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high precision, wideband rms-to-dc converter ad637 rev. k 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 ?2011 analog devices, inc. all rights reserved. features high accuracy 0.02% maximum nonlinearity, 0 v to 2 v rms input 0.10% additional error to crest factor of 3 wide bandwidth 8 mhz at 2 v rms input 600 khz at 100 mv rms computes true rms square mean square absolute value db output (60 db range) chip select/power-down feature allows analog three-state operation quiescent current reduction from 2.2 ma to 350 a 14-lead sbdip, 14-lead low cost cerdip, and 16-lead soic_w functional block diagram absolute value den input rms out db output buff in buff out 25k ? 25k ? common cs output offset 00788-001 squarer/ divider bias v in c av ad637 figure 1. general description the ad637 is a complete, high accuracy, monolithic rms-to-dc converter that computes the true rms value of any complex waveform. it offers performance that is unprecedented in integrated circuit rms-to-dc converters and comparable to discrete and modular techniques in accuracy, bandwidth, and dynamic range. a crest factor compensation scheme in the ad637 permits measurements of signals with crest factors of up to 10 with less than 1% additional error. the wide band- width of the ad637 permits the measurement of signals up to 600 khz with inputs of 200 mv rms and up to 8 mhz when the input levels are above 1 v rms. as with previous monolithic rms converters from analog devices, inc., the ad637 has an auxiliary db output available to users. the logarithm of the rms output signal is brought out to a separate pin, allowing direct db measurement with a useful range of 60 db. an externally programmed reference current allows the user to select the 0 db reference voltage to correspond to any level between 0.1 v and 2.0 v rms. a chip select connection on the ad637 permits the user to decrease the supply current from 2.2 ma to 350 a during periods when the rms function is not in use. this feature facilitates the addition of precision rms measurement to remote or handheld applications where minimum power consumption is critical. in addition, when the ad637 is powered down, the output goes to a high impedance state. this allows several ad637s to be tied together to form a wideband true rms multiplexer. the input circuitry of the ad637 is protected from overload voltages in excess of the supply levels. the inputs are not damaged by input signals if the supply voltages are lost. the ad637 is available in accuracy grade j and grade k for commercial temperature range (0c to 70c) applications, accuracy grade a and grade b for industrial range (?40c to +85c) appli- cations, and accuracy grade s rated over the ?55c to +125c temperature range. all versions are available in hermetically sealed, 14-lead sbdip, 14-lead cerdip, and 16-lead soic_w packages. the ad637 computes the true root mean square, mean square, or absolute value of any complex ac (or ac plus dc) input waveform and gives an equivalent dc output voltage. the true rms value of a waveform is more useful than an average rectified signal because it relates directly to the power of the signal. the rms value of a statistical signal is also related to the standard deviation of the signal. the ad637 is laser wafer trimmed to achieve rated performance without external trimming. the only external component required is a capacitor that sets the averaging time period. the value of this capacitor also determines low frequency accuracy, ripple level, and settling time. the on-chip buffer amplifier can be used either as an input buffer or in an active filter configuration. the filter can be used to reduce the amount of ac ripple, thereby increasing accuracy.
ad637 rev. k | page 2 of 20 table of contents features .............................................................................................. 1 ? functional block diagram .............................................................. 1 ? general description ......................................................................... 1 ? revision history ............................................................................... 2 ? specifications..................................................................................... 3 ? absolute maximum ratings............................................................ 5 ? esd caution.................................................................................. 5 ? pin configurations and function descriptions ........................... 6 ? functional description .................................................................... 7 ? standard connection ................................................................... 8 ? chip select..................................................................................... 8 ? optional trims for high accuracy ............................................ 8 ? choosing the averaging time constant....................................9 ? frequency response .................................................................. 11 ? ac measurement accuracy and crest factor ........................ 12 ? connection for db output........................................................ 12 ? db calibration............................................................................. 13 ? low frequency measurements................................................. 14 ? vector summation ..................................................................... 14 ? evaluation board ............................................................................ 16 ? outline dimensions ....................................................................... 19 ? ordering guide .......................................................................... 20 ? revision history 2/11rev. j to rev. k changes to figure 15...................................................................... 11 changes to figure 16...................................................................... 12 changes to evaluation board section and figure 23................. 16 added figure 24; renumbered sequentially .............................. 17 changes to figure 25 through figure 29.................................... 17 changes to figure 30...................................................................... 18 added figure 31.............................................................................. 18 deleted table 6; renumbered sequentially ................................ 18 changes to ordering guide .......................................................... 20 4/07rev. i to rev. j added evaluation board section ................................................. 16 updated outline dimensions ....................................................... 20 10/06rev. h to rev. i changes to table 1............................................................................ 3 changes to figure 4.......................................................................... 7 changes to figure 7.......................................................................... 9 changes to figure 16, figure 18, and figure 19 ......................... 12 changes to figure 20...................................................................... 13 12/05rev. g to rev. h updated format..................................................................universal changes to figure 1.......................................................................... 1 changes to figure 11...................................................................... 10 updated outline dimensions ....................................................... 16 changes to ordering guide .......................................................... 17 4/05rev. f to rev. g updated format..................................................................universal changes to figure 1...........................................................................1 changes to general description .....................................................1 deleted product highlights .............................................................1 moved figure 4 to page ....................................................................8 changes to figure 5...........................................................................9 changes to figure 8........................................................................ 10 changes to figure 11, figure 12, figure 13, and figure 14....... 11 changes to figure 19...................................................................... 14 changes to figure 20...................................................................... 14 changes to figure 21...................................................................... 16 updated outline dimensions....................................................... 17 changes to ordering guide .......................................................... 18 3/02rev. e to rev. f edits to ordering guide ...................................................................3
ad637 rev. k | page 3 of 20 specifications at 25c and 15 v dc, unless otherwise noted. 1 table 1. ad637j/ad637a ad637k/ad637b ad637s parameter min typ max min typ max min typ max unit transfer function v out = 2 in )(v avg v out = 2 in )(v avg v out = 2 in )(v avg conversion accuracy total error, internal trim 2 ( figure 5 ) 1 0.5 0.5 0.2 1 0.5 mv % of reading t min to t max 3.0 0.6 2.0 0.3 6 0.7 mv % of reading vs. supply +v in = 300 mv 30 150 30 150 30 150 v/v vs. supply ?v in = ?300 mv 100 300 100 300 100 300 v/v dc reversal error at 2 v 0.25 0.1 0.25 % of reading nonlinearity 2 v full scale 3 0.04 0.02 0.04 % of fsr nonlinearity 7 v full scale 0.05 0.05 0.05 % of fsr total error, external trim 0.5 0.1 0.25 0.05 0.5 0.1 mv % of reading error vs. crest factor 4 crest factor 1 to 2 specified accuracy specified accuracy specified accuracy crest factor = 3 0.1 0.1 0.1 % of reading crest factor = 10 1.0 1.0 1.0 % of reading averaging time constant 25 25 25 ms/f c av input characteristics signal range, 15 v supply continuous rms level 0 to 7 0 to 7 0 to 7 v rms peak transient input 15 15 15 v p-p signal range, 5 v supply continuous rms level 0 to 4 0 to 4 0 to 4 v rms peak transient input 6 6 6 v p-p maximum continuous nondestructive input level (all supply voltages) 15 15 15 v p-p input resistance 6.4 8 9.6 6.4 8 9.6 6.4 8 9.6 k input offset voltage 0.5 0.2 0.5 mv frequency response 5 bandwidth for 1% additional error (0.09 db) v in = 20 mv 11 11 11 khz v in = 200 mv 66 66 66 khz v in = 2 v 200 200 200 khz 3 db bandwidth v in = 20 mv 150 150 150 khz v in = 200 mv 1 1 1 mhz v in = 2 v 8 8 8 mhz
ad637 rev. k | page 4 of 20 ad637j/ad637a ad637k/ad637b ad637s parameter min typ max min typ max min typ max unit output characteristics offset voltage 1 0.5 1 mv vs. temperature 0.05 0.089 0.04 0.056 0.04 0.07 mv/c voltage swing, 15 v supply, 2 k load 0 to 12.0 13.5 0 to 12.0 13.5 0 to 12.0 13.5 v voltage swing, 3 v supply, 2 k load 0 to 2 2.2 0 to 2 2.2 0 to 2 2.2 v output current 6 6 6 ma short-circuit current 20 20 20 ma resistance chip select high 0.5 0.5 0.5 resistance chip select low 100 100 100 k db output error, v in 7 mv to 7 v rms, 0 db = 1 v rms 0.5 0.3 0.5 db scale factor ?3 ?3 ?3 mv/db scale factor temperature coefficient +0.33 +0.33 +0.33 % of reading/c ?0.033 ?0.033 ?0.033 db/c i ref for 0 db = 1 v rms 5 20 80 5 20 80 5 20 80 a i ref range 1 100 1 100 1 100 a buffer amplifier input output voltage range ?v s to (+v s ? 2.5 v) ?v s to (+v s ? 2.5 v) ?v s to (+v s ? 2.5 v) v input offset voltage 0.8 2 0.5 1 0.8 2 mv input current 2 10 2 5 2 10 na input resistance 10 8 10 8 10 8 output current ?0.13 +5 ?0.13 +5 ?0.13 +5 ma short-circuit current 20 20 20 ma small signal bandwidth 1 1 1 mhz slew rate 6 5 5 5 v/s denominator input input range 0 to 10 0 to 10 0 to 10 v input resistance 20 25 30 20 25 30 20 25 30 k offset voltage 0.2 0.5 0.2 0.5 0.2 0.5 mv chip select (cs) rms on level open or 2.4 v < v c < +v s open or 2.4 v < v c < +v s open or 2.4 v < v c < +v s rms off level v c < 0.2 v v c < 0.2 v v c < 0.2 v i out of chip select cs low 10 10 10 a cs high 0 0 0 a on time constant 10 + ((25 k) c av ) 10 + ((25 k) c av ) 10 + ((25 k) c av ) s off time constant 10 + ((25 k) c av ) 10 + ((25 k) c av ) 10 + ((25 k) c av ) s power supply operating voltage range 3.0 18 3.0 18 3.0 18 v quiescent current 2.2 3 2.2 3 2.2 3 ma standby current 350 450 350 450 350 450 a 1 specifications shown in bold are tested on all production units at final electrical test. results from those tests are used to calculate outgoing quality l evels. all minimum and maximum specifications are guara nteed, although only those shown in boldface are tested on all production units . 2 accuracy specified 0 v rms to 7 v rms dc with ad637 connected, as shown in . figure 5 3 nonlinearity is defined as the maximum deviation from the straight line connec ting the readings at 10 mv and 2 v. 4 error vs. crest factor is specified as additional error for 1 v rms. 5 input voltages are expressed in volts rms. percent is in % of reading. 6 with external 2 k pull-down resistor tied to ?v s .
ad637 rev. k | page 5 of 20 absolute maximum ratings table 2. parameter rating esd rating 500 v supply voltage 18 v dc internal quiescent power dissipation 108 mw output short-circuit duration indefinite storage temperature range ?65c to +150c lead temperature (soldering 10 sec) 300c rated operating temperature range ad637j, ad637k 0c to 70c ad637a, ad637b ?40c to +85c ad637s, 5962-8963701ca ?55c to +125c 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. esd caution
ad637 rev. k | page 6 of 20 pin configurations and function descriptions buff in 1 nc 2 common 3 output offset 4 buff out 14 v in 13 nc 12 +v s 11 cs 5 ?v s 10 den input 6 rms out 9 db output 7 c av 8 nc = no connect ad637 top view (not to scale) 00788-002 figure 2. 14-lead sbdip/ cerdip pin configuration buff in 1 nc 2 common 3 output offset 4 buff ou t 16 v in 15 nc 14 +v s 13 cs 5 ?v s 12 den input 6 rms out 11 db output 7 c av 10 nc 8 nc 9 nc = no connect ad637 top view (not to scale) 00788-003 figure 3. 16-lead soic_w pin configuration table 3. 14-lead sbdip/cerdip pin function descriptions pin o. nemonic description 1 buff in buffer input 2, 12 nc no connection 3 common analog common 4 output offset output offset 5 cs chip select 6 den input denominator input 7 db output db output 8 c av averaging capacitor connection 9 rms out rms output 10 ?v s negative supply rail 11 +v s positive supply rail 13 v in signal input 14 buff out buffer output table 4. 16-lead soic_w pin function descriptions pin o. nemonic description 1 buff in buffer input 2, 8, 9, 14 nc no connection 3 common analog common 4 output offset output offset 5 cs chip select 6 den input denominator input 7 db output db output 10 c av averaging capacitor connection 11 rms out rms output 12 ?v s negative supply rail 13 +v s positive supply rail 15 v in signal input 16 buff out buffer output
ad637 rev. k | page 7 of 20 functional description filter/amplifier 24k? 24k? one quadrant squarer/divider buffer amplifier q1 q2 q3 q4 125? 6k ? 6k? 12k? 24k ? a5 a1 a2 absolute value voltage to current converter i 1 i 3 i 4 a4 a3 bias q5 c av +v s rms out common cs den input output offset db output ad637 buff out buff in ?v s 00788-004 14 1 13 10 4 6 5 3 7 9 11 8 v in figure 4. simplified schematic the ad637 embodies an implicit solution of the rms equation that overcomes the inherent limitations of straightforward rms computation. the actual computation performed by the ad637 follows the equation rmsv v avgrmsv in 2 figure 4 is a simplified schematic of the ad637, subdivided into four major sections: absolute value circuit (active rectifier), squarer/divider, filter circuit, and buffer amplifier. the input voltage (v in ), which can be ac or dc, is converted to a unipolar current i 1 by the active rectifiers a1 and a2. i 1 drives one input of the squarer/divider, which has the transfer function 3 1 4 i i i 2 the output current of the squarer/divider i 4 drives a4, forming a low-pass filter with the external averaging capacitor. if the rc time constant of the filter is much greater than the longest period of the input signal, then the a4 output is proportional to the average of i 4 . the output of this filter amplifier is used by a3 to provide the denominator current i 3 , which equals avg i 4 and is returned to the squarer/divider to complete the implicit rms computation rmsi i i avgi 1 4 1 4 2 and v out = v in rms to compute the absolute value of the input signal, the averaging capacitor is omitted. however, a small capacitance value at the averaging capacitor pin is recommended to maintain stability; 5 pf is sufficient for this purpose. the circuit operates identically to that of the rms configuration, except that i 3 is now equal to i 4 , giving 4 1 i i 2 4 i i 4 = | i 1 | the denominator current can also be supplied externally by providing a reference voltage (v ref ) to pin 6. the circuit operates identically to the rms case, except that i 3 is now proportional to v ref . therefore, 3 1 i i avg 2 4 i and den in out v v v 2 this is the mean square of the input signal.
ad637 rev. k | page 8 of 20 standard connection the ad637 is simple to connect for a majority of rms measurements. in the standard rms connection shown in figure 5 , only a single external capacitor is required to set the averaging time constant. in this configuration, the ad637 computes the true rms of any input signal. an averaging error, the magnitude of which is dependent on the value of the averaging capacitor, is present at low frequencies. for example, if the filter capacitor, c av , is 4 f, the error is 0.1% at 10 hz and increases to 1% at 3 hz. to measure ac signals, the ad637 can be ac-coupled by adding a nonpolar capacitor in series with the input, as shown in figure 5 . 1 25k ? den input buff in buff out squarer/ divider ?v s cs db output 3 common bias 2nc 4 output offset 4.7k ? +v s 5 25k? 6 7 8 9 10 +v s 11 12 nc 13 14 nc v in v out =v in 2 (optional) c av + ad637 absolute value 00788-005 ?v s +v s c av v in figure 5. standard rms connection the performance of the ad637 is tolerant of minor variations in the power supply voltages; however, if the supplies used exhibit a considerable amount of high frequency ripple, it is advisable to bypass both supplies to ground through a 0.1 f ceramic disc capacitor placed as close to the device as possible. the output signal range of the ad637 is a function of the supply voltages, as shown in figure 6 . the output signal can be used buffered or nonbuffered, depending on the characteristics of the load. if no buffer is needed, tie the buffer input (pin 1) to common. the output of the ad637 is capable of driving 5 ma into a 2 k load without degrading the accuracy of the device. 20 15 0 0 10 5 00788-006 supply voltage ? dual supply (v) 3 5 10 15 18 max v out (volts 2k ? load) figure 6. maximum v out vs. supply voltage chip select the ad637 includes a chip select feature that allows the user to decrease the quiescent current of the device from 2.2 ma to 350 a. this is done by driving cs, pin 5, to below 0.2 v dc. under these conditions, the output goes into a high impedance state. in addition to reducing the power consumption, the outputs of multiple devices can be connected in parallel to form a wide bandwidth rms multiplexer. tie pin 5 high to disable the chip select. optional trims for high accuracy the ad637 includes provisions for trimming out output offset and scale factor errors resulting in significant reduction in the maximum total error, as shown in figure 7 . the residual error is due to a nontrimmable input offset in the absolute value circuit and the irreducible nonlinearity of the device. referring to figure 8 , the trimming process is as follows: x offset trim: ground the input signal (v in ) and adjust r1 to give 0 v output from pin 9. alternatively, r1 can be adjusted to give the correct output with the lowest expected value of v in . x scale factor trim: resistor r4 is inserted in series with the input to lower the range of the scale factor. connect the desired full-scale input to v in , using either a dc or a calibrated ac signal, and trim resistor r3 to give the correct output at pin 9 (that is, 1 v dc at the input results in a dc output voltage of l.000 v dc). a 2 v p-p sine wave input yields 0.707 v dc at the output. remaining errors are due to the nonlinearity.
ad637 rev. k | page 9 of 20 5.0 2.5 ?5.0 0 0.5 error (mv) 1.0 0 ?2.5 1.5 2 . 0 ad637k max internal trim 00788-007 ad637k external trim ad637k: 0.5mv 0.2% 0.25mv 0.05% external input level (v) figure 7. maximum total error vs. input level ad637k intern al and external trims 1 25k ? den input buff in buff out squarer/ divider ?v s cs db output 3 common bias 2nc 4 output offset r2 1m ? 5 25k ? 6 7 8 9 10 +v s 11 12 nc 13 14 nc v in v out =v in 2 v in c av + ad637 absolute value 00788-008 ?v s +v s c av r4 147 ? scale factor trim r1 5 0k ? output offset trim 4.7k ? ?v s +v s +v s r3 1k ? figure 8. optional extern al gain and offset trims choosing the averaging time constant the ad637 computes the true rms value of both dc and ac input signals. at dc, the output tracks the absolute value of the input exactly; with ac signals, the ad637 output approaches the true rms value of the input. the deviation from the ideal rms value is due to an averaging error. the averaging error comprises an ac component and a dc component. both components are functions of input signal frequency f and the averaging time constant (: 25 ms/f of averaging capacitance). figure 9 shows that the averaging error is defined as the peak value of the ac component (ripple) and the value of the dc error. the peak value of the ac ripple component of the averaging error is defined approximately by the relationship f where readingof%in f 1 6.3 50 double-frequency ripple e o time average error ideal e o 00788-009 dc error = average of output ? ideal figure 9. typical output waveform for a sinusoidal input this ripple can add a significant amount of uncertainty to the accuracy of the measurement being made. the uncertainty can be significantly reduced through the use of a postfiltering network or by increasing the value of the averaging capacitor. the dc error appears as a frequency dependent offset at the output of the ad637 and follows the relationship readingof%in f 22 4.616.0 1 w because the averaging time constant, set by c av , directly sets the time that the rms converter holds the input signal during computation, the magnitude of the dc error is determined only by c av and is not affected by postfiltering. sine wave input frequency (hz) 100 0.1 1.0 10 10k dc error or ripple (% of reading) 1k 100 10 dc error peak ripple 00788-010 figure 10. comparison of percent dc error to the percent peak ripple over frequency using the ad637 in the standard rms connection with a 1 f c av the ac ripple component of averaging error is greatly reduced by increasing the value of the averaging capacitor. there are two major disadvantages to this: the value of the averaging capacitor becomes extremely large and the settling time of the ad637 increases in direct proportion to the value of the averaging capacitor (t s = 115 ms/f of averaging capacitance). a preferable method of reducing the ripple is by using the postfilter network, as shown in figure 11 . this network can be used in either a 1- pole or 2-pole configuration. for most applications, the 1-pole filter gives the best overall compromise between ripple and settling time.
ad637 rev. k | page 10 of 20 for a single-pole filter short rx and remove c3 1 25k ? den input buff in buff out ?v s cs db output 3common bias 2nc 4 output offset +v s 5 25k ? 6 7 8 9 10 +v s 11 12 nc 13 14 rms out v in c av + ad637 absolute value 00788-011 ?v s +v s c av v in + c2 4.7k ? c3 24k? + rx 24k ? squarer/ divider for 1% settling time in seconds multiply reading by 0.115 input frequency (hz) 100 0.01 1 100k 10 100 1k 10k 10 1.0 0.1 10 % err or 1 % error 0.1% e rror 0. 0 1 % error 100 0.01 10 1.0 0.1 00788-012 *%dc error + %ripple (peak) required c av (f) figure 12. values for c av and 1% settling time for stated % of reading averaging error* accuracy includes 2% comp onent tolerance (see * in figure) input frequency (hz) 100 0.01 1 100k 10 100 1k 10k 10 1 0.1 5 % e r r o r 1 % e r r o r 0 . 1 % e r r o r 0 . 0 1 % e r r o r 100 0.01 10 1 0.1 00788-013 required c av (and c2) c2 = 3.3 c av for 1% settling time in seconds multiply reading by 0.400 *%dc error + %ripple (peak) accuracy 20% due to component tolerance figure 11. 2-pole sallen-key filter figure 12 shows values of c av and the corresponding averaging error as a function of sine wave frequency for the standard rms connection. the 1% settling time is shown on the right side of figure 12 . figure 13 shows the relationship between the averaging error, signal frequency settling time, and averaging capacitor value. figure 13 is drawn for filter capacitor values of 3.3 the averaging capacitor value. this ratio sets the magnitude of the ac and dc errors equal at 50 hz. as an example, by using a 1 f averaging capacitor and a 3.3 f filter capacitor, the ripple for a 60 hz input signal is reduced from 5.3% of the reading using the averaging capacitor alone to 0.15% using the 1-pole filter. this gives a factor of 30 reduction in ripple, and yet the settling time only increases by a factor of 3. the values of filter capacitor c av and filter capacitor c2 can be calculated for the desired value of averaging error and settling time by using figure 13 . figure 13. values of c av , c2, and 1% settling time for stated % of reading averaging error* for 1-pole post filter (see * in figure) input frequency (hz) 100 0.01 11 10 100 1k 10k 10 1 0.1 0 0 k 5 % err o r 1% er ror 0 .1% error 0 . 0 1% e rror 100 0.01 10 1 0.1 00788-014 *%dc error + %ripple (peak) accuracy 20% due to component tolerance required c av (and c2 + c3) c2 = c3 = 2.2 c av for 1% settling time in seconds multiply reading by 0.365 the symmetry of the input signal also has an effect on the magnitude of the averaging error. tabl e 5 gives the practical component values for various types of 60 hz input signals. these capacitor values can be directly scaled for frequencies other than 60 hzthat is, for 30 hz, these values are doubled, and for 120 hz they are halved. for applications that are extremely sensitive to ripple, the 2-pole configuration is suggested. this configuration minimizes capacitor values and the settling time while maximizing performance. figure 14. values of c av , c2, and c3 and 1% settling time for stated % of reading averaging error* for 2-pole sallen-key filter (see * in figure) figure 14 can be used to determine the required value of c av , c2, and c3 for the desired level of ripple and settling time.
ad637 rev. k | page 11 of 20 table 5. practical values of c av and c2 for various input waveforms recommended standard values for c av and c2 for 1% averaging error @ 60 hz with t = 16.6 ms input waveform and period absolute value circuit waveform and period minimum r c av time constant c av (f) c2 (f) 1% settling time symmetrical sine wave a 0v t 1/2t 1/2t 0.47 1.5 181 ms sine wave with dc offset b 0v t t t 0.82 2.7 325 ms pulse train waveform c t 2 0v t t 2 t 10 (t ? t 2 ) 6.8 22 2.67 sec d t 2 0v t t 2 t 10 (t ? 2t 2 ) 5.6 18 2.17 sec frequency response the frequency response of the ad637 at various signal levels is shown in figure 15 . the dashed lines show the upper frequency limits for 1%, 10%, and 3 db of additional error. for example, note that for 1% additional error with a 2 v rms input, the highest frequency allowable is 200 khz. a 200 mv signal can be measured with 1% error at signal frequencies up to 100 khz. to take full advantage of the wide bandwidth of the ad637, care must be taken in the selection of the input buffer amplifier. to ensure that the input signal is accurately presented to the converter, the input buffer must have a ?3 db bandwidth that is wider than that of the ad637. note the importance of slew rate in this application. for example, the minimum slew rate required for a 1 v rms, 5 mhz, sine wave input signal is 44 v/s. the user is cautioned that this is the minimum rising or falling slew rate and that care must be exercised in the selection of the buffer amplifier, because some amplifiers exhibit a two-to-one difference between rising and falling slew rates. the ad845 is recommended as a precision input buffer. 10 1k 10m 10k v out (v) 100k 1m 1 0.1 0.01 1% 3db 10% 7v rms input 2v rms input 1v rms input 100mv rms input 10mv rms input 00788-015 input frequency (hz) figure 15. frequency response
ad637 rev. k | page 12 of 20 ac measurement accuracy and crest factor crest factor is often overlooked in determining the accuracy of an ac measurement. crest factor is defined as the ratio of the peak signal amplitude to the rms value of the signal (cf = v p /v rms). most common waveforms, such as sine and triangle waves, have relatively low crest factors (2). waveforms that resemble low duty cycle pulse trains, such as those occurring in switching power supplies and scr circuits, have high crest factors. for example, a rectangular pulse train with a 1% duty cycle has a crest factor of 10 (cf = 1 ). 0 vp t t cf = 1/ e in (rms) = 1 v rms 100s = duty cycle = 100s e 0 00788-016 figure 16. duty cycle timing pulse width (s) 10 1 0.01 1 1000 10 increase in error (%) 100 0.1 cf = 10 cf = 3 00788-017 c av = 22f figure 17. ad637 error vs. pulse width rectangular pulse figure 18 is a curve of additional reading error for the ad637 for a 1 v rms input signal with crest factors from 1 to 11. a rectangular pulse train (pulse width 100 s) is used for this test because it is the worst-case waveform for rms measurement (all the energy is contained in the peaks). the duty cycle and peak amplitude were varied to produce crest factors from l to 10 while maintaining a constant 1 v rms input amplitude. crest factor 1.5 0 ?1.5 1 11 2 increase in error (%) 37 8910 1.0 0.5 ?0.5 ?1.0 0 0788-018 456 positive input pulse c av = 22f figure 18. additional error vs. crest factor 2.0 1.8 0 2.0 0.5 1.0 1.5 1.2 0.6 0.4 0.2 1.6 1.4 0.8 1.0 cf = 10 cf = 7 cf = 3 0 00788-019 v in (v rms) magnitude of error (% of rms level) figure 19. error vs. rms input level for three common crest factors connection for db output another feature of the ad637 is the logarithmic, or decibel, output. the internal circuit that computes db works well over a 60 db range. figure 20 shows the db measurement connection. the user selects the 0 db level by setting r1 for the proper 0 db reference current, which is set to cancel the log output current from the squarer/divider circuit at the desired 0 db point. the external op amp is used to provide a more convenient scale and to allow compensation of the +0.33%/c temperature drift of the db circuit. the temperature resistor r3, as shown in figure 20 , is available from precision resistor co., inc., in largo, fla. (model pt146). consult its website for additional information.
ad637 rev. k | page 13 of 20 db calibration refer to figure 20 : ? set v in = 1.00 v dc or 1.00 v rms ? adjust r1 for 0 db out = 0.00 v ? set v in = 0.1 v dc or 0.10 v rms ? adjust r2 for db out = ?2.00 v any other db reference can be used by setting v in and r1 accordingly. ad707jn +v s r2 5k ? 33.2k ? r3 1k? * buffer ad637 squarer/divider filter 25k ? 25k ? 1 2 3 4 5 6 7 14 13 12 11 10 9 8 c av ?v s +v s + buff out common output offset buff in nc cs den input db output v in nc 1f v out +v s ad508j +2.5 v r1 500k ? 0db adjust 10k ? 00788-020 4.7k ? +v s ?v s +v s signal input db scale factor adjust compensated db output + 100mv/db ?v s 60.4 ? 2 3 4 6 7 absolute value bias section *1k ? + 3500ppm see text nc = no connect figure 20. db connection
ad637 rev. k | page 14 of 20 buffer ad637 squarer/divider bias section filter 25k? 1 2 3 4 5 6 7 14 13 12 11 10 9 8 c av + absolute value nc signal input nc ad548jn filtered v rms output 1f 1000pf 6.8m ? 1m ? r 3.3m ? v rms 0 0788-021 4.7k ? +v s buff out common output offset buff in cs den input db output v in v out ?v s +v s notes 1. values chosen to give 0.1% averaging error @ 1hz. 2. nc = no connect. c av1 3.3f 499k? 1% v in 2 ?v s +v s 100f 25k ? v? 3 2 4 7 6 v+ 3.3m ? 1f +v s ?v s 50k ? output offset adjust figure 21. ad637 as a low frequency rms converter low frequency measurements if the frequencies of the signals to be measured are below 10 hz, the value of the averaging capacitor required to deliver even 1% averaging error in the standard rms connection becomes extremely large. figure 21 shows an alternative method of obtaining low frequency rms measurements. the averaging time constant is determined by the product of r and c av1 , in this circuit, 0.5 sec/f of c av . this circuit permits a 20:1 reduction in the value of the averaging capacitor, permitting the use of high quality tantalum capacitors. it is suggested that the 2-pole, sallen-key filter shown in figure 21 be used to obtain a low ripple level and minimize the value of the averaging capacitor. if the frequency of interest is below 1 hz, or if the value of the averaging capacitor is still too large, the 20:1 ratio can be increased. this is accomplished by increasing the value of r. if this is done, it is suggested that a low input current, low offset voltage amplifier, such as the ad548, be used instead of the internal buffer amplifier. this is necessary to minimize the offset error introduced by the combination of amplifier input currents and the larger resistance. vector summation vector summation can be accomplished through the use of two ad637s, as shown in figure 22 . here, the averaging capacitors are omitted (nominal 100 pf capacitors are used to ensure stability of the filter amplifier), and the outputs are summed as shown. the output of the circuit is 22 out vvv this concept can be expanded to include additional terms by feeding the signal from pin 9 of each additional ad637 through a 10 k resistor to the summing junction of the ad711 and tying all of the denominator inputs (pin 6) together. if c av is added to ic1 in this configuration, then the output is 22 vv if the averaging capacitor is included on both ic1 and ic2, the output is 22 vv this circuit has a dynamic range of 10 v to 10 mv and is limited only by the 0.5 mv offset voltage of the ad637. the useful bandwidth is 100 khz.
ad637 rev. k | page 15 of 20 buffer ad637 squarer/divider bias section filter 1 2 3 4 5 6 7 14 13 12 11 10 9 absolute value 100pf v x in buffer ad637 squarer/divider filter 25k? 25k? 1 2 3 4 5 6 7 14 13 12 11 10 9 8 absolute value 100pf v y in 5pf 10k ? ad711k expandable 10k ? 10k ? ic1 ic2 00788-022 c av buff out common output offset buff in nc cs den input db output nc v out 4.7k ? +v s common output offset buff in nc cs den input db output 4.7k ? +v s buff out nc v out 8 ?v s ?v s ?v s +v s bias section v x 2 + v y 2 v out = ?v s +v s 20k ? +v s +v s 25k ? 25k ? figure 22. vector sum configuration
ad637 rev. k | page 16 of 20 evaluation board figure 23 shows a digital image of the AD637-EVALZ, an evaluation board specially designed for the ad637. it is available at www.analog.com and is fully tested and ready for bench testing after connecting power and signal i/o. the circuit is configured for dual power supplies, and standard bnc connectors serve as the signal input and output ports. referring to the schematic in figure 30 , the input connector rms_in is capacitively coupled to pin 15 (v in of soic package) of the ad637. the dc_out connector is connected to pin 11, rms out, with provisions for connections to the output buffer between pin 1 and pin 16. the buffer is an uncommitted op amp, and is configured on the AD637-EVALZ as a low-pass sallen-key filter whose f c < 0.5 hz. users can connect to the buffer by moving the filter switch to the on position. dc_out is still the output of the ad637, and the test loop, buf_out, is the output of the buffer. the r2 trimmer adjusts the output offset voltage. the lpf frequency is changed by changing the component values of cf1, cf2, r4, and r5. see figure 24 and figure 30 to locate these components. note that a wide range of capacitor and resistor values can be used with the ad637 buffer amplifier. 00788-123 figure 23. AD637-EVALZ
ad637 rev. k | page 17 of 20 0 0788-124 figure 24. AD637-EVALZ assembly 00788-125 figure 25. component side silkscreen 00788-126 figure 26. evaluation boardcomponent side copper 00788-127 figure 27. evaluation bo ardsecondary side copper 00788-128 figure 28. evaluation boardinternal power plane 00788-129 figure 29. evaluation boardinternal ground plane
ad637 rev. k | page 18 of 20 00788-130 gnd1 gnd2 gnd3 gnd4 c1 10f 25v c2 10f 25v ? v s + + v s ?v s +v s + + cf2 47f 25v c av 22f 16v + cf1 47f 25v + + c3 0.1f cin 22f 16v buf_out dc_out rms_in dc_out 2 1 4 3 6 5 8 7 15 16 13 14 11 12 9 10 nc buff in output offset common den input cs nc z1 ad637 db output v in buff out +v s nc rms out ?v s nc c av r3 4.7k ? +v s c4 0.1f ?v s r4 24.3k ? r1 1m ? r2 50k ? db_out buf_in rms_in r5 24.3k ? +v s +v s ?v s 4 1 2 3 5 6 in out filter figure 30. evaluation board schematic 00788-131 precision dmm to monitor vout power supply a c or dc input signal source from precision calibrator or function generator figure 31. AD637-EVALZ ty pical bench configuration
ad637 rev. k | page 19 of 20 outline dimensions c ontrolling dimensions are in inches; millimeter dimension s (in parentheses) are rounded-off inch equivalents fo r reference only and are not appropriate for use in design . 14 1 7 8 0.310 (7.87) 0.220 (5.59) pin 1 0.080 (2.03) max 0.005 (0.13) min seating plane 0.023 (0.58) 0.014 (0.36) 0.060 (1.52) 0.015 (0.38) 0.200 (5.08) max 0.200 (5.08) 0.125 (3.18) 0.070 (1.78) 0.030 (0.76) 0.100 (2.54) bsc 0.150 (3.81) min 0.765 (19.43) max 0.320 (8.13) 0.290 (7.37) 0.015 (0.38) 0.008 (0.20) figure 32. 14-lead side-brazed cera mic dual in-line package [sbdip] (d-14) 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 0.098 (2.49) 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 0.785 (19.94) 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) pin 1 1 7 8 14 figure 33. 14-lead ceramic dual in-line package [cerdip] (q-14) dimensions shown in inches and (millimeters)
ad637 rev. k | page 20 of 20 controlling dimensions are in millimeters; inch dimensions (in parentheses) are rounded-off millimeter equivalents for reference only and are not appropriate for use in design. compliant to jedec standards ms-013-aa 10.50 (0.4134) 10.10 (0.3976) 0.30 (0.0118) 0.10 (0.0039) 2.65 (0.1043) 2.35 (0.0925) 10.65 (0.4193) 10.00 (0.3937) 7.60 (0.2992) 7.40 (0.2913) 0 . 7 5 ( 0 . 0 2 9 5 ) 0 . 2 5 ( 0 . 0 0 9 8 ) 45 1.27 (0.0500) 0.40 (0.0157) c oplanarity 0.10 0.33 (0.0130) 0.20 (0.0079) 0.51 (0.0201) 0.31 (0.0122) seating plane 8 0 16 9 8 1 1.27 (0.0500) bsc 03-27-2007-b figure 34. 16-lead standard small outline package [soic_w] wide body (rw-16) dimensions shown in millimeters and (inches) ordering guide model 1 notes temperature range package description package option 5962-8963701ca 2 ?55c to +125c 14-lead cerdip q-14 ad637aq ?40c to +85c 14-lead cerdip q-14 ad637ar ?40c to +85c 16-lead soic_w rw-16 ad637arz ?40c to +85c 16-lead soic_w rw-16 ad637bq ?40c to +85c 14-lead cerdip q-14 ad637br ?40c to +85c 16-lead soic_w rw-16 ad637brz ?40c to +85c 16-lead soic_w rw-16 ad637jd 0c to 70c 14-lead sbdip d-14 ad637jdz 0c to 70c 14-lead sbdip d-14 ad637jq 0c to 70c 14-lead cerdip q-14 ad637jr 0c to 70c 16-lead soic_w rw-16 ad637jr-reel 0c to 70c 16-lead soic_w rw-16 ad637jr-reel7 0c to 70c 16-lead soic_w rw-16 ad637jrz 0c to 70c 16-lead soic_w rw-16 ad637jrz-rl 0c to 70c 16-lead soic_w rw-16 ad637jrz-r7 0c to 70c 16-lead soic_w rw-16 ad637kd 0c to 70c 14-lead sbdip d-14 ad637kdz 0c to 70c 14-lead sbdip d-14 ad637kq 0c to 70c 14-lead cerdip q-14 ad637krz 0c to 70c 16-lead soic_w rw-16 ad637sd ?55c to +125c 14-lead sbdip d-14 ad637sd/883b ?55c to +125c 14-lead sbdip d-14 ad637sq/883b ?55c to +125c 14-lead cerdip q-14 AD637-EVALZ evaluation board 1 z = rohs compliant part. 2 a standard microcircuit drawing is available. ?2007C2011 analog devices, inc. all rights reserved. trademarks and registered trademarks are the property of their respective owners. d00788-0-2/11(k)

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