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low cost, low power, true rms - to - dc converter data sheet ad736 rev. i 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 f rom 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 te chnology way, p.o. box 9106, norwood, ma 02062 - 9106, u.s.a. tel: 781.329.4700 www.analog.com fax: 781.461.3113 ? 1988 C 2012 analog devices, inc. all rights reserved. features converts an ac voltage waveform to a dc voltage and then converts to the t rue rms, average rectified, or absolute value 200 mv rms full - scale input range (larger inputs with input attenuator) high input impedance: 10 12 ? low input bias current: 2 5 pa maximum high accuracy: 0.3 mv 0.3% of reading rms conversion with signal crest factors up to 5 wide power supply range: +2.8 v, ? 3.2 v to 16.5 v low power: 200 a maximum supply current buffered voltage output no external trims needed for sp ecified accuracy related device: the ad737 features a power - down control with standby current of only 25 a; the dc output voltage is negative and the output impedance is 8 k ? general description the ad736 is a low power, precision, monolithic true rms - to - dc converter. it is laser trimmed to provide a maximum error of 0.3 mv 0.3% of reading with sine wave inputs. furthermore, it maintains high accuracy while measuring a wide range of input wav eforms, including variable duty - cycle pulses and triac (phase) - controlled sine waves. the low cost and small size of this converter make it suitable for upgrading the performance of non - rms precision rectifiers in many applications. compared to these circuits, the ad736 of fers higher accuracy at an equal or lower cost. the ad736 can compute the rms value of both ac and dc input voltages. it can also be operated as an ac - coupled device by adding one external capacitor. in this mode, the ad736 can resolve input signal levels of 100 v rms or less, despite variations in temperature or supply voltage. high accuracy is also maintained for input waveforms with crest factors of 1 to 3. in addition, crest factors as high as 5 can be measured (introducing only 2.5% additional error ) at the 200 mv full - scale input level. the ad736 has its own output buffer amplifier, thereby pro - viding a great deal of design flexibility. requiring only 200 a of power supply current, the ad736 is optimized for use in portable multimeters and other b attery - powered applications. functional block dia gram cc 8k? ?vs cav com vin cav out full wave rectifier rms core 8k? cf (opt) cf bias section + vs 00834-001 figure 1. the ad736 allows the choice of two signal input terminals: a high impedance fet input (10 12 ?) that directly interfaces with high - z input attenuators and a low impedance input (8 k?) that allows the measurement of 300 mv input levels while operating from the minimum power supply voltage of +2.8 v, ?3.2 v. the two inputs can be used either single ended or differentially. the ad736 has a 1% reading error bandwidth that exceeds 10 khz for the input amplitudes from 20 mv rms to 200 mv rms while consuming only 1 mw. the ad736 is available in four performance grades. the ad736j and ad736k grades are rated over the 0c to +70c and ?20c to +85c commercial temperature ranges. the ad736a and ad736b grades are rated over the ?40c to +85c industrial temperature range. the ad736 is available in three low cost, 8 - lead packages: pdip, soic, and cerdip. p roduct highlights 1. the ad736 is capable of computing the average rectified value, absolute value, or true rms value of various input signals. 2. only one external component, an averaging capacitor, is required for the ad736 to perform true rms measurement. 3. t he low power consumption of 1 mw makes the ad736 suitable for many battery - powered applications. 4. a high input impedance of 10 12 ? eliminates the need for an external buffer when interfacing with input attenuators. 5. a low impedance input is available for those applications that require an input signal up to 300 mv rms operating from low power supply voltages.
ad736 data sheet rev. i | page 2 of 20 table of contents features .............................................................................................. 1 general description ......................................................................... 1 functional block diagram .............................................................. 1 product highlights ........................................................................... 1 revision history ............................................................................... 2 specifications ..................................................................................... 3 absolute maximum ratings ............................................................ 5 thermal resistance ...................................................................... 5 esd caution .................................................................................. 5 pin configuration and function descriptions ............................. 6 typical performance characteristics ............................................. 7 theory of operation ...................................................................... 10 types of ac measurement ........................................................ 10 calculating settling time using figure 16 ............................. 11 rms measurement choosing the optimum value for c av .... 11 rapid settling times via the average responding connection .................................................................................. 12 dc error, output ripple, and averaging error ..................... 12 ac measurement accuracy and crest factor ............................ 12 applications ..................................................................................... 13 connecting the input ................................................................. 13 selecting practical values for input coupling (c c ), averaging (c av ), and filtering (c f ) capacitors ...................... 14 additional application concepts ............................................. 15 evaluation board ............................................................................ 17 outline dim ensions ....................................................................... 19 ordering guide .......................................................................... 20 revision history 1 2 / 12 rev. h to rev. i changes to features and figure 1 .................................................. 1 c hange to error vs. crest factor parameter, table 1 .................. 3 changes to operating voltage range parameter, table 1 .......... 4 changes to table 2 ........................................................................... 5 added table 3; renumbered sequentially ................................... 5 changes to figure 9 ......................................................................... 8 changes to figure 16 ....................................................................... 9 changes to figure 18 ..................................................................... 10 added additional application concepts section and changes to figure 25 ..................................................................... 15 changes to figure 29 ..................................................................... 17 deleted table 6 ............................................................................... 17 changes to ordering guide ......................................................... 20 2/07 rev. g to rev. h updated layout ....................................................................... 9 to 1 2 added applications section ......................................................... 13 inserted figure 21 to figure 24; renumbered sequentially ..... 13 delete d figure 25 ........................................................................... 15 add ed evaluation boar d section ................................................. 1 6 inserted figure 29 to figure 34; renumbered sequentially ..... 1 6 inserted figure 35; renumbered sequentially ........................... 1 7 add ed table 6 ................................................................................. 1 7 2/06 rev. f to rev. g updated format ................................................................. universal changes to features ......................................................................... 1 added table 3 ................................................................................... 6 changes to figure 21 and figure 22 ........................................... 14 changes to figure 23, figure 24, and figure 25 ........................ 15 updated outline dimensions ...................................................... 16 changes to ordering guide ......................................................... 17 5/04 rev. e to rev. f changes to specifications ................................................................ 2 replaced figure 18 ........................................................................ 10 updated outline dimensions ...................................................... 16 changes to ordering guide ......................................................... 16 4/03 rev. d to rev. e changes to general description ................................................. 1 changes to specifications ............................................................. 3 changes to absolute maximum ratings .................................... 4 changes to ordering guide ......................................................... 4 11/02 rev. c to rev. d changes to functional block diagram ....................................... 1 changes to pin configuration ..................................................... 3 figure 1 replaced .......................................................................... 6 changes to figure 2 ....................................................................... 6 changes to application circuits figures 4 to 8 ......................... 8 outline dimensions updated ...................................................... 8
data sheet ad736 rev. i | page 3 of 20 specifications at 25c 5 v supplies, ac - coupled with 1 khz sine wave input applied, unless otherwise noted. specifications in bold are tested on all producti on units at final electrical test. results from those tests are used to calculate outgoing quality levels. table 1 . ad736j/ad736a ad736k/ad736b parameter conditions min typ max min typ max unit transfer function v out = avg ( v in 2 ) conversion accuracy 1 khz sine wave total error, internal trim 1 using c c all grades 0 mv rms to 200 mv rms 0.3/0.3 0.5/0.5 0.2/0.2 0.3/0.3 mv/% of reading 200 mv to 1 v rms ?1.2 2.0 ?1.2 2.0 % of reading t min to t max a and b grades @ 200 mv rms 0.7/0.7 0.5/0.5 mv/% of reading j and k grades @ 200 mv rms 0.007 0.007 % of reading/c vs. supply voltage @ 200 mv rms input v s = 5 v to 16.5 v 0 +0.06 +0.1 0 +0.06 +0.1 %/v v s = 5 v to 3 v 0 ?0 .18 ? 0.3 0 ?0.18 ?0.3 %/v dc reversal error, dc - coupled @ 600 mv dc 1.3 2.5 1.3 2.5 % of reading nonlinearity 2 , 0 mv to 200 mv @ 100 mv rms 0 0.25 0.35 0 0.25 0.35 % of reading total error, external trim 0 mv rms to 200 mv rms 0.1/0.5 0.1/0.3 mv/ % of reading error vs. crest factor 3 crest factor = 1 to 3 c av , c f = 100 f 0.7 0.7 % additional error crest factor = 3 to 5 c av , c f = 100 f 2.5 2.5 % additional error input characteristics high impedance input signal range ( pin 2) continuous rms level v s = +2. 8 v, ? 3 .2 v 200 200 mv rms v s = 5 v to 16.5 v 1 1 v rms peak transient input v s = +2.8 v, ? 3. 2 v 0.9 0.9 v v s = 5 v 2.7 2.7 v v s = 16.5 v 4.0 4.0 v input resistance 10 12 10 12 ? input bias current v s = 3 v to 16 .5 v 1 25 1 25 pa low impedance input signal range (pin 1) continuous rms level v s = +2.8 v, C 3.2 v 300 300 mv rms v s = 5 v to 16.5 v 1 1 v rms peak transient input v s = +2.8 v, ?3.2 v 1.7 1.7 v v s = 5 v 3.8 3.8 v v s = 16.5 v 11 11 v input resistance 6.4 8 9.6 6.4 8 9.6 k? maximum continuous nondestructive input all supply voltages 12 12 v p -p input offset voltage 4 j and k grades 3 3 mv a and b grades 3 3 mv vs. temperature 8 30 8 30 v/c vs. supply v s = 5 v to 16.5 v 50 150 50 150 v/v v s = 5 v to 3 v 80 80 v/v
ad736 data sheet rev. i | page 4 of 20 ad736j/ad736a ad736k/ad736b parameter conditions min typ max min typ max unit output characteristics output offset voltage j and k grades 0.1 0.5 0.1 0.3 mv a and b grades 0.5 0.3 mv vs. tem perature 1 20 1 20 v/c vs. supply v s = 5 v to 16.5 v 50 130 50 130 v/v v s = 5 v to 3 v 50 50 v/v output voltage swing 2 k ? load v s = +2.8 v, ?3.2 v 0 to 1.6 1.7 0 to 1.6 1.7 v v s = 5 v 0 to 3.6 3.8 0 to 3.6 3.8 v v s = 16.5 v 0 to 4 5 0 to 4 5 v no load v s = 16.5 v 0 to 4 12 0 to 4 12 v output current 2 2 ma short - circuit current 3 3 ma output re sistance @ dc 0.2 0.2 ? frequency response high impedance input (pin 2) for 1% additional error sine wave input v in = 1 mv rms 1 1 khz v in = 10 mv rms 6 6 khz v in = 100 mv rms 37 37 khz v in = 200 mv rms 33 33 kh z 3 db bandwidth sine wave input v in = 1 mv rms 5 5 khz v in = 10 mv rms 55 55 khz v in = 100 mv rms 170 170 khz v in = 200 mv rms 190 190 khz low impedance input (pin 1) for 1% additional error sine wave input v in = 1 mv rms 1 1 khz v in = 10 mv rms 6 6 khz v in = 100 mv rms 90 90 khz v in = 200 mv rms 90 90 khz 3 db bandwidth sine wave input v in = 1 mv rms 5 5 khz v in = 10 mv rms 55 55 khz v in = 100 mv rms 350 350 khz v in = 200 mv rms 460 460 khz power supply operating voltage range +2.8, ?3.2 5 16.5 +2.8, ?3.2 5 16.5 v quiescent current zero signal 160 200 160 200 a 200 mv rms, no load sine wave input 230 270 230 270 a temperature range operating, rated performance commercial 0c to 70c ad736jn, ad736jr ad736kn, ad736kr industrial ?40c to +85c ad736aq, ad736ar ad736bq, ad736br 1 accuracy is specified with the ad736 connected as shown in figure 18 with c apacitor c c . 2 nonlinearity is defined as the maximum deviatio n (in percent error) from a straight line connecting the readings at 0 mv rms and 200 mv rms. output offset voltage is adjust ed to zero. 3 error vs. crest factor is specified as additional error for a 200 mv rms signal. crest factor = v peak /v rms. 4 dc off set does not limit ac resolution.
data sheet ad736 rev. i | page 5 of 20 absolute maximum rat ings table 2 . parameter rating supply vol tage 16.5 v internal power dissipation 200 mw input voltage pin 2 through pin 8 v s pin 1 12 v output short - circuit duration indefinite differential input voltage +v s and C v s storage temperature range (q) C 65c to +150c storage temperature rang e (n, r) C 65c to +125c lead temperature (soldering , 60 sec) 300c esd rating 500 v stresses above those listed under absolute maximum ratings may cause permanent damage to the device. this is a stress rating only; functional operation of the device a t these or any other conditions above those indicated in the operational section of this specification is not implied. exposure to absolute maximum rating conditions for extended periods may affect device reliability. thermal resistance ja is specified fo r the worst - case conditions, that is, a device soldered in a circuit board for surface - mount packages. table 3 . thermal resistance package type ja unit 8 - lead pdip 165 c/w 8 - lead cerdip 110 c/w 8 - lead soic 155 c/w esd cauti on
ad736 data sheet rev. i | page 6 of 20 pin configuration an d function descripti ons c c 1 v in 2 c f 3 ?v s 4 com 8 +v s 7 output 6 c av 5 ad736 top view (not to scale) 00834-025 figure 2 . pin configuration table 4 . pin function descriptions pin o. mnemonic description 1 c c coupling capacitor. if dc coupling is desired at pin 2, connect a coupling capacitor to this pin. if the coupli ng at pin 2 is ac, connect this pin to ground. note that this pin is also an input, with an input impedance of 8 k?. such an input is useful for applications with high input voltages and low supply voltages. 2 v in high input impedance pin. 3 c f connect an auxiliary low - pass filter capacitor from the output. 4 ?v s negative supply voltage if dual supplies are used, or ground if connected to a single - supply source. 5 c av connect the averaging capacitor here. 6 output dc output voltage. 7 +v s positive s upply voltage. 8 com common.
data sheet ad736 rev. i | page 7 of 20 typical performance characteristics ?0.5 ?0.3 ?0.1 0 0.3 0.1 0.5 0.7 0 4 2 8 6 12 14 10 16 supp l y vo lt age (v) additiona l error (% of reading) v in = 200mv rms 1khz sine wave c av = 100f c f = 22f 00834-002 figure 3 . additional error vs. supply voltage 0 2 4 6 8 12 10 14 16 0 4 2 8 6 12 14 10 16 supply voltage (v) peak input before clipping (v) pin 1 pin 2 dc-coupled 00834-003 figure 4 . maximum input level vs. supply voltage 0 4 2 10 16 14 12 8 6 0 2 4 6 10 8 12 14 16 supply voltage (v) peak buffer output (v) 1khz sine wave input 00834-004 figure 5 . peak buffer output vs. supply voltage 100v 1mv 10mv 1v 100mv 10v 0.1 1 100 10 1000 ?3db frequency (khz) input level (rms) sine wave input, v s = 5v, c av = 22f, c f = 4.7f, c c = 22f 1% error ?3db 10% error 00834-005 figure 6 . frequency response driving pin 1 100v 1mv 10mv 1v 100mv 10v 0.1 1 100 10 1000 ?3db frequency (khz) input level (rms) sine wave input, v s = 5v, c av = 22f, c f = 4.7f, c c = 22f 1% error 10% error ?3db 00834-006 figure 7 . frequency response driving pin 2 c av = 100f c av = 250f 0 1 2 3 4 5 6 1 2 3 4 5 crest factor (v peak /v rms) additional error (% of reading) c av = 10f c av = 33f 3ms burst of 1khz = 3 cycles 200mv rms signal v s = 5v c c = 22f c f = 100f 00834-007 figure 8 . additional error vs. crest factor with various values of c av
ad736 data sheet rev. i | page 8 of 20 ?0.8 ?0.6 ?0.2 ?0.4 0 0.4 0.2 0.6 0.8 ?60 ?20 ?40 20 0 60 80 100 120 40 140 temperature (c) additional error (% of reading) v in = 200mv rms 1khz sine wave c av = 100 f c f = 22 f v s = 5v 00834-008 figure 9 . additional error vs. temperature 100 200 300 400 500 600 0 0.2 0.4 0.6 0.8 1.0 rms input level (v) dc supply current (a) v in = 200mv rms 1khz sine wave c av = 100f c f = 22f v s = 5v 00834-009 figure 10 . dc supply current vs. rms input level 10v 100v 1mv 10mv 100 1k 10k 100k ?3db frequency (hz) input level (rms) v in = 1khz sine wave input ac-coupled v s = 5v 00834-010 figure 11 . rms input level (pin 2) vs. ? 3 db frequency ?2.5 ?2.0 ?1.5 ?1.0 ?0.5 0 0.5 1.0 10mv 100mv 1v 2v input level (rms) error (% of reading) v in = sine wave @ 1khz c av = 22f, c c = 47f, c f = 4.7f, v s = 5v 00834-011 figure 12 . error vs. rms input voltage (pin 2), output buffer offset is adjusted to zero 1 10 100 10 100 1k frequency (hz) c av (f) ?1% ?0.5% v in = 200mv rms c c = 47f c f = 47f v s = 5v 00834-012 figure 13 . c av vs. frequency for specified averaging error 1mv 10mv 100mv 1v 1 10 100 1k frequency (hz) input level (rms) ?0.5% ?1% v in sine wave ac-coupled c av = 10f, c c = 47f, c f = 47f, v s = 5v 00834-013 figure 14 . rms input level vs. frequency for specified averaging error
data sheet ad736 rev. i | page 9 of 20 1.0 1.5 2.0 2.5 3.0 4.0 3.5 0 2 4 6 8 12 14 10 16 supply voltage (v) input bias current (pa) 00834-014 figure 15 . pin 2 input bias current vs. supply voltage 00834-015 100v 1 mv 10mv 100mv 1v 1ms 10ms 100ms 1s 10s 100s settling time input level (rms) cav = 10f cav = 33f cav = 100f vs = 5v cc = 22f cf = 0f figure 16 . rms input level for various values of c av vs. settling time 100fa 10na 1na 100pa 10pa 1pa ?55 ?35 ?15 5 25 65 85 105 45 125 temperature (c) input bias current 00834-016 figure 17 . pin 2 input bias current vs. temperature
ad736 data sheet rev. i | page 10 of 20 theory of operation ac coupled c c = 10f 8k? 00834-017 input amplifier i b < 10 pa full-wave rectifier rms translinear core ? vs cav + v s com 1 2 3 4 vin 5 cav 33 f c f output 6 7 cf 8 + + 8k ? ( optional lpf ) dc + coupled ad736 0.1 f to com pin 0.1 f 10 f output amplifier bias section c c figure 18 . ad736 true rms circuit as shown by figure 18 , the ad736 has five functional subsections: the input amplifier, full - wave rectifier (fwr), rms core, output amplifier, and bias section. the fet input amplifier allows both a high impedance, buffered input (pin 2) and a low impedance, wide dynamic range input (pin 1). the h igh impedance input, with its low input bias current, is well suited for use with high impedance input attenuators. the output of the input amplifier drives a full - wave precision rectifier that , in turn, drives the rms core. the essential rms operations of squaring, averaging, and square rooting are performed in the core using an external averaging capacitor, c av . without c av , the rectified input signal travels through the core unprocessed, as is done with the average responding connection (see figure 19). a final subsection, an output amplifier, buffers the output from the core and allows optional low - pass filtering to be performed via the external capacitor, c f , which is connected across the feedback path of the amp lifier. in the average responding connection, this is where all of the averaging is carried out. in the rms circuit, this additional filtering stage helps reduce any output ripple that was not removed by the averaging capacitor, c av . types of ac measurem e nt the ad736 is capable of measuring ac signals by operating as either an average responding converter or a true rms - to - dc converter. as its name implies, an average responding converter computes the average absolute value of an ac (or ac and dc) voltage o r current by full - wave rectifying and low - pass filtering the input signal; this approximates the average. the resulting output, a dc average level, is scaled by adding (or reducing) gain; this scale factor converts the dc average reading to an rms equivale nt value for the waveform being measured. for example, the average absolute value of a sine wave voltage is 0.636 times v peak ; the corresponding rms value is 0.707 v peak . therefore, for sine wave voltages, the required scale factor is 1.11 (0.707 / 0.636). in contrast to measuring the average value, true rms measurement is a universal language among waveforms, allowing the magnitudes of all types of voltage (or current) waveforms to be compared to one another and to dc. rms is a direct measure of the power or heating value of an ac voltage compared to that of a dc voltage; an ac signal of 1 v rms produces the same amount of heat in a resistor as a 1 v dc signal.
data sheet ad736 rev. i | page 11 of 20 mathematically, the rms value of a voltage is defined (using a simplified equation) as ( ) 2 rms v avg v = this involves squaring the signal, taking the average, and then obtaining the square root. true rms converters are smart rectifiers; they provide an accurate rms reading regardless of the type of waveform being measured. however, average responding converters can exhibit very high errors when their input signals deviate from their precalibrated waveform; the magnitude of the error depends on the type of waveform being measured. for example, if an average responding converter is calibrated to measure the rms value of sine wave voltages and then is used to measure either symmetrical square waves or dc voltages, the converter has a computational error 11% (of reading) higher than the true rms value (see table 5 ). calculating settling time using figure 16 figure 16 can be used to closely approximate the time required for the ad736 to settle when its input level is reduced in amplitude. the net time required for t he rms converter to settle is the difference between two times extracted from the graph ( the initial time minus the final settling time ) . as an example, consider the following conditions: a 33 f averaging capacitor, a 100 mv initial rms input level, and a final (reduced) 1 mv input level. from figure 16 , the initial settling time (where the 100 mv line intersects the 33 f line) is approximately 80 ms. the settling time corresponding to the new or final input level of 1 mv is approximately 8 seconds. therefore, the net time for the circuit to settle to its new value is 8 seconds minus 80 ms, which is 7.92 seconds. note that because of the smooth decay characteristic inherent with a capacitor/diode combination, this is the total settling time to the final value (that is, not the settling time to 1%, 0.1%, and so on, of the final value). in addition, this graph provides the worst - case settling time because the ad736 settles very quickly with increasing input levels. rm s measurement choosing the optimum value for c av because the external averaging capacitor, c av , holds the rectified input signal during rms computation, its value directly affects the accuracy of the rms measurement, especially at low frequencies. further more, because the averaging capacitor appears across a diode in the rms core, the averaging time constant increases exponentially as the input signal is reduced. this means that as the input level decreases, errors due to nonideal averaging decrease, and t he time required for the circuit to settle to the new rms level increases. therefore, lower input levels allow the circuit to perform better (due to increased averaging) but increase the waiting time between measurements. obviously, when selecting c av , a t rade - off between computational accuracy and settling time is required. table 5 . error introduced by an average responding circuit when measuring common waveforms waveform type 1 v peak amplitude crest factor (v peak /v rms) true rm s value (v) average responding circuit calibrated to read rms value of sine waves (v) % of reading error using average responding circuit undistorted sine wave 1.414 0.707 0.707 0 symmetrical square wave 1.00 1.00 1.11 +11.0 undistorted triangle wave 1.73 0.577 0.555 ?3.8 gaussian noise (98% of peaks <1 v) 3 0.333 0.295 ?11.4 rectangular 2 0.5 0.278 ?44 pulse train 10 0.1 0.011 ?89 scr waveforms 50% duty cycle 2 0.495 0.354 ?28 25% duty cycle 4.7 0.212 0.150 ?30
ad736 data sheet rev. i | page 12 of 20 rapid settling times via the average responding connectio n because the average responding connection shown in figure 19 does not use the c av averaging capacitor, its settling time does not vary with the input signal level. it is determi ned solely by the rc time constant of c f and the internal 8 k ? resistor in the output amplifiers feedback path. +v s +v s c f 33f c c 10f com output (optional) positive supp l y +v s 0.1f ?v s 0.1f common neg a tive supp l y v out 8 7 6 5 1 2 3 4 ad736 + rms core + c c v in v in full wave rectifier c f ?v s ?v s c a v bias section input amplifier 8k? output amplifier 8k? 00834-018 figure 19 . ad736 average responding circuit dc error, output rip ple, and averaging error figure 20 shows the typical output waveform of the ad736 with a sine wave input applied. as with all real - world devices, the ideal output of v out = v in is never achieved exactly. instead, the output contains both a dc and an ac error component. as shown in figu re 20 , the dc error is the difference between the average of the output signal (when all the ripple in the output is removed by external filtering) and the ideal dc output. the dc error component is therefore set solely by the value of the averaging capac itor used . n o amount of post filtering (that is, using a very large c f ) allows the output voltage to equal its ideal value. the ac error component, an output ripple, can be easily remo ved by using a large enough post filtering capacitor, c f . in most case s, the combined magnitudes of both the dc and ac error components need to be considered when selecting appropriate values for capacitor c av and capacitor c f . this combined error, representing the maximum uncertainty of the measurement, is termed the avera ging error and is equal to the peak value of the output ripple plus the dc error. dc error = e o ? e o (ideal) average e o = e o e o ideal e o double-frequency ripple time 00834-019 figure 20 . output waveform for sine wave input voltage as the input frequency increases, both error components decrease rapidly; if the input frequ ency doubles, the dc error and ripple reduce to one quarter and one half of their original values, respectively, and rapidly become insignificant. ac measurement accur acy and crest factor the crest factor of the input waveform is often overlooked when dete rmining the accuracy of an ac measurement. crest factor is defined as the ratio of the peak signal amplitude to the rms amplitude (crest factor = v peak /v rms). many common waveforms, such as sine and triangle waves, have relatively low crest factors (2). ot her waveforms, such as low duty - cycle pulse trains and scr waveforms, have high crest factors. these types of waveforms require a long averaging time constant (to average out the long periods between pulses). figur e 8 shows the additional error vs. the crest factor of the ad736 for various values of c av .
data sheet ad736 rev. i | page 13 of 20 applications connecting the input the in p uts of the ad736 resemble an op amp, with noninverting and inverting inputs. the input stages are jfets accessible at p in 1 and pin 2. designated as the high impedance input, p in 2 is connected directly to a jfet gate. pin 1 is the low impedance input because of the scaling resistor connected to the gate of the second jfet. this gate - resistor junction is not externally acc essible and is servo - ed to the voltage level of the gate of the first jfet , as in a classic feedback circuit. this action results in the typical 8 k ? input impedance referred to ground or reference level. this input structure provides four input configurations as shown in figure 21, figure 22, figure 23, and figure 24. figure 21 and figure 22 show the high impedance configurations, and figure 23 and figure 24 show the low impedance connections used to extend the input voltage range. 00834-026 ad736 com +v s +v s output c f 1m vout dc c av c c v in ?v s 1 2 3 4 8 7 6 5 c av ?v s figure 21 . high - z ac - coupled input connection (default) 00834-027 ad736 com +v s +v s output vout dc c av c c v in ?v s 1 2 3 4 8 7 6 5 c av c f ?v s figure 22 . high - z dc - coupled input connection 00834-028 ad736 com +v s +v s output vout dc c av c c v in ?v s 1 2 3 4 8 7 6 5 c av c f ?v s figure 23 . low - z ac - coupled input connection 00834-029 ad736 com +v s +v s output vout dc c av c c v in ?v s 1 2 3 4 8 7 6 5 c av c f ?v s figure 24 . low - z dc - coupled input connection
ad736 data sheet rev. i | page 14 of 20 selecting practical values for input coupling (c c ), averaging (c av ), and filtering (c f ) capacitors table 6 provides practical values of c av and c f for several common applications. the input coupling capacitor, c c , in conjunction with the 8 k? internal input scaling resistor, determine the ?3 db low frequency roll - off. this frequency, f l , is equal to ) (8000)( 2 1 farads in c of value f c l = note that at f l , the amplitude error is approximately ?30% ( 3 db) of the reading. to reduce this error to 0.5% of t he reading, choose a value of c c that sets f l at one - tenth of the lowest frequency to be measured. in addition, if the input voltage has more than 100 mv of dc offset, then the ac - coupling network shown in figure 27 should be used in addition to c c . table 6 . capacitor selection chart application rms input level low frequency cutoff (?3 db) max crest factor c av (f) c f (f) settling time 1 to 1% general - purpose rms computation 0 v to 1 v 20 hz 5 150 10 360 ms 200 hz 5 15 1 36 ms 0 mv to 200 mv 20 hz 5 33 10 360 ms 200 hz 5 3.3 1 36 ms ge neral purpose 0 v to 1 v 20 hz none 33 1.2 sec average 200 hz none 3.3 120 ms responding 0 mv to 200 mv 20 hz none 33 1.2 sec 200 hz none 3.3 120 ms scr waveform measurement 0 mv to 200 mv 50 hz 5 100 33 1.2 sec 60 hz 5 82 27 1.0 sec 0 mv to 100 mv 50 hz 5 50 33 1.2 sec 60 hz 5 47 27 1.0 sec audio applications speech 0 mv to 200 mv 300 hz 3 1.5 0.5 18 ms music 0 mv to 100 mv 20 hz 10 100 68 2.4 sec 1 settling time is specified over the stated rms input level with the input signal increasing from zero. settling times are gre ater for decreasing amplitude input signals .
data sheet ad736 rev. i | page 15 of 20 additional applicati on conc epts figure 25 through figure 28 show four application concepts. figure 25 shows the high input impedance fet input connected to a multitap attenuator network used in various types of instruments requiring wide ranges of voltages. for a direct network connection, the gate - charge bleeding resistor is not required. the impedance of the fet input is high enough (10 12 ) so that the load ing error is negligible. manufacturers and distributors of the matched precision resistor networks shown in these figures can easily be found on the web. the voltages shown in the diagrams are the input levels corresponding to 200 mv at each tap. finally, the 47 k, 1 w resistor and diode pair are a practical input protection scheme for ac line connection measurements. figure 26 shows both inputs connected differentially. figu re 27 shows additional components used for offset correction of the output amplifier, and figure 28 shows connections for single - supply operation such as is the case for handheld devices. further information can b e found in the an - 268 application note rms - to - dc converters ease measurement tasks and the rms to dc converter application guide , both of which can be found on the analog devices, inc., website. +v s +v s c av 33f 47k? 1w c c 10f com (optional) output 8 7 6 5 1 2 3 4 ad736 + rms core + c c v in full wave rectifier c f ?v s ?v s +v s ?v s c av bias section input amplifier 8k? output amplifier 8k? c f 10f 1f 1f (optional) + optional ac coupling capacitor vin for full scale output 0.01f 1kv 2v 20v 200v 9m? 900k? 90k? 10k? v in 200mv bav199 00834-020 figure 25 . ad736 with a high impedance input attenuator +v s +v s c av 33f com output output 8 7 6 5 1 2 3 4 ad736 + rms core c c v in full wave rectifier c f ?v s ?v s c av bias section input amplifier 8k? output amplifier 8k? c f 10f c c 10f 1f 1f (optional) + +in input impedance: 10 12 ?||10pf ?in ad711 + 3 2 6 00834-021 figure 26 . differential input connection
ad736 data sheet rev. i | page 16 of 20 7 +v s +v s c av 33f c c 10f com output (optional) output 8 6 5 1 2 3 4 ad736 + rms core + c c v in full wave rectifier c f ?v s ?v s +v s c av bias section input amplifier 8k? output amplifier 8k? c f 10f 1f 1f (optional) + v in output v os adjust 1m? 39m? ac-coupled dc-coupled 1m? 0.1f 00834-022 figure 27 . external output v os adjus tment +v s c c 10f com output 8 7 6 5 1 2 3 4 ad736 rms core + c c v in v in full wave rectifier c f ?v s c a v bias section input amplifier 8k? output amplifier 8k? c f 10f (optional) + + 1m? 0.1f 33f 9v 100k? 100k? 4.7f 4.7f v s 2 v s 2 00834-023 figure 28 . battery - powered option
data sheet ad736 rev. i | page 17 of 20 evaluation board an evaluation board, ad736 - evalz, is available for experimentation or becoming familiar with rms - to - dc converters. figure 29 is a photograph of the board , and figure 30 is the top silk screen showing the component locations. figure 31 , figure 32 , figure 33 , and figure 34 show the layers of copper , and figure 35 shows the schematic of the board configured as shipped. the board is designe d for multipurpose applications and can be used for the ad737 as well. 00834-030 figure 29 . ad736 e valuation board 00834-032 figure 30 . evaluation board component - side silkscreen as shipped, the board is configured for du al supplies and high impedance input. optional jumper locations enable low impedance and dc input connections. using the low impedance input ( p in 1) often enables higher input signals than other wise possible. a dc connection enables an ac plus dc measureme nt, but care must be taken so that the op posite polarity input is not dc - coupled to ground. figure 35 shows the board schematic with all movable jumpers. the jumper positions in black are default connections; the dotted - outline jumpers are optional connections. the board is tested prior to shipment and only requires a power supply connection and a precision meter to perform measurements. 00834-033 figure 31 . evaluation board component - side copper 00834-034 figure 32 . evaluation board secondary - side copper 00834-035 figure 33 . evaluation board internal power plane 00834-036 figure 34 . evaluation board internal ground plane
ad736 data sheet rev. i | page 18 of 20 00834-032 ad736 com c av +v s +v s out j2 c c v in c f ?v s ?v s 1 2 3 4 7 6 5 c a v c6 0.1f c4 0.1f r1 1m w2 in j1 vin lo-z w1 dc cou p p2 hi-z se l hi-z gnd cav 33f 16v+ cin 0.1f cf2 gnd1 gnd2 gnd3 gnd4 r4 0 8 r3 0 + c1 10f 25v c2 10f 25v ?v s ?v s +v s +v s + + vout cf1 norm sel j3 pd +v s filt w3 ac cou p c c w4 lo-z in figure 35 . evaluation board schematic
data sheet ad736 rev. i | page 19 of 20 outline dimensions compliant t o jedec s t andards ms-001 controlling dimensions are in inches; millimeter dimensions (in p arentheses) are rounded-off inch equi v alents for reference on l y and are not appropri a te for use in design. corner leads m a y be configured as whole or half leads. 070606- a 0.022 (0.56) 0.018 (0.46) 0.014 (0.36) se a ting plane 0.015 (0.38) min 0.210 (5.33) max 0.150 (3.81) 0.130 (3.30) 0. 1 15 (2.92) 0.070 (1.78) 0.060 (1.52) 0.045 (1.14) 8 1 4 5 0.280 (7. 1 1) 0.250 (6.35) 0.240 (6.10) 0.100 (2.54) bsc 0.400 (10.16) 0.365 (9.27) 0.355 (9.02) 0.060 (1.52) max 0.430 (10.92) max 0.014 (0.36) 0.010 (0.25) 0.008 (0.20) 0.325 (8.26) 0.310 (7.87) 0.300 (7.62) 0.195 (4.95) 0.130 (3.30) 0. 1 15 (2.92) 0.015 (0.38) gauge plane 0.005 (0.13) min figure 36 . 8 - lead plastic dual in - line package [pdip] narrow body (n - 8) dimensions shown in inches and (millimeters) controlling dimensions are in inches; millimeter dimensions (in parentheses) are rounded-off inch equivalents for reference only and are not appropriate for use in design. 0.310 (7.87) 0.220 (5.59) 0.005 (0.13) min 0.055 (1.40) max 0.100 (2.54) bsc 15 0 0.320 (8.13) 0.290 (7.37) 0.015 (0.38) 0.008 (0.20) seating plane 0.200 (5.08) max 0.405 (10.29) max 0.150 (3.81) min 0.200 (5.08) 0.125 (3.18) 0.023 (0.58) 0.014 (0.36) 0.070 (1.78) 0.030 (0.76) 0.060 (1.52) 0.015 (0.38) 1 4 5 8 figure 37 . 8 - lead ceramic dual in - line package [cerdip] (q - 8) dimensions shown in inches and (millimeters) controlling dimensions are in millimeters; inch dimensions (in p arentheses) are rounded-off millimeter equi v alents for reference on l y and are not appropri a te for use in design. compliant t o jedec s t andards ms-012-a a 012407- a 0.25 (0.0098) 0.17 (0.0067) 1.27 (0.0500) 0.40 (0.0157) 0.50 (0.0196) 0.25 (0.0099) ? 45 8 0 1.75 (0.0688) 1.35 (0.0532) se a ting plane 0.25 (0.0098) 0.10 (0.0040) 4 1 8 5 5.00 (0.1968) 4.80 (0.1890) 4.00 (0.1574) 3.80 (0.1497) 1.27 (0.0500) bsc 6.20 (0.2441) 5.80 (0.2284) 0.51 (0.0201) 0.31 (0.0122) coplanarit y 0.10 figure 38 . 8 - lead standard small outline package [soic_n] narrow body (r - 8) dimensions shown in millimeters and (inches)
ad736 data sheet rev. i | page 20 of 20 ordering guide model 1 temperature range package description package option ad736aq C 40c to +85c 8 - lead cerdip q -8 ad736bq C 40c to +85c 8 - lead cerdip q -8 ad736ar - reel C 40c to +85c 8 - lead soic_n r -8 ad736ar - reel7 C 40c to +85c 8 - lead soic_n r -8 ad736arz C 40c to +85c 8 - lead soic_n r -8 ad736arz -r7 C 40c to +85c 8 - lead soic_n r -8 ad736arz -rl C 40c to +85c 8 - lead soic_n r -8 ad736br - reel C 40c to +85c 8 - lead soic_n r -8 ad736br - reel7 C 40c to +85c 8 - lead soic_n r -8 AD736BRZ C 40c to +85c 8 - lead soic_n r -8 AD736BRZ -r7 C 40c to +85c 8 - lead soic_n r -8 AD736BRZ -rl C 40c to +85c 8 - lead soic_n r -8 ad736jn 0c to +70c 8 - lead pdip n - 8 ad736jnz 0c to +70c 8 - lead pdip n -8 ad736knz 0c to +70c 8 - lead pdip n -8 ad736jr 0c t o +70c 8 - lead soic_n r -8 ad736jr - reel 0c to +70c 8 - lead soic_n r -8 ad736jr - reel7 0c to +70c 8 - lead soic_n r -8 ad736jrz 0c to +70c 8 - lead soic_n r -8 ad736jrz -rl 0c to +70c 8 - lead soic_n r -8 ad736jrz -r7 0c to +70c 8 - lead soic_n r -8 ad73 6krz 0c to +70c 8 - lead soic_n r -8 ad736krz -rl 0c to +70c 8 - lead soic_n r -8 ad736krz -r7 0c to +70c 8 - lead soic_n r -8 ad736 - evalz evaluation board 1 z = rohs compliant part . ? 1988 C 2012 analog devices, inc. all rights re served. trademarks and registered trademarks are the property of their respective owners. d00834 - 0- 12/12(i)

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