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low cost, low power, true rms-to-dc converter ad737 rev. h information furnished by analog devices is believed to be accurate and reliable. however, no responsibility is assumed by analog devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. specifications subject to change without notice. no license is granted by implication or otherwise under any patent or patent rights of analog devices. trademarks and registered trademarks are the property of their respective owners. one technology way, p.o. box 9106, norwood, ma 02062-9106, u.s.a. tel: 781.329.4700 www.analog.com fax: 781.461.3113 ?2008 analog devices, inc. all rights reserved. features computes true rms value average rectified value absolute value provides 200 mv full-scale input range (larger inputs with input attenuator) direct interfacing with 3? digit cmos adcs high input impedance: 10 12 low input bias current: 25 pa maximum 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: 160 a maximum supply current no external trims needed for specified accuracy a general-purpose, buffered voltage output version also available (ad736) functional block diagram c c v in ad737 com output full-wave rectifier bias section rms core input amplifier 8k ? 8k ? power down ?v s +v s c av 1 2 3 4 8 7 6 5 00828-001 figure 1. general description the ad737 1 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. furthermore, it maintains high accuracy while measuring a wide range of input waveforms, including variable duty cycle pulses and triac (phase) controlled sine waves. the low cost and small physical size of this converter make it suitable for upgrading the performance 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 compute 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 portable multimeters and other battery- powered applications. this converter also provides a power-down feature that reduces the power-supply standby current to less than 30 a. two signal input terminals are provided in the ad737. a high impedance (10 12 ) fet input interfaces directly with high r 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 inputs can be 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 four performance grades. the ad737j and ad737k grades are rated over the commercial temperature range of 0c to 70c. the ad737jr-5 is tested with supply voltages of 2.5 v dc. the ad737a and ad737b grades are rated over the industrial temperature range of ?40c to +85c. the ad737 is available in three low cost, 8-lead packages: pdip, soic_n, and cerdip. product highlights 1. 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 ad737 to perform true rms measurement. 3. the low power consumption of 0.72 mw makes the ad737 suitable for battery-powered applications. 1 protected under u.s. patent number 5,495,245.
ad737 rev. h | 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 va lu e 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 .......................................................................... 22 ? revision history 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
ad737 rev. h | 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 specified. 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. ad737a, ad737j ad737b, ad737k ad737j-5 parameter conditions min typ max min typ max min typ max unit 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 aq and bq e in = 200 mv rms 0.5/0.7 0.3/0.5 por/c jn, jr, kn, 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 vs. 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 in = 200 mv dc, v s = 2.5 v 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 factors 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 factors 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 rev. h | page 4 of 24 ad737a, ad737j ad737b, ad737k ad737j-5 parameter conditions min typ max min typ max min typ max unit 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 input (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 vs. 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 voltage swing v s = +2.8 v/?3.2 v ?1.6 ?1.7 ?1.6 ?1.7 v v s = 5 v ?3.3 ?3.4 ?3.3 ?3.4 v v s = 16.5 v ?4 ?5 ?4 ?5 v v s = 2.5 v, input to pin 1 ?1.1 C0.9 v 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
ad737 rev. h | page 5 of 24 ad737a, ad737j ad737b, ad737k ad737j-5 parameter conditions min typ max min typ max min typ max unit 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 reading. 2 nonlinearity is defined as the maximum de viation (in percent error) fr om a straight line connecting the readings at 0 v and at 200 mv rms. 3 after fourth-order error correction using the equation y = ? 0.31009 x 4 ? 0.21692 x 3 ? 0.06939 x 2 + 0.99756 x + 11.1 10 ?6 where y is the corrected result and x is the device output between 0.01 v and 0.3 v. 4 crest factor error is specifie d as the additional erro r resulting from the specific crest factor, using a 200 mv rms signal as a reference. the crest factor is defined as v peak /v rms. 5 dc offset does not limit ac resolution.
ad737 rev. h | page 6 of 24 absolute maximum ratings table 2. parameter rating supply voltage 16.5 v internal power dissipation 200 mw input voltage v s output short-circuit duration indefinite differential input voltage +v s and ?v s storage temperature range cerdip (q-8) ?65c to +150c pdip (n-8) and soic_n (r-8) ?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 resistance 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 cerdip (q-8) 110 c/w 8-lead pdip (n-8) 165 c/w 8-lead soic_n (r-8) 155 c/w esd caution
ad737 rev. h | page 7 of 24 pin configurations and function descriptions c c 1 v in 2 power down 3 ?v s 4 com 8 +v s 7 output 6 c av 5 ad737 top view (not to scale) 00828-002 1 2 3 4 8 7 6 5 ad737 c c v com +v in power down ?v s s output c av 00828-003 top view (not to scale) 1 8 2 3 4 7 6 5 ad737 top view (not to scale) c c v in power down ?v s com +v s output c av 00828-004 figure 2. soic_n pin configuration (r-8) figure 3. cerdip pin configuration (q-8) figure 4. pdip pin configuration (n-8) table 4. pin function descriptions 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 Cv s negative power supply. 5 c av averaging capacitor. 6 output output. 7 +v s positive power supply. 8 com common.
ad737 rev. h | page 8 of 24 typical performance characteristics t a = 25c, v s = 5 v (except ad737j-5, where v s = 2.5 v), c av = 33 f, cc = 10 f, f = 1 khz, sine wave input applied to pin 2, unless otherwise specified. v in = 200mv rms c av = 100f c f = 22f ?0.5 04 28 61 2 1 4 10 16 additional error (% of reading) 00828-005 ?0.3 ?0.1 0 0.3 0.1 0.5 0.7 supply voltage (v) figure 5. additional error vs. supply voltage 100v 1mv 10mv 1v 100mv 10 v 0 04 28 61 2 1 4 10 16 peak input before clipping (v) 00828-006 2 4 6 8 12 10 14 16 supply voltage (v) pin 1 pin 2 dc coupled figure 6. maximum input level vs. supply voltage 5 024681012141618 dual supply voltage (v) 00 10 20 15 25 supply current (a) 828-007 figure 7. supply current (power-down mode) vs. supply voltage (dual) 0.1 1 100 10 1000 frequency (khz) input level (rms) 00828-008 c av = 22f, c f = 4.7f, c c = 22f 1% error ?3db 10% error figure 8. frequency response driving pin 1 100v 1mv 10mv 1v 100mv 10 v 0.1 1 100 10 1000 frequency (khz) input level (rms) 00828-009 c av = 22f, c f = 4.7f, c c = 22f 1% error 10% error ?3db figure 9. frequency response driving pin 2 c av = 100f c av = 250f 0 1 2 3 4 5 6 12345 additional error (% of reading) 00828-010 crest factor (v peak /v rms) c av = 10f c av = 33f 3ms burst of 1khz = 3 cycles 200mv rms signal c c = 22f c f = 100f figure 10. additional error vs. crest factor
ad737 rev. h | page 9 of 24 v in = 200mv rms c av = 100f c f = 22f ?0.8 ?60 ?20?40 20 0 60 80 100 120 40 140 temperature (c) 0082 ?0.6 ?0.2 ?0.4 0 0.4 0.2 0.6 0.8 additional error (% of reading) 8-011 figure 11. additional error vs. temperature ?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) 00828-014 c av = 22f, c c = 47f, c f = 4.7f 1 10 100 10 100 1k frequency (hz) averaging capacitor (f) 00828-015 figure 14. error vs. rms input level using circuit in figure 30 0 dc supply current (a) 00828-012 200 100 400 300 500 0 0.2 0.4 0.6 0.8 1.0 rms input level (v) figure 12. dc supply current vs. rms input level 10v 100v 1mv 10m v 100 1k 10k 100k ?3db frequency (hz) 00828-013 input level (rms) ac coupled figure 13. rms input level vs. C3 db frequency ?1% ?0.5% v in = 200mv rms c c = 47f c f = 47f figure 15. value of averaging capacitor vs. frequency for specified averaging error 1mv 10mv 100mv 1 v 1 10 100 1k frequency (hz) 0082 input level (rms) 8-016 ?0.5% ?1% ac coupled c av = 10f, c c = 47f, c = 47f f figure 16. rms input level vs. frequency for specified averaging error
ad737 rev. h | page 10 of 24 100fa 10n a 1.0 02468 1214 10 16 supply voltage (v) 0082 1.5 2.0 2.5 3.0 4.0 3.5 input bias current (pa) 8-017 figure 17. input bias current vs. supply voltage 1na 100pa 10pa 1pa ?55 ?35 ?15 5 25 65 85 105 45 125 temperature (c) input bias current 00828-019 100v 1mv 10mv 100mv 1 v 1ms 10ms 100ms 1s 10s 100s settling time input level (rms) 00828-018 c c = 22f c f = 0f c av = 10f c av = 33f c av = 100f figure 18. rms input level vs. settling time for three values of c av figure 19. input bias current vs. temperature 100v 10mv 1mv 1v 100mv 10 v 0.1 1 10 100 1000 frequency (khz) input level (rms) 00828-020 v s =2.5v, c av = 22f, c f = 4.7f, c c = 22f figure 20. frequency response driving pin 1
ad737 rev. h | page 11 of 24 100v 10mv 1mv 1v 100mv 10 v 0.1 1 10 100 1000 frequency (khz) 0082 ?2.5 0.5 ?0.5 ?1.0 ?1.5 ?2.0 0 1.0 10mv 100mv 1v 2v input level (rms) error (% of reading) 00828-023 v s =2.5v, c av = 22f, c f = 4.7f, c c = 22f input level (rms) 8-021 0.5% ?3db 10% 1% c av = 22f, v s = 2.5v c c = 47f, c f = 4.7f figure 21. error contours driving pin 1 figure 23. error vs. rms input level driving pin 1 0 1 2 3 4 5 12345 crest factor additional error (% of reading) 00828-022 c av = 22f c av = 10f c av = 100f c av = 220f c av = 33f 3 cycles of 1khz 200mv rms v s = 2.5v c c = 22f c f = 100f figure 22. additional error vs. crest factor for various values of c av
ad737 rev. h | page 12 of 24 theory of operation as shown in figure 24 , 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 impedance, buffered input at pin 2 or a low impedance, wide dynamic range input at pin 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 p o wer down 4 c a 33f ac c c = 10f c f 10f (optional lpf) v in ?v s +v s v in c c ?v s + optional return path 8k ? + + dc bias section fet op amp 1 b <10pa 8k ? 00828- 024 0.1f 0.1f common positive supply negative supply figure 24. ad737 true rms circuit (test circuit) the output of the input amplifier 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 input signal passes through the core unprocessed, as is done with the average responding connection (see figure 26 ). in the average responding mode, averaging is carried out by an rc post filter consisting of an 8 k internal scale factor resistor connected 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 removed by the averaging capacitor. finally, the bias subsection permits a power-down function. this reduces the idle current of the ad737 from 160 a to 30 a. this feature is selected by connecting pin 3 to pin 7 (+v s ). types of ac measurement 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 current 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 converts the dc average reading to an rms equivalent 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 voltages, 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 vavg 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 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. as 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 than the true rms value (see table 5 ). the transfer function for the ad737 is )( 2 in out vavg v =
ad737 rev. h | page 13 of 24 average e o = e o dc error, output ripple, and averaging error figure 25 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) e o ideal e o double-frequency ripple time 00828-026 figure 25. 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 10 shows the additional error vs. the crest factor of the ad737 for various values of c av . calculating settling time figure 18 can be used to closely approximate the time required for the ad737 to settle when its input level is reduced in amplitude. 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 18 , 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 rev. h | page 14 of 24 applications information rms measurementchoosing an optimum value for c av vin rms ?2.5v 1 8 7 6 5 4 3 2 cc v in com +v s out c av ?v s 33f 33f ad737 vout dc +2.5v 1m ? because the external averaging capacitor, c av , holds the rec- tified input signal during rms computation, its value directly affects the accuracy of the rms measurement, especially at low frequencies. 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. thus, diminishing input values allow the circuit to perform better (due to increased averaging) while increasing the 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 connection because the average responding connection shown in figure 26 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 supply +v s 0.1f common ?v s 0.1f negative supply 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 av 1 2 3 4 8 7 6 5 full-wave rectifier rms core figure 26. ad737 average responding circuit selectable average or rms conversion for some applications, it is desirable to be able to select between rms-value-to-dc conversion and average-value-to-dc conversion. if c av is disconnected from the root-mean core, the 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 rms avg ntr4501nt1 assumed to be a logic source figure 27. cmos switch is used to se lect rms or average responding modes selecting practical values for 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, determines the ?3 db low frequency roll-off. this frequency, f l , is equal to () faradsinc f c l = 80002 1 (1) note that, at f l , the amplitude error is approximately ?30% (?3 db) of reading. to reduce 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 in addition to capacitor c c . scaling input and output voltages the ad737 is an extremely flexible device. with minimal external circuitry, it can be powered with single- or dual- polarity power supplies, and input and output voltages are independently scalable to accommodate nonmatching i/o devices. this section describes a few such applications. extending or scaling the input range for low supply voltage applications, the maximum peak voltage to the device is extended by simply applying the input voltage to pin 1 across the internal 8 k input resistor. the ad737 input circuit functions quasi-differentially, with a high impedance fet input at pin 2 (noninverting) and a low impedance input at pin 1 (inverting, see figure 26 ). 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 feedback loop acts to servo the summing node voltage to match the voltage at pin 2, the maximum peak input voltage increases until the internal circuit runs out of headroom, approximately double for a symmetrical dual supply.
ad737 rev. h | page 15 of 24 battery operation all the level-shifting for battery operation is provided by the 3? digit converter, shown in figure 28 . alternatively, an external op amp adds flexibility by accommodating nonzero common-mode voltages and providing output scaling and offset to zero. when an external operational amplifier is used, the output polarity is positive going. figure 29 shows an op amp used in a single-supply application. note that the combined input resistor value (r1 + r2 + 8 k) matches that of the r5 feedback resistor. in this instance, the magnitudes of the output dc voltage and 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 ad737 is retrofitted to an existing application using an averaging responding circuit (full-wave rectifier). the power supply is 12 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 29 . 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 outmag value from equation 2, calculate the feedback resistor required for 6 v output using k 48.1 a 125 v 6 = = fb r (3) select the closest-value standard 1% resistor, 47.5 k. because the supply is 12 v, the common-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 used to calibrate the transfer function (gain), and r4 sets the output voltage 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 must bea rail-to-rail type, for example an ad8541, as shown in figure 29 . 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 amplifier must be taken into account. adjust r4 slightly higher as appropriate. table 6. ad737 capacitor selection application rms input level low frequency cutoff (?3 db) maximum 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 general-purpose average responding 0 v to 1 v 20 hz none 33 1.2 sec 200 hz none 3.3 120 ms 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 stat ed rms input level with the input signal increasing from zero. settling times are grea ter for decreasing amplitude input signals.
ad737 rev. h | 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 40a in this mode 2v 20v 200v 9m ? 900k? 90k ? 10k ? v in 200mv v in ?v s + + +v s + 1f output 1m ? +v s 1n4148 1n4148 ?v s 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 full-wave rectifier 00828-027 figure 28. 3? digit dvm circuit input scale factor adj com inpu c av c av 33f c f 0.47f c1 0.47f c5 c4 2.2f r7 100k ? r4 5k? r2 5k? r3 78.7k ? r5 80.6k ? r1 69.8k ? 1% r8 c3 0.01f 0.01f c2 0.01f c c power down t 1f 100k ? v in ?v s output +v s + output ad737 + 5v 5v 2.5v ad8541ar 5v nc nc = no connect 1 8 output zero adjust 2 3 4 7 1 6 2 7 5 4 5 3 6 00828-028 figure 29. 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 factor adjust 8k ? 8k? power down ?v s +v s + c av 1 2 3 4 8 7 6 5 100? 200? c av 33f c c 10f + full-wave rectifier + figure 30. external scale factor trim
ad737 rev. h | page 17 of 24 + r cal ** r1** i ref 10 * 11 9 q2 **r1 + r cal in ? = 10,000 4.3v 0db input level in v ad711 1k ? 3500ppm/c 60.4 ? 13 q1 12 14 * precision resistor corp type pt/st 2k ? 31.6k ? scale factor trim db outpu t 100mv/db 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 av c av + 00828-030 c c 10f nc = no connect *q1, q2 part of rca ca3046 or similar npn transistor array. full-wave rectifier figure 31. 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 a djust 500k ? 8k? scale factor adjust 1k? 1m ? 1k? 499 ? 00828-031 full-wave rectifier figure 32. dc-coupled offset voltage and scale factor trims
ad737 rev. h | page 18 of ad737 evaluation board an evaluation board, ad737-evalz, is available for experi- ments or for becoming familiar with rms-to-dc converters. figure 33 is a photograph of the board; figure 35 to figure 38 show the signal and power plane copper patterns. the board is designed for multipurpose applications and can be used for the ad736 as well. although not shipped with the board, an optional socket that accepts the 8-lead surface mount package is available from enplas corp. 00828-033 figure 35. ad737 evaluation boardcomponent-side copper 00828- 038 figure 33. ad737 evaluation board 00828-034 figure 36. ad737 evaluation boardsecondar y-side copper 00828- 032 figure 34. ad737 evaluation boardcomponent-side silkscreen as described in the applications information section, the ad737 can be connected in a variety of ways. as shipped, the board is configured for dual supplies with the high impedance input connected and the power-down feature disabled. jumpers are provided for connecting the input to the low impedance input (pin 1) and for dc connections to either input. the schematic with movable jumpers is shown in figure 39 . the jumper positions in black are default connections; the dotted-outline jumpers are optional connections. the board is tested prior to shipment and requires only a power supply connection and a precision meter to perform measurements. tabl e 7 provides a bill of materials for the ad737 evaluation board. 00828-035 figure 37. ad737 evaluation boardinternal power plane 00828-036 figure 38. ad737 evaluation boardinternal ground plane 24
ad737 rev. h | 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 33 f 16v + 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 0.1f cc + r3 0 ? c f2 r4 0 ? w2 r1 1m ? ?v s + v v in s ?v s +v s gnd1 gnd3 gnd2 gnd4 c f1 w3 ac coup cav lo-z v in +v s 00828-037 j3 figure 39. ad737 evaluation board schematic table 7. ad737 evaluation board bill of materials qty name description reference designator manufacturer mfg. part number 1 test loop red +v s components corp. tp-104-01-02 1 test loop green ?v s components corp. tp-104-01-05 2 capacitor tantalum 10 f, 25 v c1, c2 nichicon f931e106mcc 3 capacitor 0.1 f, 16 v, 0603, x7r c4, c6, c in kemet c0603c104k4ractu 1 capacitor tantalum 33 f, 16v, 20%, 6032 cav nichicon f931c336mcc 5 test loop purple c av , hi-z, lo-z, v in , v out components corp. tp-104-01-07 1 integrated circuit rms-to-dc converter dut analog devices, inc. ad737jrz 4 test loop black gnd1, gnd2, gnd3, gnd4 components corp. tp-104-01-00 2 connector bnc, right angle j1, j2 amp 227161-1 1 header 6 pins, 2 3 j3 3m 929836-09-03 1 header 3 pins p2 molex 22-10-2031 1 resistor 1 m, 1/10 w, 1%, 0603 r1 panasonic erj3ekf1004v 2 resistor 0 , 5%, 0603 r3, r4 panasonic erj3gey0r00v 4 header 2 pins, 0.1" center w1, w2, w3, w4 molex 22-10-2021
ad737 rev. h | page 20 of 24 (in parentheses) are rounded-off millimeter equivalents for reference only and are not appropriate for use in design. 012407- outline dimensions controlling dimensions are in millimeters; inch dimensions compliant to jedec standards ms-012-a a 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) seating plane 0.25 (0.0098) 0.10 (0.0040) 4 1 85 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 40. 8-lead standard small outline package [soic_n] narrow body (r-8) dimensions shown in millimeters and (inches) controlling dimensions are in inches; millimeter dimensions 0.310 (7.87) 0.220 (5.59) (in parentheses) are rounded-off inch equivalents for reference only and are not appropriate for use in design. 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) 14 5 8 figure 41. 8-lead ceramic dual in-line package [cerdip] (q-8) dimensions shown in inches and (millimeters)
ad737 rev. h | page 21 of 24 compliant to jedec standards ms-001 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. corner leads may be configured as whole or half leads. 070606-a 0.022 (0.56) 0.018 (0.46) 0.014 (0.36) seating plane 0.015 (0.38) min 0.210 (5.33) max 0.150 (3.81) 0.130 (3.30) 0.115 (2.92) 0.070 (1.78) 0.060 (1.52) 0.045 (1.14) 8 1 4 5 0.280 (7.11) 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) 0.115 (2.92) 0.015 (0.38) gauge plane 0.005 (0.13) min figure 42. 8-lead plastic dual-in-line package [pdip] (n-8) dimensions shown in inches and (millimeters)
ad737 rev. h | page 22 of 24 ordering guide model temperature range package description package option ad737an ?40c to +85c 8-lead plastic dual in-line package [pdip] n-8 ad737anz 1 ?40c to +85c 8-lead plastic dual in-line package [pdip] n-8 ad737aq ?40c to +85c 8-lead ceramic dual in-line package [cerdip] q-8 ad737ar ?40c to +85c 8-lead standard small outline package [soic_n] r-8 ad737arz 1 ?40c to +85c 8-lead standard small outline package [soic_n] r-8 ad737bq ?40c to +85c 8-lead ceramic dual in-line package [cerdip] q-8 ad737jn 0c to 70c 8-lead plastic dual in-line package [pdip] n-8 ad737jnz 1 0c to 70c 8-lead plastic dual in-line package [pdip] n-8 ad737jr 0c to 70c 8-lead standard small outline package [soic_n] r-8 ad737jr-reel 0c to 70c 8-lead standard small outline package [soic_n] r-8 ad737jr-reel7 0c to 70c 8-lead standard small outline package [soic_n] r-8 ad737jr-5 0c to 70c 8-lead standard small outline package [soic_n] r-8 ad737jr-5-reel 0c to 70c 8-lead standard small outline package [soic_n] r-8 ad737jr-5-reel7 0c to 70c 8-lead standard small outline package [soic_n] r-8 ad737jrz 1 0c to 70c 8-lead standard small outline package [soic_n] r-8 ad737jrz-r7 1 0c to 70c 8-lead standard small outline package [soic_n] r-8 ad737jrz-rl 1 0c to 70c 8-lead standard small outline package [soic_n] r-8 ad737jrz-5 1 0c to 70c 8-lead standard small outline package [soic_n] r-8 ad737jrz-5-r7 1 0c to 70c 8-lead standard small outline package [soic_n] r-8 AD737JRZ-5-RL 1 0c to 70c 8-lead standard small outline package [soic_n] r-8 ad737kn 0c to 70c 8-lead plastic dual in-line package [pdip] n-8 ad737knz 1 0c to 70c 8-lead plastic dual in-line package [pdip] n-8 ad737kr 0c to 70c 8-lead standard small outline package [soic_n] r-8 ad737kr-reel 0c to 70c 8-lead standard small outline package [soic_n] r-8 ad737kr-reel7 0c to 70c 8-lead standard small outline package [soic_n] r-8 ad737krz 1 0c to 70c 8-lead standard small outline package [soic_n] r-8 ad737krz-rl 1 0c to 70c 8-lead standard small outline package [soic_n] r-8 ad737krz-r7 1 0c to 70c 8-lead standard small outline package [soic_n] r-8 ad737-evalz 1 evaluation board 1 z = rohs compliant part.
ad737 rev. h | page 23 of 24 notes
ad737 rev. h | page 24 of 24 notes ?2008 analog devices, inc. all rights reserved. trademarks and registered trademarks are the prop erty of their respective owners. d00828-0-10/08(h)

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