r1 and r2 is greater than about 30 k?, the resistors start to contribute to the noise . for best output swing and linearity , keep (r1 + r2) || r l 1 0 k?.
data sheet ad8237 rev. 0 | page 21 of 28 the bias current at the fb pin is dependent on the common - mode and differential input impedance. fb bias current errors from the commo n- mode input impedance can be reduced by placing a resistor value of r1||r2 in series with the ref terminal, as shown in figure 66 . at higher gains, this resistor can simply be the same value as r1. ad8237 +in ?in ref fb v out g = 1 + r2 r1 i b + i b ? v ref r1 r2 r1 || r2 + ? i b r i b f 10289-068 figure 66 . cancelling error from fb input bias current s ome applications may be able to take advantage of the symmetry of the input transconductance amplifiers by canceling the differential input impedance errors , as shown in figure 67 . if the source resistance is well known, setting the parallel combination of r1 and r2 equal to r s accomplishes this. if practical resistor values force the parallel combination of r1 and r2 to be less than r s , add a series resistor to th e fb input to make up for the difference. ad8237 +in ?in ref fb v out r1 r2 v in r s r in r in if r1||r2 = r s , v out = v in (1 + r2 r1 ) v +in = v in r in r s + r in 10289-069 figure 67 . canceling input impedance errors g ain accuracy unlike most instrumentation amplifiers, the relative match of the two gain setting resistors determines the gain accuracy of the ad8237 rather than a single external resistor. for example, if two resistors have exactly the same absolute error, there is no error in gain. conversely, two 1% resistors can cause approximately 2% maximum gain error at high gains. temperature coefficient mismatch of the gain setting resistors increases the gain drift of the instrumentation amplifier circuit according to the gain equation . because these external resistors do no t have to match any on - chip resistor s, resistors with good tc r tracking can achieve excellent gain drift without the need for a low absolute tcr . for the best performance, keep the two input pairs (+in and ?in, and fb and ref) at similar dc and ac common - mode potentials . this has two bene fits . f or dc common - mode, this minimizes the gain error of the ad8237 . f or ac common - mode, this yie lds improved frequency response. there is a maximum rate at which the als circuit can shift the common - mode volt age , which is shown in figure 27 . because of this limit, the best large signal frequency response is achieved when the ac common - mode voltage of the two input pairs are matched. for example, if the negative input i s at a fixed voltage and the positive input is driven with a signal, the feedback input mo ves with the positive input; therefore, the ac common - mode voltage of the two input pairs is the same. th e effect of this is shown in figure 25 and figure 26. clock feedthrough the ad8237 uses nonoverlapping clocks to perform the chopping and als functions. the input voltage - to - current ampli fiers are chopped at approximately 27 khz. although there is internal ripple - suppression circuitry, trace amounts of these clock frequencies and their harmonics can be observed at the output in some configurations. these ripples are typically 100 v rti w hen the bandwidth is greater than the clock frequency. t hey can be larger after a transient pulse but settle back to nominal , which is included in the settling time specifications . the amount of feedthrough at the output is dependent upon the gain and band width mode. the worst case is in high bandwidth mode when the gain can be almost 40 before the clock ripple is outside the bandwidth of the amplifier. for some applications, it may be necessary to use additional filtering after the ad8237 to remove this ripple. input voltage range the allow able input range of the ad8237 is much simpler than traditional architectures. for the transfer function of the ad8237 to be valid, the input voltage must follow two rules ? keep the differential input voltage within the limits shown in figure 14 ; approximately ( total supply voltage C 1.2 ) v. ? keep the voltage of the inputs (including the ref and fb pins ) and the output with in the specified voltage range, which are approximately the supply rails . because the output swing is completely independent of the input common - mode voltage, there are no he xago nal figures or complicated formulas to follow, and no limitation for the output swing the amplifier has for input signals with changing common mode.
ad8237 data sheet rev. 0 | page 22 of 28 input protection if no external protection is used, keep the inputs of the ad8237 within the voltages specified in the absolute maximum ratings. if the application requires voltages beyond these ratings, input protection resistors can be placed in series with the inputs of the ad8237 to limit the current to 5 ma. for example, if +v s is 3 v and a 10 v overload voltage can occur at the inputs, place a protection resistor of at least (10 v ? 3 v)/5 ma = 1.4 k in series with the inputs. ad8237 r protect r protect v +in + ? v ?in + ? +v s ?v s positive voltage protection: r protect > v in ? +v s 5m a negative voltage protection: r protect > ?v s ? v in 5m a 10289-070 figure 68 . protection resistors for large input voltages filtering radio frequency interferen ce the ad8237 contains an on - chip rfi filter that is sufficient for a majority of ap plications. for applications where additional radio frequency immunity is needed, an external rfi filter can also be applied as shown in figure 69. +in +v s ?v s c c 1nf 5% c d 10nf c c 1nf 5% 10f 10f 0.1f 0.1f r 10k? 1% r 10k? 1% ad8237 ?in 10289-071 differential filter cutoff = 1 2 r (2c d + c c ) common-mode filter cutoff = 1 2 r c c figure 69 . adding e xtra rfi f iltering using t he reference p in in general, in strumentation a mplifier reference pins can be useful for a few reasons. they provide a means of physically separating the input and output grounds to reject ground bounce common to the inputs. they can also be used to precisely level shift the output signal . in the configuration shown in figure 65 through figure 67 , the gain of the reference pin to the output is unity , as is common in a typical in - amp . because the reference pin is functionally no different from the positive input , it can be used with gain , as shown in figure 70 . this configuration can be very useful in certain cases, such as dc removal servo loops , which typically u s e an inverting integrator to drive ref and compensate for a dc offset . this requires special attention to the input range ( especially at ref ) and the output range . all three input voltages are referred to the one ground shown , which may need to be a low impedance midsupply . ad8237 +in ?in ref fb v out r2 r1 v out = (v ref + v +in ? v ?in ) (1 + r2 r1 ) 10289-072 figure 70 . applying gain to the reference voltage traditional instrumentation amplifier architectures require the reference pin to be driven with a low impedance source. in these traditional architectures, impedance at the reference pin degrades both cmrr and gain accuracy. with the ad8237 architecture, resistance at the reference pin has no effect on cmrr. ad8237 +in ?in ref fb v out g = 1 + r2 + r ref r1 v ref r1 r2 r ref 10289-073 figure 71 . calculating gain with reference resistance
data sheet ad8237 rev. 0 | page 23 of 28 r esistance at the reference pin does affect the gain of the ad8237 ; however, if this resistance is constant, the gain setting resistors can be adjust ed to compensate. for example, the ad8237 can be driven with a voltage divider , as shown in figure 72. ad8237 +in ?in ref fb v out g = 1 + r2 + r3 || r4 r1 r1 r2 r3 r4 v s 10289-074 figure 72 . using voltage divider to set reference volta ge layout common - mode rejection ratio o ver frequency poor layout can cause some of the common - mode signal to be converted to a differential signal before reaching the in - amp. this conversion can occur when the path to the positive input pin has a different frequency response than the path to the negative input pin. for best cmrr vs. frequency performance, closely match the impedance of each path . place a dditional source resistance in the input path (for example, for input protection) close to the in - amp inp uts to minimize interaction between the resistors and the parasitic capacitance from the printed circuit board (pcb) traces. power supplies use a stable dc voltage to power the instrumentation amplifier. noise on the supply pins can adversely affec t perform ance. for more information, see the psrr performance curves in figure 17 through figure 20 . place a 0.1 f capacitor as close as possible to each supply pin. as shown in figure 73 , a 10 f tantalum capacitor can be used farther away from the part. this capacitor, which is intended to be effective at low frequencies , can usually be shared by other precision integrated circuits. keep the traces between these integrated circuits short to minimize interaction of the trace parasitic inductance with the shared capacitor. if a single supply is used, decoupling capacitors at ?v s can be omitted. r1 r2 ad8237 +v s +in ?in 0.1f 10f 0.1f 10f ?v s v out 10289-075 ref fb figure 73 . supply decoupling, ref, and outp ut referred to local ground reference the output voltage of the ad8237 is developed with respect to the potential on the reference terminal. take c are to tie ref to the appropriate local ground. input bias curr ent return path the input bias current of the ad8237 must have a return path to ground. when the source, such as a thermocouple, cannot provide a return current path, create one, as shown in figure 74.
ad8237 data sheet rev. 0 | page 24 of 28 capacitively coupled +v s c r r c ?v s ad8237 1 f high-pass = 2rc thermocouple +v s ?v s 10m? ad8237 transformer +v s ?v s ad8237 correct v out v out thermocouple +v s ?v s ad8237 capacitively coupled +v s c c ?v s ad8237 transformer +v s ?v s ad8237 incorrect v out v out v out v out 10289-076 figure 74 . creating an i bias path
data sheet ad8237 rev. 0 | page 25 of 28 applications informa tion battery current moni tor the micropower current consumption, unique topology , and rail - to - rail input of the ad8237 make it ideal for battery - powered current sensing applications. when configured as shown in figure 75 , the ad8237 is able to obtain an accur ate high - side current measurement for both charging and discharging . depending on the nature of the load, +v s may require rc decoupling. use kelvin sensing methods to achieve the most accurate results. ad8237 +in ?in ref fb v out v ref r1 r2 r shunt v bat + ? v out = g(i r shunt ) + v ref +v s ?v s load 10289-077 figure 75 . battery - powered current sense programmable gain in - amp most integrated circuit instrumentation amplifiers use a single resistor to set the gain, which is in a low impedance path. any co mponent placed between the gain setting pins has cu rrent flowing through it, which adds to the gain resistance . typical cmos switches have on resistance, r on . r on is not well controlled, is nonlinear with input voltage, and has high drift. this creates large gain errors and distortion at the output of the in - amp. this r on problem has made it difficult to build a precision programmable gain in - amp in the past. with the ad8237 topology, the switches can be placed in a high impedance sense path, eliminating the para sitic resistance effects. figure 76 shows one way to accomplish programmable gain. some applications may benefit from using a digi tal pot entiometer instead of a multiplexer. ad8237 +in ?in 470pf v out fb 10289-078 20k? adg604 200? 4:1 mux 2k? 200? 22.1? g = 10 g = 100 g = 1000 ref 2k? g = 1 figure 76 . programmable gain with a multiplexer
ad8237 data sheet rev. 0 | page 26 of 28 ad8237 in an ecg front e nd electrocardiogram ( ecg ) circuits must operate with a differential dc offset due to the half - cell potential of the electrodes . the tolerance for this over potential is typically 300 mv ; however, it can be a volt or more in some situations. as ecg circuits move to lower supply voltages, the half - cell potential problem becomes more difficult , strictly limiting the gain that can be applied in the first st age. the ad8237 architecture provides a unique solution to this problem. if the ref pin is left unconnected to the gain setting network, a low frequency inverting integrator can be connected from the output to t he ref pin. because the ad8237 applies gain to the integrator output, the integrator only has to swing as far as the dc offset to compensate for it , rather than the dc offset multiplied by the gain. with this sy stem architecture, large gains can be applied at the in - amp stage, and the requirements of the rest of the system can be greatly reduced. this also reduces noise and offset error contributions from devices after the in - amp in the signal path. the circuit i n figure 77 illustrate s the core concept. additional op amps can be added for improved performance, such as input b uffering, filtering, and driven lead, if it is required by the system. proper decoupling is not sho wn . instrumentation amplifier g = +100 +5v +5v +5v 3.3f a b c +5v ad8607 ad8607 ref fb 100k? 1k? 110k? 22nf 10289-079 ecg out 100k? 100k? patient protection v mid v mid 2m? ad8237 47nf figure 77 . ad8237 in ecg
data sheet ad8237 rev. 0 | page 27 of 28 outline dimensions compliant to jedec standards mo-187-aa 0.80 0.60 0.40 8 0 4 8 1 5 pin 1 0.65 bsc seating plane 0.38 0.22 1.10 max 3.20 3.00 2.80 coplanarity 0.10 0.23 0.08 3.20 3.00 2.80 5.15 4.90 4.65 0.15 0.00 0.95 0.85 0.75 figure 78 . 8 - lead mini small outline package [msop] (rm - 8) dimensions shown in millimeters ordering g uide model 1 temperature range package description package branding AD8237ARMZ ? 40c to +125c 8- lead mini small outline package [msop] , t ube rm -8 y4h AD8237ARMZ -r7 ? 40c to +125c 8- lead mini small outline package [msop], 7 -i nch tape and reel rm -8 y4h AD8237ARMZ - rl ? 40c to +125c 8 - lead mini small outline package [msop] , 13 - i nch tape and reel rm - 8 y4h 1 z = rohs compliant part.
ad8237 data sheet rev. 0 | page 28 of 28 notes ? 2012 analog devices, inc. all rights reserved. trademarks and registered trademarks are the property of their respective owners. d10289 -0- 8/12(0)

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