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low cost, low power, true rms - to - dc conve rter data sheet a d737 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 from its use. specifications subject to change without notice. no license is granted by implication or otherwise under any patent or patent rights of analog devices. trademarks and registered trademarks are the property of their respective owners. one technology way, p.o. box 9106, norwood, ma 02062 - 9106, u.s.a. tel: 781.329.4700 www.analog.com fax: 7 81.461.3113 ? 2012 analog devices, inc. all rights reserved. f eatures computes true rms value average rectified value absolute value provides 200 mv full - scale input range (larger inputs with input scaling ) direct interfacing with 3? digit cmos adcs high input impedance: 10 12 ? low input bias current: 25 pa max imum high accuracy: 0.2 mv 0.3% of reading rms conversion with signal crest factors up to 5 wide power supply range: 2.5 v to 16.5 v low power: 25 a (typ ical ) s tandby current no external trims needed for specified accuracy the ad737 output is negative - going ; t he ad736 is a positive output - going version of the same basic device functional block diagram 00828-001 com output c av ?v s 8k? bias section absolute value circuit squarer divider c c v in +v s power down c f c av 8k? figure 1. general description the ad737 is a low power, precision, monolithic , true rms - to - dc converter. it is laser trimmed to provide a maximum error of 0.2 mv 0.3% of reading with sine wave inputs. fur thermore, 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 physical size of this converter make it sui table for upgrading the perform ance of non - rms precision rectifiers in many applications. compared to these circuits, the ad737 offers higher accuracy at equal or lower cost. the ad737 can c ompute the rms value of both ac and dc input voltages. it can also be operated ac - coupled by adding one external capacitor. in this mode, the ad737 can resolve input signal levels of 100 v rms or less, despite variations in tem - perature 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 (while introducing only 2.5% additional error) at the 200 mv full - scale input level. the ad737 has no output buffer amplifier, thereby significantly reducing dc offset errors occurring at the output, which makes the device highly compatible with high input impedance adcs. requiring only 160 a of power supply current, the ad737 is optimized for use in portabl e multimeters and other battery - powered applications. in power - down mode , the stan dby supply current in is typically 25 a. the ad737 has both high (10 12 ?) and low impedance input options. the high - z fet input connects high source impedance input attenuators, and a low impedance (8 k?) input accepts rms voltages to 0.9 v while operating from the minimum power supply voltage of 2.5 v. the two i nputs can b e used either single ended or differentially. the ad737 achieves 1% of reading error bandwidth, exceeding 10 khz for input amplitudes from 20 mv rms to 200 mv rms , while consuming only 0.72 mw. the ad737 is available in two perfor mance grades. the ad737j and ad737k grades operate over the commercial temperature range of 0c to 70 c. the ad737jr -5 is tested with supply voltages of 2.5 v dc. the ad737a grade operates over the industrial temperature range of ?40c to +85c. the ad737 is available in t wo low cost, 8 - lead packages: pdip and soic _n . product highlights 1. c omput es average rectified, absolute, or true rms value of a signal regardless of waveform. 2. only one external component, an averaging capac itor, is required for the ad737 to perform true rms measurement. 3. the standby power consumption of 125 w makes the ad737 suitable for battery - powered applications.
ad737 data sheet rev. i | page 2 of 24 table of contents features .............................................................................................. 1 functional block diagram .............................................................. 1 general description ......................................................................... 1 product highlights ........................................................................... 1 revision history ............................................................................... 2 specifications ..................................................................................... 3 absolute maximum ratings ............................................................ 6 thermal resistance ...................................................................... 6 esd caution .................................................................................. 6 pin configurations and function descriptions ........................... 7 typical performance characteristics ............................................. 8 theory of operation ...................................................................... 12 types of ac measurement ........................................................ 12 dc error, output ripple, and averaging error ..................... 13 ac measurement accuracy and crest factor ........................ 13 calculating settling time .......................................................... 13 applications information .............................................................. 14 rms measurementchoosing an optimum value for c av ... 14 rapid settling times via the average responding connection .................................................................................. 14 selecting practical values for capacitors ................................ 14 scaling input and output voltages .......................................... 14 ad737 evaluation board ............................................................... 18 outline dimensions ....................................................................... 20 ordering guide .......................................................................... 21 revision history 6/12rev. h to rev. i removed cerdip package throughout ........................ universal changes to features, general description, product highlights sections and figure 1 ....................................................................... 1 changes to table 1 ............................................................................ 3 changes to table 2 ............................................................................ 6 deleted figure 3, renumbered sequentially ................................. 7 changes to figure 5, figure 7, and figure 8 captions ................. 8 changes to figure 12 caption ......................................................... 9 changes to figure 19 caption ....................................................... 10 changes to figure 23 ...................................................................... 12 changes to figure 26 ...................................................................... 14 changes to scaling the output voltage section ......................... 15 changes to figure 27 ...................................................................... 16 deleted table 7 ................................................................................ 19 updated outline dimensions ....................................................... 20 changes to ordering guide .......................................................... 21 10/08rev. g to rev. h added selectable average or rms conversion section and figure 27 .......................................................................................... 14 updated outline dimensions ....................................................... 20 changes to ordering guide .......................................................... 22 12/06rev. f to rev. g changes to specifications ................................................................ 3 reorganized typical performance characteristics ...................... 8 changes to figure 21 ...................................................................... 11 reorganized theory of operation section ................................. 12 reorganized applications section ................................................ 14 added scaling input and output voltages section .................... 14 deleted application circuits heading ......................................... 16 changes to figure 28 ...................................................................... 16 added ad737 evaluation board section .................................... 18 updated outline dimensions ....................................................... 20 changes to ordering guide .......................................................... 21 1/05rev. e to rev. f updated format .................................................................. universal added functional block diagram .................................................. 1 changes to general description section ....................................... 1 changes to pin configurations and function descriptions section ......................................................................... 6 changes to typical performance characteristics section ........... 7 changes to table 4 .......................................................................... 11 change to figure 24 ....................................................................... 12 change to figure 27 ....................................................................... 15 changes to ordering guide .......................................................... 18 6/03rev. d to rev. e added ad737jr-5 .............................................................. universal changes to features .......................................................................... 1 changes to general description ..................................................... 1 changes to specifications ................................................................. 2 changes to absolute maximum ratings ........................................ 4 changes to ordering guide ............................................................. 4 added tpcs 16 through 19 ............................................................. 6 changes to figures 1 and 2 .............................................................. 8 changes to figure 8 ........................................................................ 11 updated outline dimensions ....................................................... 12 12/02rev. c to rev. d changes to functional block diagram ........................................... 1 changes to pin configuration ......................................................... 4 figure 1 replaced .............................................................................. 8 changes to figure 2 ........................................................................... 8 figure 5 replaced ........................................................................... 10 changes to application circuits figures 4, 6C8 ......................... 10 outline dimensions updated ....................................................... 12 12/99rev. b to rev. c
data shee t ad737 rev. i | page 3 of 24 specifications t a = 25c, v s = 5 v except as noted , c av = 33 f, c c = 10 f, f = 1 khz, sine wave input applied to pin 2, unless otherwise spe cified. specifications shown in boldface are tested on all production units at final electrical test. results from these tests are used to calculate outgoing quality levels. table 1 . parameter test conditions/ comments ad737a, ad737 j ad737k ad737j - 5 unit min typ max min typ max min typ max accuracy total error e in = 0 to 200 mv rms 0.2/0.3 0.4/0.5 0.2/0.2 0.2/0.3 mv/por 1 v s = 2.5 v 0.2/0.3 0.4/0.5 mv/por 1 v s = 2.5 v, input to pin 1 0.2/0.3 0.4/0.5 mv/por 1 e in = 200 mv to 1 v rms ?1.2 2.0 ?1.2 2.0 por over temperature jn, jr, kr e in = 200 mv rms, v s = 2.5 v 0.007 0.007 0.02 por/c an and ar e in = 200 mv rms , v s = 2.5 v 0.014 0.014 por/c v s. supply voltage e in = 200 mv rms, v s = 2.5 v to 5 v 0 ?0.18 ?0.3 0 ?0.18 ?0.3 0 ?0.18 ?0.3 %/v e in = 200 mv rms, v s = 5 v to 16.5 v 0 0.06 0.1 0 0.06 0.1 0 0.06 0.1 %/v dc reversal error dc - coupled, v in = 600 mv dc 1.3 2.5 1.3 2.5 por v s = 2.5 v v in = 200 mv dc 1.7 2.5 por nonlinearity 2 e in = 0 mv to 200 mv rms , @ 100 mv rms 0 0.25 0.35 0 0.25 0.35 por input to pin 1 3 ac coupled, e in = 100 mv rms, after correction, v s = 2.5 v 0.02 0.1 por total error, external trim e in = 0 mv to 200 mv rms 0.1/0.2 0. 1/0.2 0.1/0.2 mv/por additional crest factor error 4 for crest factor s from 1 to 3 c av = c f = 100 f 0.7 0.7 % c av = 22 f, c f = 100 f, v s = 2.5 v, input to pin 1 1.7 % for crest factor s from 3 to 5 c av = c f = 100 f 2.5 2.5 % input characteristics high - z input (pin 2) signal range continuous rms level v s = +2.5 v 200 mv rms v s = +2.8 v / ?3.2 v 200 200 mv rms v s = 5 v to 16.5 v 1 1 v rms
ad737 data sheet rev. i | page 4 of 24 parameter test conditions/ comments ad737a, ad737 j ad737k ad737j - 5 unit min typ max min typ max min typ max peak transient input v s = +2.5 v input to pin 1 0.6 v 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 1012 1012 1012 ? input bias current v s = 5 v 1 25 1 25 1 25 pa low - z in put (pin 1) signal range continuous rms level v s = +2.5 v 300 mv rms v s = +2.8 v / ?3.2 v 300 300 mv rms v s = 5 v to 16.5 v 1 1 v rms peak transient input v s = +2.5 v 1.7 v 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 6.4 8 9.6 k? maximum continuous nondestructive input all supply voltages 12 12 12 v p - p input offset voltage 5 ac - coupled 3 3 3 mv over the rated operating temperature range 8 30 8 30 8 30 v/c v s. supply v s = 2.5 v to 5 v 80 80 80 v/v v s = 5 v to 16.5 v 50 150 50 150 v/v output characteristics no load , output is n egative w ith r espect t o com output voltage range v s = +2.8 v / ?3.2 v ?1.6 ?1.7 ?1.6 ?1.7 v 6 v s = 5 v ?3.3 ?3.4 ?3.3 ?3.4 v 6 v s = 16.5 v ?4 ?5 ?4 ?5 v v s = 2.5 v, input to pin 1 ?1.1 C 0.9 v 6 output resistance dc 6.4 8 9.6 6.4 8 9.6 6.4 8 9.6 k? frequency response high - z input (pin 2) 1% additional error v in = 1 mv rms 1 1 1 khz v in = 10 mv rms 6 6 6 khz v in = 100 mv rms 37 37 37 khz v in = 200 mv rms 33 33 33 khz
data shee t ad737 rev. i | page 5 of 24 parameter test conditions/ comments ad737a, ad737 j ad737k ad737j - 5 unit min typ max min typ max min typ max 3 db bandwidth v in = 1 mv rms 5 5 5 khz v in = 10 mv rms 55 55 55 khz v in = 100 mv rms 170 170 170 khz v in = 200 mv rms 190 190 190 khz low - z input (pin 1) 1% additional error v in = 1 mv rms 1 1 1 khz v in = 10 mv rms 6 6 6 khz v in = 40 mv rms 25 khz v in = 100 mv rms 90 90 90 khz v in = 200 mv rms 90 90 90 khz 3 db bandwidth v in = 1 mv rms 5 5 5 khz v in = 10 mv rms 55 55 55 khz v in = 100 mv rms 350 350 350 khz v in = 200 mv rms 460 460 460 khz power - down mode disable voltage 0 0 v input current, pd enabled v pd = v s 11 11 a power supply operating voltage range +2.8 / ?3.2 5 16.5 +2.8 / ?3.2 5 16.5 2.5 5 16.5 v current no input 120 160 120 160 120 160 a rated input 170 210 170 210 170 210 a powered down 25 40 25 40 25 40 a 1 por is % of r eading. 2 nonlinearity is defined as the maximum deviation (in percent error) from a straight line connecting the readings at 0 v and at 200 mv rms . 3 after fourth - order erro r correction using the equation y = ? 0.31009 x 4 ? 0.21692 x 3 ? 0.06939 x 2 + 0.99756 x + 11.1 10 ?6 wher e y is the corrected result and x is the device output between 0.01 v and 0.3 v . 4 crest factor error is specified as the addit ional error resulting from the specific crest factor, using a 200 mv rms signal as a reference. the crest factor is defined a s v peak /v rms . 5 dc offset does not limit ac resolution . 6 value is measured with respect to com .
ad737 data sheet rev. i | page 6 of 24 absolute maximum rat ings table 2 . parameter rating supply voltage 16.5 v internal po wer dissipation 200 mw input voltage pin 1 12 v pin 2 to pin 8 v s output short - circuit duration indefinite differential input voltage +v s and ?v s storage temperature range ?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 at these or any other conditions above those indicated in the operational section of this specification is not implied. exposure to absolute maximum rating conditions for extended periods may affect device reliability. thermal res istance ja is specified for 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 (n - 8) 165 c/w 8 - lead soic_n (r - 8) 155 c/w esd caution
data shee t ad737 rev. i | page 7 of 24 pin configurations a nd function descript ions c c 1 v in 2 power down 3 ?v s 4 com 8 +v s 7 output 6 c a v 5 ad737 t op view (not to scale) 00828-002 1 2 3 4 8 7 6 5 ad737 t o p view (not to scale) c c v in power down ?v s com +v s output c a v 00828-004 figure 2 . soic_n pin configuration (r - 8) figure 3 . pdip pin configuration (n- 8) table 4 . pin function descript ions pin no. mnemonic description 1 c c coupling capacitor for indirect dc coupling. 2 v in rms input. 3 power down disables the ad737 . low is enabled; high is powered down. 4 C v s negative power supply . 5 c av averaging capacitor. 6 output output. 7 +v s positive power supply. 8 com common.
ad737 data sheet rev. i | page 8 of 24 typical performance characteristics t a = 25 c, v s = 5 v (except ad737j - 5 , where v s = 2.5 v), c av = 33 f, c c = 10 f, f = 1 khz, sine wave input applied to pin 2, unless otherwise specified. v in = 200mv rms c a v = 100f c f = 22f ?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) 00828-005 figure 4 . additional error vs. supply voltage 0 2 4 6 8 12 10 14 16 0 4 2 8 6 12 14 10 16 supp l y vo lt age (v) peak input before clipping (v) 00828-006 pin 1 pin 2 dc coupled figure 5. peak input level for 1% saturation vs. supply voltage 5 10 20 15 25 0 2 4 6 8 10 12 14 16 18 dua l supp l y vo lt age (v) supp l y current (a) 00828-007 figure 6. supply current (power - down mode) vs. supply voltage ( dual) 100v 1mv 10mv 1v 100mv 10v 0.1 1 100 10 1000 frequenc y (khz) input leve l (rms) 00828-008 c a v = 22 f , c f = 4.7 f , c c = 22f 1% error ?3db 10% error figure 7 . frequency response driving pin 1 ; negative dc output 100v 1mv 10mv 1v 100mv 10v 0.1 1 100 10 1000 frequenc y (khz) input leve l (rms) 00828-009 c a v = 22 f , c f = 4.7 f , c c = 22f 1% error 10% error ?3db figure 8 . frequency response driving pin 2 ; negative dc output c a v = 100f c a v = 250f 0 1 2 3 4 5 6 1 2 3 4 5 crest f ac t or (v peak /v rms) additiona l error (% of reading) 00828-010 c a v = 10f c a v = 33f 3ms burst of 1khz = 3 cycles 200mv rms signa l c c = 22f c f = 100f figure 9 . additional error vs. crest factor
data shee t ad737 rev. i | page 9 of 24 v in = 200mv rms c a v = 100f c f = 22f ?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 temper a ture (c) additiona l error (% of reading) 00828-0 1 1 figure 10 . additional error vs. temperature 0 200 100 400 300 500 0 0.2 0.4 0.6 0.8 1.0 rms input leve l (v) dc supp l y current (a) 00828-012 figure 11 . dc supply current vs. rms input level 10v 100v 1mv 10mv 100 1k 10k 100k ?3db frequency (hz) input level (rms) 00828-013 ac-coupled figure 12 . rms input level vs. C 3 db frequency ; negative dc output ?2.5 ?2.0 ?1.5 ?1.0 ?0.5 0 0.5 1.0 10mv 100mv 1v 2v input leve l (rms) error (% of reading) 00828-014 c a v = 22 f , c c = 47 f , c f = 4.7f figure 13 . error vs. rms input level using circuit in figure 29 1 10 100 10 100 1k frequenc y (hz) a veraging ca p aci t or (f) 00828-015 ?1% ?0.5% v in = 200mv rms c c = 47f c f = 47f figure 14 . value of averaging capac itor vs. frequency for specified averaging error 1mv 10mv 100mv 1v 1 10 100 1k frequency (hz) input level (rms) 00828-016 ac-coupled c av = 10f, c c = 47f, c f = 47f ?0.5% ?1% figure 15 . rms input level vs. frequency for specified averaging error
ad737 data sheet rev. i | page 10 of 24 1.0 1.5 2.0 2.5 3.0 4.0 3.5 0 2 4 6 8 12 14 10 16 supp l y vo lt age (v) input bias current (pa) 00828-017 figure 16 . input bias current vs. supply voltage 100v 1mv 10mv 100mv 1v 1ms 10ms 100ms 1s 10s 100s settling time input leve l (rms) 00828-018 c c = 22f c f = 0f c a v = 10f c a v = 33f c a v = 100f figure 17 . rms input level vs. settling time for three values of c av 100f a 10n a 1n a 100p a 10p a 1p a ?55 ?35 ?15 5 25 65 85 105 45 125 temper a ture (c) input bias current 00828-019 figure 18 . input bias current vs. temperature 100v 10mv 1mv 1v 100mv 10v 0.1 1 10 100 1000 frequenc y (khz) input leve l (rms) 00828-020 v s = 2.5 v , c a v = 22 f , c f = 4.7 f , c c = 22f figure 19 . frequency response driving pin 1 ; negative dc output
data shee t ad737 rev. i | page 11 of 24 100v 10mv 1mv 1v 100mv 10v 0.1 1 10 100 1000 frequenc y (khz) input leve l (rms) 00828-021 v s = 2.5 v , c a v = 22 f , c f = 4.7 f , c c = 22f ?3db 0.5% 10% 1% figure 20 . error contours driving pin 1 0 1 2 3 4 5 1 2 3 4 5 crest f ac t or additiona l error (% of reading) 00828-022 c a v = 22f c a v = 10f c a v = 100f c a v = 220f c a v = 33f 3 cycles of 1khz 200mv rms v s = 2.5v c c = 22f c f = 100f figure 21 . additional error vs. crest factor for various values of c av ?2.5 0.5 ?0.5 ?1.0 ?1.5 ?2.0 0 1.0 10mv 100mv 1v 2v input leve l (rms) error (% of reading) 00828-023 c a v = 22 f , v s = 2.5v c c = 47 f , c f = 4.7f figure 22 . error vs. rms input level driving pin 1
ad737 data sheet rev. i | page 12 of 24 theory of operation as shown in figure 23 , the ad737 has four functional subsec - tions: an input amplifier, a full - wave rectifier, an rms core, and a bias section. the fet input amplifier allows a high imped ance, buffered input at p in 2 or a low impedance, wide dynamic range input at p in 1. the high impedance input, with its low input bias current, is ideal for use with high impedance input attenuators. the input signal can be either dc - coupled or ac - coupled to the input amplifier. unlike other rms converters, the ad737 permits both direct and indirect ac coupling of the inputs. ac coupling is provided by placing a series capacitor between the input signal and pin 2 (or pin 1) for direct coupling and between pin 1 and ground (while driving pin 2) for indirect coupling. rms translinear core 8 com +v s 7 6 output 5 c av current mode absolute value 1 2 3 power down 4 c a 33f ac c c = 10f c f 10f (optional lpf) v in ?v s ?v s +v s v in c c + optional return path 8k? + + dc bias section fet op amp i b < 10pa 8k? 00828-024 0.1 f 0.1 f common positive supply negative supply figure 23 . ad737 true rms circuit (test circuit) the output of the input ampli fier drives a full - wave precision rectifier, which , in turn, drives the rms core. it is the core that provides the essential rms operations of squaring, averaging, and square rooting, using an external averaging capacitor, c av . without c av , the rectified i nput signal passes through the core unprocessed, as is done with the average responding connection ( see figure 25 ). in the average responding mode, averaging is carried out by an rc post filter consi sting of an 8 k? internal scale factor resistor co nnected between pin 6 and pin 8 and an external averaging capacitor, c f . in the rms circuit, this addi - tional filtering stage reduces any output ripple that was not remov ed by the averaging capacitor . fin ally, the bias subsection permits a power - down function. this reduces the idle curre nt of the ad737 from 160 a to 30 a. this feature is selected b y connecting pin 3 to pin 7 ( +v s ) . types of ac measurem ent the ad737 is capable of measuring ac signals by operating as either an average responding converter or a true rms - to - dc con - verter. as its name implies, an average responding converter computes the average absolute value of an ac (or ac and dc) voltage or cur rent by full - wave rectifying and low - pass filtering the input signal; this approximates the average. the resulting output, a dc average level, is then scaled by adding (or reducing) gain; this scale factor conver ts the dc a verage reading to an rms equiva lent value for the waveform being measured. for example, the average absolute value of a sine wave voltage is 0.636 that of v peak ; the corresponding rms value is 0.707 times v peak . therefore, for sine wave voltage s, the required scale factor is 1.11 (0.707 divided by 0.636). in contrast to measuring the average value, true rms measure - ment 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. mathematically, the rms value of a voltage is defined (using a simplified equation) as ) ( 2 v avg v rms = this involves squaring the signal, taking the average, and then obtaining the square root. true rms converters are smart recti - fiers; they provide an accurate rms reading regardl ess of the type of waveform being mea sured. however, average respond ing converters can exhibit very high errors when their input signals deviate from their pr ecalibrated waveform; the magni tude of the error depends on the type of waveform being measured. a s an 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 tha n the true rms value (see table 5 ). the tra nsfer function for the ad737 is ) ( 2 in out v avg v =
data sheet ad737 rev. i | page 13 of 24 dc error, output ripple, and averaging error figure 24 shows the typical output waveform of the ad737 with a sine wave input voltage applied. as with all real-world devices, the ideal output of v out = v in is never exactly achieved; instead, the output contains both a dc and an ac error component. dc error = e o ? e o (ideal) average e o = e o e o ideal e o double-frequency ripple time 00828-026 figure 24. output waveform for sine wave input voltage as shown, the dc error is the difference between the average of the output signal (when all the ripple in the output has been removed by external filtering) and the ideal dc output. the dc error component is, therefore, set solely by the value of the averaging capacitor usedno amount of post filtering (using a very large postfiltering capacitor, c f ) allows the output voltage to equal its ideal value. the ac error component, an output ripple, can be easily removed using a large enough c f . in most cases, the combined magnitudes of the dc and ac error components must be considered when selecting appropriate values for c av and c f capacitors. this combined error, repre- senting the maximum uncertainty of the measurement, is termed the averaging error and is equal to the peak value of the output ripple plus the dc error. as the input frequency increases, both error components decrease rapidly. if the input frequency doubles, the dc error and ripple reduce to one-quarter and one-half of their original values, respectively, and rapidly become insignificant. ac measurement accuracy and crest factor the crest factor of the input waveform is often overlooked when determining 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). other 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 time periods between pulses. figure 9 shows the additional error vs. the crest factor of the ad737 for various values of c av . calculating settling time figure 17 can be used to closely approximate the time required for the ad737 to settle when its input level is reduced in ampli- tude. the net time required for the 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, an initial rms input level of 100 mv, and a final (reduced) input level of 1 mv. from figure 17, 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 inherent smoothness of the decay characteristic of a capacitor/diode combination, this is the total settling time to the final value (not the settling time to 1%, 0.1%, and so on, of the final value). also, this graph provides the worst-case settling time because the ad737 settles very quickly with increasing input levels. table 5. error introduced by an average respo nding circuit when measuring common waveforms type of waveform 1 v peak amplitude crest factor (v peak /v rms) true rms value (v) reading of an average responding circuit calibrated to an rms sine wave value (v) error (%) 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
ad737 data sheet rev. i | page 14 of 24 applications information rms meas urement choosing an optimum value f or c av because the external averaging capacitor, c av , holds the rec - tified input signal during rms computation, its value directly affects the accuracy of th e rms measure ment, especially at low frequen cies. furthermore, because the averaging capacitor is connected across a diode in the rms core, the averaging time constant ( av ) increases exponentially as the input signal decreases . it follows that decreasing the input signal decreases errors due to nonideal averaging but increases the settling time approaching the decreased rms - computed dc value. th us , diminishing input values allow the circuit to perform better (due to increased averaging) while increas ing th e waiting time between measurements. a trade - off must be made between computational accuracy and settling time when selecting c av . rapid settling times via the average responding connectio n because the average responding connection shown in figure 25 does not use an averaging capacitor, its settling time does not vary with input signal level; it is determined solely by the rc time constant of c f and the internal 8 k? output scaling resistor. positive supp l y +v s 0.1f ?v s 0.1f common neg a tive supp l y v out c c v in c f 33f 00828-025 com output ad737 bias section input amplifier 8k? 8k? power down ?v s +v s + c a v 1 2 3 4 8 7 6 5 full- w a ve rectifier rms core figure 25 . ad737 average responding circuit selectable a verage or rms c onversion for some applications, it is desirable to be able to select between rms - value - to - dc co nversion and average - value - to - dc conversion . if c av is dis connected from the root - mean core, t he ad737 full - wave rectifier is a highly accurate absolute value circuit . a cmos switch whose gate is controlled by a logic level selects between average and rms values. 00828-039 vin rms ?2.5v 1 8 7 6 5 4 3 2 c c v in com +v s out c av ?v s 33f 33f ad737 vout dc +2.5v 1m? rms avg ntr4501nt1 assumed to be a logic source fig ure 26 . cmos switch i s used to select rms or average responding modes selecting practical values f or capacitors table 6 provides practical values of c av and c f for several common applications. the input co upling capacitor, c c , in conjunction with the 8 k? internal input scaling resisto r, determines the ?3 db low fre quency roll - off. this frequency, f l , is equal to ( ) = 8000 2 1 (1) note that , at f l , the amplitude error is approximately ?30% (?3 db) of reading. to reduc e this error to 0.5% of 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, the ac coupling network at pin 2 is required i n addition to c apacit or c c . scaling input and output voltages the ad737 is an extremely flexible d evice . with minimal external circuitry, i t can be powered with single - or dual - polarity power supplies , and i nput and output voltages are independent ly scal able to accommodate non matching i/o devices. this section describes a few such applications . extending or scaling the i nput range for low supply voltage applications, the maximum peak voltage to the device is extended by simply ap plyi ng the input voltage to pin 1 across the internal 8 k ? input resistor. t he ad737 input circuit functions quasi - differentially , with a high impedance fet input at pin 2 (non inverting) and a low impedance input at pin 1 (inverting, see figure 25) . the internal 8 k ? resistor behaves as a voltage - to - current converter connected to the summing node of a feedback loop around the input amplifier . because the feedbac k loop acts to serv o the summing node volt age to match the voltage at pin 2, the max imum peak input voltage increase s until the internal circuit runs out of headroom , approximately double f or a symmetrical dual supply .
data shee t ad737 rev. i | page 15 of 24 battery operation all the level - shifting for battery operation is provided by the 3? digit converter , shown in figure 27 . alternatively, an external op amp adds flexibility by accommodating non zero common - mode voltage s and provi d in g output scaling and offset to zero. when an external operational amplifier is used, the output polarity is positive go ing . figure 28 shows an op amp us ed in a single - supply application. note that th e combined input resistor value ( r1 + r2 + 8 k? ) matches that of the r5 feedback resistor . in this instance , the magnitude s of the output dc voltage a nd the rms of the ac input are equal . r3 and r4 provide current to offset the output to 0 v. scaling the output voltage the output voltage can be scaled to the input rms voltage. for example, assume that the ad73 7 is retrofitted to an existing application using an averaging responding circuit (full - wave rectifier). t he power supply is 1 2 v, the input voltage is 10 v ac , and the desired output is 6 v dc . for convenience, use the same combined input resistance as shown in figure 28. calculate the rms input current as outmag inmag i i = = + + = a 125 k 8 k 2.5 k 69.8 v 10 (2) next, using the i out mag value from equation 2 , calculate the new feedback r esistor value (r5) required for 6 v output using k 48.1 a 125 v 6 = = r5 ( 3) select the closest - value standard 1% resistor, 47.5 k ? . because the supply is 12 v, the co mmon - mode voltage at the r7/r8 divider is 6 v , and the combined resistor value ( r3 + r4 ) is equal to the feedback resistor, or 47.5 k ? . r2 is us ed to calibrate the transfer function (gain) , and r4 sets the output volta ge to zero with no input voltage . perform calibration as follows: 1. with no ac input applied, adjust r4 for 0 v. 2. apply a known input to the input. 3. adjust the r2 trimmer until the input and output match. the op amp selected for any single - supply application m ust be a rail - to - rail type, for example a n ad8541 , as shown in figure 28. for higher voltages , a higher voltage part , such as an op196 , can be used. when calibrating to 0 v, the specified voltage above ground for the operational a mplifier must be taken into account. a djust r4 slightly higher as appropriate. table 6 . ad737 capacitor selection application rms inp ut evel o freuency cutoff 3 db maimum crest f actor c a 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 general - purpose average responding 0 v to 1 v 20 hz none 33 1.2 s ec 200 hz none 3.3 120 ms 0 mv to 200 mv 20 hz none 33 1.2 s ec 200 hz none 3.3 120 ms scr waveform measurement 0 mv to 200 mv 50 hz 5 100 33 1.2 s ec 60 hz 5 82 27 1.0 s ec 0 mv to 100 mv 50 hz 5 50 33 1.2 s ec 60 hz 5 47 27 1.0 s ec 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 s ec 1 settling time is specified over the stated rms input level with the input signal increasing from zero. settling times are greater for decreasing amplitude in put signals .
ad737 data sheet rev. i | page 16 of 24 com +v ad589 1.23v c av c c power down 0.1f c c 10f switch closed activates power-down mode. ad737 draws just 40 a in this mode 2v 20v 200v 9m? 900k? 90k? 10k? v in 200mv v in ?v s + + +v s + 1f output 1m? ?v s +v s 1n4148 1n4148 47k? 1w 1f + common 33f ref low ref high 3 1 / 2 digit icl7136 type converter low high analog 9v 200k? 20k? 50k? + 1prv 0.01f rms core ad737 bias section input amplifier 8k? 8k? 1 2 3 4 8 7 6 5 00828-027 full-wave rectifier figure 27 . 3? digit dvm circuit com input c a v c a v 33f c f 0.47f c1 0.47f c5 1f c4 2.2f r7 100k? r4 5k? r2 5k? r3 78.7k? r5 80.6k? r1 69.8k? 1% r8 100k? c3 0.01f 0.01f c2 0.01f c c power down input scale f ac t or adj v in ?v s output +v s + output ad737 + 5v 5v 2.5v ad8541ar 5v nc nc = no connect output zero adjust 1 2 3 4 8 7 6 5 1 4 7 5 3 6 2 00828-028 figure 28 . battery - powered operation for 200 mv maximum rms full - scale input v out rms core c c v in c f 10f 00828-029 com output ad737 bias section input amplifier scale f ac t or adjust 8k? 8k? power down ?v s +v s + c a v 1 2 3 4 8 7 6 5 100? 200? c a v 33f c c 10f + + full- w a ve rectifier figure 29 . external scale factor trim
data shee t ad737 rev. i | page 17 of 24 + r cal ** r1** i ref 10 * 1 1 9 q2 **r1 + r cal in ? = 10,000 4.3v 0db input leve l in v ad7 1 1 1k? 3500ppm/c 60.4? db output 100mv/db precision resis t or cor p type pt/st 2k? 31.6k? scale f ac t or trim 13 q1 12 14 * rms core ad737 bias section input amplifier 8k? 8k? 1 2 3 4 8 7 6 5 3 6 2 v in power down ?v s c c com output +v s nc c a v c a v + 00828-030 c c 10f nc = no connect *q1, q2 p art of rc a ca3046 or similar npn transis t or arr a y . full- w a ve rectifier figure 30 . db output connection com 1 2 3 8 7 6 v out +v s ?v s +v s c c v in power down ad737 input amplifier offset adjust 500k? 8k? 00828-031 scale f ac t or adjust 1k? 1m? 1k? 499? full- w a ve rectifier figure 31 . dc - coupled offset voltage and scale facto r trims
ad737 data sheet rev. i | page 18 of 24 ad737 eval uation board an evaluation board , ad737 - e va l z , is available f or experi - ments or for becoming familiar with rms - to - dc converters . figure 32 is a pho t ograph of the board ; figure 34 to figure 37 show the signal and power plane copper patterns. the board is designed for multi p urpose applications and can be used for the ad736 a s well. although not shipped with the board, a n o ptional socket t hat accepts the 8-lead surface - mount package is av ailable from enplas corp . 00828-038 figure 32 . ad737 evaluation board 00828-032 figure 33 . ad737 evaluation board component - side s ilkscreen as described in the applications information section, the ad73 7 can be connected in a variety of ways. a s shipped , the board is configured for dual supplies with the high imped ance input connected and the power - down feature disa bled . jumpers are provided for connecting the in put to the low impedance input ( p in 1 ) a nd for dc connec tions to either input. the schematic with movable jumpers is shown in figure 38 . the jumper positions in black are default connections ; the dotted - outline jumpers are optional connections. the board is tested prio r to shipment and requires only a power supply connection and a precision meter to perform measurements. 00828-033 figure 34 . ad737 evaluation board component - side copper 00828-034 figure 35 . ad737 evaluation board secondary - side copper 00828-035 figure 36 . ad737 evaluation board internal power plane 00828-036 figure 37 . ad737 evaluation board internal ground p lane
data shee t ad737 rev. i | page 19 of 24 j1 c1 10f 25v c2 10f 25v ?v s 1 8 7 6 5 4 3 2 c c power down com +v s ?v s output c av + v s c in 0.1f c av 3 3 f 1 6 v + w1 dc coup dut ad737 p2 hi-z sel gnd in hi-z w4 lo-z in sel pin3 filt pd norm + c4 0.1f j2 v out + c6 0.1 f cc + r3 0? c f2 r4 0? w2 r1 1m? v in ?v s +v s ?v s +v s gnd1 gnd3 gnd2 gnd4 c f1 w3 ac coup cav lo-z j3 v in +v s 00828-037 figure 38 . ad737 evaluation board s chematic
ad737 data sheet rev. i | page 20 of 24 outline dimensions controlling dimensions are in millimeters; inch dimensions (in p arentheses) are rounded-off millimeter equiv alents for reference onl y and are not appropria te for use in design. compliant t o jedec st andards ms-012-aa 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) sea 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) coplanarity 0.10 figure 39 . 8 - lead standard small outline package [soic_n] narrow body (r - 8) dimensions shown in millimeters and (inches) 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) sea 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 40 . 8 - lead plastic dual - in - line package [pdip] (n - 8) dimensions shown in inches an d (millimeters)
data shee t ad737 rev. i | page 21 of 24 ordering guide model 1 temperature range package description package option ad737 anz ?40c to +85c 8 - lead plastic dual in - line package [pdip] n -8 ad737 arz ?40c to +85c 8 - lead standard small outline package [soic _n ] r -8 ad737 jnz 0c to 70c 8 - lead plastic dual in - line package [pdip] n -8 ad737 jrz 0c to 70c 8 - lead standard small outline package [soic _n ] r -8 ad737 jrz -r7 0c to 70c 8 - lead standard small outline package [soic _n ] r -8 ad737 jrz -rl 0c to 70c 8 - lead standard small outli ne package [soic _n ] r -8 ad737 jrz -5 0c to 70c 8 - lead standard small outline package [soic _n ] r -8 ad737 jrz -5 - r7 0c to 70c 8 - lead standard small outline package [soic _n ] r -8 ad737 jrz -5 - rl 0c to 70c 8 - lead standard small outline package [soic _n ] r -8 ad737 kr - reel 0c to 70c 8 - lead standard small outline package [soic _n ] r -8 ad737 kr - reel7 0c to 70c 8 - lead standard small outline package [soic _n ] r -8 ad737 krz -rl 0c to 70c 8 - lead standard small outline pa ckage [soic _n ] r -8 ad737 krz - r7 0c to 70c 8 - lead standard small outline package [soic_n] r - 8 ad737 - evalz evaluation board 1 z = rohs compliant part.
ad737 data sheet rev. i | page 22 of 24 notes
data shee t ad737 rev. i | page 23 of 24 notes
ad737 data sheet rev. i | page 24 of 24 notes ? 2012 analog devices, inc. all rights reserved. trademarks and registered trademarks are the property of their respective owners. d00828 - 0 - 6/12(i)

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