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low level, true rms-to-dc converter data sheet ad636 rev. e document feedback 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 ?2013 analog devices, inc. all rights reserved. technical support www.analog.com features true rms-to-dc conversion 200 mv full scale laser-trimmed to high accuracy 0.5% maximum error (ad636k) 1.0% maximum error (ad636j) wide response capability computes rms of ac and dc signals 1 mhz, ?3 db bandwidth: v rms > 100 mv signal crest factor of 6 for 0.5% error db output with 50 db range low power: 800 a quiescent current single or dual supply operation monolithic integrated circuit low cost general description the ad636 is a low power monolithic ic that performs true rms-to-dc conversion on low level signals. it offers performance that is comparable or superior to that of hybrid and modular converters costing much more. the ad636 is specified for a signal range of 0 mv to 200 mv rms. crest factors up to 6 can be accommodated with less than 0.5% additional error, allowing accurate measurement of complex input waveforms. the low power supply current requirement of the ad636, typically 800 a, is ideal for battery-powered portable instruments. it operates from a wide range of dual and single power supplies, from 2.5 v to 16.5 v or from +5 v to +24 v. the input and output terminals are fully protected; the input signal can exceed the power supply with no damage to the device (allowing the presence of input signals in the absence of supply voltage), and the output buffer amplifier is short-circuit protected. the ad636 includes an auxiliary db output derived from an internal circuit point that represents the logarithm of the rms output. the 0 db reference level is set by an externally supplied current and can be selected to correspond to any input level from 0 dbm (774.6 mv) to ?20 dbm (77.46 mv). frequency response ranges from 1.2 mhz at 0 dbm to greater than 10 khz at ?50 dbm. the ad636 is easy to use. the device is factory-trimmed at the wafer level for input and output offset, positive and negative waveform symmetry (dc reversal error), and full-scale accuracy at 200 mv rms. therefore, no external trims are required to achieve full-rated accuracy. functional block diagram r l db buffer in buffer out i out 10k ? 10k ? 40k ? +v s +v s +v s ?v s c av v in com current mirror squarer divider absolute value ad636 00787-001 ?v s buf figure 1. the ad636 is available in two accuracy grades. the total error of the j-version is typically less than 0.5 mv 1.0% of reading, while the total error of the ad636k is less than 0.2 mv to 0.5% of reading. both versions are temperature rated for operation between 0c and 70c and available in 14-lead sbdip and 10-lead to-100 metal can. the ad636 computes the true root-mean-square of a complex ac (or ac plus dc) input signal and gives an equivalent dc output level. the true rms value of a waveform is a more useful quantity than the average rectified value because it is a measure of the power in the signal. the rms value of an ac-coupled signal is also its standard deviation. the 200 mv full-scale range of the ad636 is compatible with many popular display-oriented adcs. the low power supply current requirement permits use in battery-powered hand-held instruments. an averaging capacitor is the only external component required to perform measurements to the fully specified accuracy is. its value optimizes the trade-off between low frequency accuracy, ripple, and settling time. an optional on-chip amplifier acts as a buffer for the input or the output signals. used in the input, it provides accurate performance from standard 10 m input attenuators. as an output buffer, it sources up to 5 ma.
ad636 data sheet rev. e | page 2 of 16 table o f 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 descript ions ........................... 6 typical performance characteristics ............................................. 7 theory of operation ........................................................................ 8 rms measurements ..................................................................... 8 the ad636 buffer amplifier ...................................................... 8 frequency response ..................................................................... 9 ac measurement accuracy and crest factor (cf) ................. 9 applications ..................................................................................... 10 standard connection ................................................................. 10 optional trims for high accuracy .......................................... 10 single - supply connection ........................................................ 10 choosing the averaging time const ant ................................. 11 a complete ac digital voltmeter ........................................... 12 a low power, high input, impedance db meter ....................... 12 circuit description ................................................................ 12 performance data .................................................................. 12 frequency response 3 dbm ............................................... 13 calibration .............................................................................. 13 outline dimensions ....................................................................... 14 ordering guide .......................................................................... 14 revis ion history 5/13 rev. d to rev. e reorganized layout ............................................................ universal changes to figure 1 ........................................................................... 1 change to table 1 .............................................................................. 4 added typical performance characteristics section ................... 7 added theory of operation section; changes to figure 7 and figure 8 ............................................................................................... 8 changed applying the ad636 s ection to applications section; changes to figure 9, figure 10, and single - supply connection section ............................................................................................... 10 changes to figure 11 ....................................................................... 11 changes to figure 13 and a complete ac digital voltmeter section ............................................................................................... 12 changes to figure 17 and figure 18 .............................................. 13 changes to ordering gu ide ........................................................... 14 11 /06 rev. c to rev. d changes to general description ..................................................... 1 changes to table 1 ............................................................................. 3 changes to ordering guide .......................................................... 13 1/06 rev b to rev. c updated format .................................................................. universal changes to figure 1 and general description .............................. 1 deleted metallization photograph .................................................. 3 added pin config uration and function description section .... 6 updated outline dimensions ....................................................... 14 changes to ordering guide .......................................................... 14 8/99 rev a to rev. b
data sheet ad636 rev. e | page 3 of 16 specifications @ 25c, +v s = +3 v, and ?v s = C 5 v, unless otherwise noted . 1 table 1 . model ad636j ad636k u nit min typ max min typ max transfer function ( ) 2 in out v avg v = ( ) 2 in out v avg v = conversion accuracy total error, internal trim 2 , 3 0.5 1.0 0.2 0.5 mv % of reading vs. temperature, 0c to +70c 0.1 0. 01 0.1 0.005 mv % of r eading/c vs. supply voltage 0.1 0.01 0.1 0.01 mv % of reading/v dc reversal error at 200 mv 0.2 0.1 % of r eading total error, external trim 2 0.3 0.3 0 .1 0.2 mv % of reading error vs. crest factor 4 crest factor 1 to 2 specified accuracy specified accuracy crest factor = 3 ?0.2 ?0.2 % of r eading crest factor = 6 ?0.5 ?0.5 % of reading averaging time constant 25 25 m s/f of c av input characteristics signal range, all supplies continuous rms level 0 to 200 0 to 200 mv rms peak transient inputs +3 v, ?5 v supply 2.8 2.8 v p -p 2.5 v supply 2.0 2.0 v p -p 5 v supp ly 5.0 5.0 v p - p maximum continuous nondestructive input level (all supply voltages) 12 12 v p -p input resistance 5.33 6.67 8 5.33 6.67 8 k input offset voltage 0.5 0.2 mv frequency response 3 , 5 bandwidth for 1% additional error (0.09 db) v in = 10 mv 14 14 khz v in = 100 mv 90 90 khz v in = 200 mv 130 130 khz 3 db bandwidth v in = 10 mv 100 100 khz v in = 100 mv 900 900 khz v in = 200 mv 1.5 1.5 mhz output characteristics 3 offset voltage, v in = com 0.5 0.2 mv vs. temperature 10 10 v/c vs. supply 0.1 0.1 mv/v voltage swing +3 v, ?5 v supply 0.3 0 to 1.0 0.3 0 to 1.0 v 5 v to 16.5 v supply 0.3 0 to 1.0 0.3 0 to 1.0 v output impedance 8 10 12 8 10 12 k
ad636 data sheet rev. e | page 4 of 16 model ad636j ad636k u nit min typ max min typ max db output error, v in = 7 mv to 300 mv rms 0.3 0.5 0.1 0.2 db scale factor ?3.0 ?3.0 mv/db scale factor temperature coefficient 0.33 0.33 % of r eading/c ?0.033 ?0.033 db/c i ref for 0 db = 0.1 v rms 2 4 8 2 4 8 a i ref range 1 50 1 50 a i out terminal i out scale factor 100 100 a/v rms i out scale factor tolerance ?20 10 +20 ?20 10 +20 % output resistance 8 10 12 8 10 12 k voltage compliance ?v s to (+v s ? 2 v) ?v s to (+v s ? 2 v) v buffer amplifier input and output voltage range ?v s to (+v s ? 2 v) ?v s to (+v s ? 2 v) v input offset voltage, r s = 10 k 0.8 2 0.5 1 mv input bias current 100 300 100 300 na input resistance 10 8 10 8 output current (+5 ma, ?130 a) (+5 ma, ?130 a) short - circuit current 20 20 ma small signal bandwidth 1 1 mhz slew rate 6 5 5 v/s power supply voltage, rated performance +3, ?5 +3, ?5 v dual supply +2, ?2.5 16.5 +2, ?2.5 16.5 v single supply 5 24 5 24 v quiescent current 7 0.80 1.00 0.80 1.00 ma temperature range rated performance 0 +70 0 +70 c storage ?55 +150 ?55 +150 c transistor count 62 62 1 all min imum and max imum specifications are guaranteed. specifications sho wn in boldface are tested on all production units at final electrical tes t and are used to calculate outgoing quality levels. 2 a ccuracy specified for 0 m v to 200 mv rms, dc or 1 khz sine wave input. accuracy is degraded at higher rms signal levels. 3 measured at pin 8 of p dip (i out ), with pin 9 tied to common. 4 error vs. cres t factor is specified as additional error for a 200 mv rms rectangular pulse train , pulse width = 200 s. 5 input voltages are expressed in v rms. 6 with 10 k pull - down resistor from pin 6 (buf out) to ? v s . 7 with buf in tied to common.
data sheet ad636 rev. e | page 5 of 16 abs olute maximum ratings table 2 . parameter ratings supply voltage dual supply 1 6.5 v single supply 24 v internal power dissipation 1 500 mw maximum input voltage 1 2 v peak storage temperature range ?55 c to +150 c operating temperature range 0 c to 70 c lead temperature range (soldering 60 sec) 300c esd rating 1000 v 1 10- lea d to : ja = 150c/ w. 14- lead p di p: ja = 95 c/w. stresses above those listed under absolute maximum ratings may cause permanent damage to the device. this is a stress rating only; functional operation of t he 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
ad636 data sheet rev. e | page 6 of 16 pin configurations and function descriptions v in 1 nc 2 ?v s 3 c av 4 +v s 14 nc 13 nc 12 nc 11 db 5 com 10 buf out 6 r l 9 buf in 7 i out 8 nc = no connect ad636 top view (not to scale) 0 0787-003 figure 2. 14-lead sb dip pin configuration buf in buf out i out ?v s +v s v in com r l db c av 6 7 8 9 10 3 4 2 1 5 ad636 00787-004 figure 3. 10-pin to-100 pin configuration table 3. pin function descriptions14-lead sbdip pin no. mnemonic description 1 v in input voltage. 2 nc no connection. 3 ?v s negative supply voltage. 4 c av averaging capacitor. 5 db log (db) value of the rms output voltage. 6 buf out buffer output. 7 buf in buffer input. 8 i out rms output current. 9 r l load resistor. 10 com common. 11, 12, 13 nc no connection. 14 +v s positive supply voltage. table 4. pin function descriptions10-pin to-100 pin no. mnemonic description 1 r l load resistor. 2 com common. 3 +v s positive supply voltage. 4 v in input voltage. 5 ?v s negative supply voltage. 6 c av averaging capacitor. 7 db log (db) value of the rms output voltage. 8 buf out buffer output. 9 buf in buffer input. 10 i out rms output current.
data sheet ad636 rev. e | page 7 of 16 typical performance characteristics 1 . 0 0 . 5 0 0 1 k 10 k 100 k 1 m r ex t e rnal (? ) ratio of v peak /v supply r l = 50k ? r l = 16 . 7k ? r l = 6 . 7k ? 00787-015 figure 4 . ratio of peak negative sw ing to ?v s vs. r external for several load resistances f r e q u e nc y ( h z) 1 v rms i n p u t 200m v rms i n p u t 100m v rms i n p u t 30m v rms i n p u t 1m v rms i n p u t 10 % 3 d b 1 % 10m v rm s i n p u t 1 k 10 k 100 k 1 m 10 m v out (v) 1 200 m 100 m 10 m 1 m 30 m 0 . 1 m 00787-016 figure 5. ad636 frequency response cr es t f ac t o r 0 . 5 0 ?1 . 0 increase in error (% of reading) ?0 . 5 t v p 0 200 s e o = du t y c y c l e = c f = 1 / e i n ( rms ) = 200m v 200 s t ? ? 1 2 3 4 5 6 7 00787-017 figure 6. error vs. crest factor
ad636 data sheet rev. e | page 8 of 16 theory of operation rms measurements the ad636 embodies an implicit solution of the rms equation that overcomes the dynamic range as well as other limitations inherent in a straightforward computation of rms. the actual computation performed by the ad636 follows the equation: ? ? ? ? ? ? ? ? ?? rmsv v avgrmsv in 2 the ad636 is comprised of four major sections: absolute value circuit (active rectifier), squarer/divider, current mirror, and buffer amplifier (see figure 7, for a simplified schematic). the input voltage, v in , which can be ac or dc, is converted to a unipolar current i1, by the active rectifier a1, a2. i1 drives one input of the squarer/divider, which has the transfer function: i3 i1 i4 2 ? the output current, i4, of the squarer/divider drives the current mirror through a low-pass filter formed by r1 and the externally connected capacitor, c av . if the r1, c av time constant is much greater than the longest period of the input signal, then i4 is effectively averaged. the current mirror returns a current i3, which equals avg. [i4], back to the squarer/divider to complete the implicit rms computation. therefore, rmsi1 i4 i2 avgi4 ? ? ? ? ? ? ? ?? 2 the current mirror also produces the output current, i out , which equals 2i4. i out can be used directly or converted to a voltage with r2 and buffered by a4 to provide a low impedance voltage output. the transfer function of the ad636 thus results v out = 2 r2 i rms = v in rms the db output is derived from the emitter of q 3 , because the voltage at this point is proportional to Clog v in . emitter follower, q5, buffers and level shifts this voltage, so that the db output voltage is zero when the externally supplied emitter current (i ref ) to q5 approximates i3. absolute value/ voltage?current converter a4 6 7 5 3 9 8 4 10 14 a1 a2 a3 1 com buffer buf in 10k ? q5 q4 q2 q1 q3 c av i out 8k ? 8k ? + |v in | r4 i1 i3 i4 i ref current mirro r v in r4 20k ? r3 10k ? one-quadrant squarer/ divider ?v s +v s r l db out buf out r2 10k ? 20a fs r1 25k ? 10a fs 00787-013 +v s c av figure 7. simplified schematic the ad636 buffer amplifier the buffer amplifier included in the ad636 offers the user additional application flexibility. it is important to understand some of the characteristics of this amplifier to obtain optimum performance. figure 8 shows a simplified schematic of the buffer. because the output of an rms-to-dc converter is always positive, it is not necessary to use a traditional complementary class ab output stage. in the ad636 buffer, a class a emitter follower is used instead. in addition to excellent positive output voltage swing, this configuration allows the output to swing fully down to ground in single-supply applications without the problems associated with most ic operational amplifiers. buffer output 10k ? r externa l (optional, see text) ?v s + v s buffer input current mirror r load r e 40k ? 5a 5a 00787-014 figure 8. buffer amplifier simplified schematic when this amplifier is used in dual-supply applications as an input buffer amplifier driving a load resistance referred to ground, steps must be taken to ensure an adequate negative voltage swing. for negative outputs, current flows from the load resistor through the 40 k emitter resistor, setting up a voltage divider between ?v s and ground. this reduced effective ?v s , limits the available negative output swing of the buffer. the addition of an external re sistor in parallel with r e alters this voltage divider such that increased negative swing is possible.
data sheet ad636 rev. e | page 9 of 16 figure 4 shows the value of r external for a particular ratio of v peak to ?v s for several values of r load . the a ddition of r external increases the quiescent current of the buffer amplifier by an amount equal to r ext /?v s . nominal buffer quiescent current with no r external is 30 a at ?v s = ?5 v. frequency response the ad636 uses a logarithmic circuit to perform the implicit rms computation. as with any log circuit, bandwidth is proportional to sign al level. the solid lines in figure 5 represent the frequency response of the ad636 at input levels from 1 mv to 1 v rms. the dashed lines indicate the upper frequency limits for 1%, 10%, and 3 db of reading additional error. for example, note that a 1 v rms signal produces less than 1% of reading additional error up to 220 khz. a 10 mv signal can be measured with 1% of reading additiona l error (100 v) up to 14 khz. ac measurement accur acy and crest factor (cf) crest factor is o ften 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 sign al (cf = v p /v rms) . most common waveforms, such as sine and triangle waves, have relatively low crest factors (<2). waveforms that res emble 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 / ) . figure 6 is a curve of reading error for the ad636 for a 200 mv rms input signal with crest factors from 1 to 7. a rectangular pulse train (pulse width 20 0 s) was u sed for this test because it is the w orst - 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 1 to 7 while maintaining a constant 200 mv rms input amplitude.
ad636 data sheet rev. e | page 10 of 16 appl ications the input and outp ut signal ranges are a function of the supply voltages as detailed in the specifications. the ad636 can also be used in an unbuffered voltage output mode by disconnecting the input to the buffer. the output then appears unbuffered across the 10 k resistor. the buffer amplifier can then be used for other purposes. further, the ad636 can be used in a current output mode by disconnecting the 10 k resistor from the ground. the output current is available at pin 8 (pin 10 on the h package) with a nom inal scale of 100 a per volt rms input, positive out. standard connection the ad636 is simple to connect for the majority of high accuracy r ms measurements, requiring only an external capacitor to set the averaging time constant. the standard connection i s shown i n figure 9 in this configuration, the ad636 measure s the rms of the ac and dc level present at the input but show s an error for low frequency inputs as a function of the filter capacitor , c av , a s shown in figure 13 . th erefore , if a 4 f capacitor is used, the additional average error at 10 hz is 0.1%, and at 3 hz it is 1%. the accuracy at higher frequencies is according to specification. if it is desired to reject the dc input, a capacitor is added in series with the input, as shown in figure 11 ; the capac itor must be nonpolar. if the ad636 is driven with power supplies with a considerable amount of high frequency ripple, it is advisable to bypass both supplies to ground wit h 0.1 f cera mic discs as near the device as possi ble. c f is a n optional output ripple filter . v in ad636 14 13 12 1 1 10 9 8 1 2 3 4 5 6 7 absolute v alue squarer divider buf + ? current mirror 10k? 10k? +v s c f (optional) squarer divider absolute v alue ad636 current mirror + buf 10k? 10k? + ? 1 2 10 9 4 5 6 8 3 7 v in ?v s c f (optional) v out ?v s c a v c a v +v s ? 00787-005 buf out buf in i out r l com +v erms ?v db +v nc nc nc com r l i out buf in buf out db c a v + ? c +v ?v nc erms nc = no connect figure 9. standard rms connection optional trims f or high accuracy if it is desired to improve the accuracy of the ad636, the external trims shown in figure 10 can be added. r4 is used to trim the offset. the scale factor is trimmed by using r1 as shown. the insertion of r2 allows r1 to either increase or decrease the scale factor by 1 .5%. the trimming procedure is as follows: ? ground the input signa l, v in , and adjust r4 to give 0 v output from pin 6. alternatively, r4 can be adjusted to give the correct output with the lowest expected value of v in . ? connect the desired full - scale input level to v in , either dc or a calibrated ac signal (1 khz is t he optimum frequency); then trim r1 to give the correct output from pin 6, that is, 200 mv dc input should give 200 mv dc output. of course, a 200 mv peak - to - peak sine wave should give a 141.4 mv dc output. the remaining errors, as given in the specificat ions, are due to the nonlinearity. r2 154? 1 2 3 4 5 6 7 ad63 6 1 4 1 3 1 2 1 1 1 0 9 8 absolute v alue squarer divider 10k? 10k? current mirror v in ?v s ?v scale f ac t or adjust ? c a v buf + r1 200? 1.5% +v s +v s r4 500k? ?v s offset adjust r3 470k? v out 00787-006 +v nc nc nc com r l i out erms nc c a v db buf out buf in nc = no connect + ? figure 10 . optional external gain and output offset trims single - supply connection although the applications illustrated in figure 9 and figure 10 assume the use of dual power supplies, three external bias components connected to the com pin enable powering the ad636 with unipolar supplies as low as 5 v. the two resistors and capacitor network shown connected to p in 10 in figure 11 are satisfactory over the same range of voltages permissible with dual supply operation. any external bias voltage applied to pin 10 is i nternally reflected to the vin pin, rendering the same ac operation as w ith a dual supply. dc or ac + dc conversion is impractical, due to the resultant dc level shift at the input. the capacitor insures that no extraneous signals are coupled into the com pin . the values of the resistors are relatively high to minimize power c onsumption because only 1 a of bias current flows into pin 10 (pin 2 on the h package). alternately, the com pin of some cmos adcs provides a suitable artificial ground for the ad636. ac input coupling requires only capacitor c2 as shown; a dc return is n ot necessary because it is provided internally. c2 is selected for the proper low frequency break point with the input resistance of 6.7 k ; for a cut - off at 10 hz, c2 should be 3.3 f. the signal ranges in this connection are
data sheet ad636 rev. e | page 11 of 16 slightly more restricted than in the dual supply connection. the load resistor, r l , is necessary to provide current sinking capability. c2 3.3f ad63 6 absolute v alue squarer divider 10k? 10k? current mirror ? c a v buf + 20k? nonpolarized 39k? 0.1f 0.1f +v s v out r l 1k? t o 10k? v in 00787-007 1 2 3 4 5 6 7 1 4 1 3 1 2 1 1 1 0 9 8 v in nc ?v s c a v db buf out buf in nc nc nc com r l i out nc = no connect + ? figure 11 . single - supply connection (see text) choosing t he averaging time co nstant the ad636 compute s the rms of both ac and dc si gnals. if the input is a slowly varying dc voltage, the output of the ad636 track s the input exactly. at higher frequencies, the average output of the ad636 approach es the rms value of the input signal. the actual output of the ad636 differ s from the ideal output by a dc (or average) error and some amount of ripple, as demonstrated in figure 12. t i me i d ea l e o dc err o r = e o ? e o (i d ea l ) a vera g e e o = e o d o ub l e-f r eq u enc y r i ppl e e o 00787-008 figure 12 . typical output waveform for sinusoidal input the dc error is dependent on the input signal frequency and the value of c av . figure 13 can be used to determine the minimum value of c av , which yield s a give n % dc error above a given frequency using the standard rms connection. the ac component of the output signal is the ripple. there are two ways to reduce the ripple. the first method involves using a large value o f c av . because the ripple is inversely prop ortional to c av , a ten fold increase in this capacitance effect s a tenfold reduction in ripple. when measuring waveforms with high crest factors (such as low duty cycle pulse trains), the averaging time constant should be at least ten times the signal perio d. for example, a 100 hz pulse rate requires a 100 ms time constant, which corresponds to a 4 f capacitor (time constant = 25 ms per f) . input frequency (hz) 100 0.01 1 10 0.1 1 10 100 0.1 0.01 0.01% error 0.1% error *% dc error + % ripple (peak) 1% error for 1% settling time in seconds multiply reading by 0.115 required c av (f) 1 10 100 1k 10k 100k values for c av and 1% settling time for stated % of reading averaging error* accuracy 20% due to component tolerance 10% error 00787-009 figure 13 . error/settling time graph for use with the standard rms connection the primary disadvantage in using a large c av to rem ove ripple is that the settling time for a step change in input level is increased proportionately. figure 13 shows the relationship betwee n c av a nd 1% settling time is 115 ms for each microfarad of c av . t he settling time is twice as great for decreasing signals as for increasing sig nals (the values in figure 13 are for decreasing signals). settling time also increases for low signal levels, as shown in figure 14. rms i n p u t l evel 10 . 0 7 . 5 0 10m v 100m v 1 . 0 5 . 0 2 . 5 1 v 1m v settling time relative to settling time @ 200mv rms 00787-010 figure 14 . settling time vs. input level a better method for reducing output ripple is the use of a post - filter. figure 15 shows a suggested circuit. if a single - pole filter is used (c3 removed, r x shorted), and c2 is approximately 5 times the value o f c av , the ripple is reduced , as shown in figure 16 , and the settling time is increased. for example, with c av = 1 f and c2 = 4.7 f, the ripple for a 60 hz input is reduced from 10% of reading to approximately 0.3% of reading. the settling time, however, is increased by approximately a factor of 3. the values of c av and c2 can therefore be reduced to permit faster settling times whi le still providing substantial ripple reduction.
ad636 data sheet rev. e | page 12 of 16 the 2 - pole post filter uses an active filter stage to provide even greater ripple reduction without substantially increasing the settling times over a circuit with a 1 - pole filter. the value s of c av , c2, a nd c3 can then be reduced to allow extremely fast settling times for a constant amount of ripple. caution should be exercised in choosing the value of c av , because the dc error is dependent upon this value and is independent of the post filter. for a more detailed explanation of these topics , refer to the rms - to - dc conversion application guide , 2nd edition . rx 10k? 1 2 3 4 5 6 7 ad636 14 13 12 1 1 10 9 8 absolute v alue squarer divider buf current mirror + ? + ? + ? + ? c2 c3 (for single pole, short rx, remove c3) c ?v v in v in +v s +v v rms out 10k? 10k? 00787-0 1 1 nc nc nc com r l i out nc ?v s c a v db buf out buf in nc = no connect +v s figure 15 . 2 - pole post filter f r eq u enc y (h z) 1 0 0 . 1 dc error or ripple (% of reading) 1 1 0 10 0 1 k 10 k p -p r i ppl e (o n e po l e) c a v = 1 f c 2 = 4 . 7 f dc err o r c a v = 1 f (a ll f i lter s) p -p r i ppl e (two po l e) c a v = 1 f , c 2 = c 3 = 4 . 7 f 00787-012 p -p r i ppl e c a v = 1 f (st andard c o nn ec t io n ) figure 16 . per formance features of various filter types a complete a c digital voltmeter figure 17 shows a design for a complete low power ac digital voltmeter circuit based on the ad636. t he 10 m inp ut attenuator allows full - scale ranges of 20 0 mv, 2 v, 20 v , and 200 v rms. signals are capacitively coupled to the ad636 buffer amplifier, which is connected in an ac bootstrapped configuration to minimize loading. the buffer then drives the 6. 7 k in put impedance of the ad6 36. the com terminal of the adc provides the false ground r equired by the ad636 for single - supply operation. an ad589 1.2 v reference diode is used to provide a stable 100 mv reference for the adc in the linear rms mode; in the db mode, a 1n4148 diode is inserted in series to pro vide correction for the temperature coefficient of the db scale factor. adjust r13 to calibrate the meter for an accurate readout at full scale. calibration of the db range is accomplished by adjusting r9 for the desired 0 db reference point, and then adj usting r14 for the desired db scale factor (a scale of 10 counts per db is convenient). total power supply current for this circuit is typically 2.8 ma using a 7106 - type adc. a low power, high in put , impedance db meter the portable db meter circuit combine s the functions of the ad636 rms converter, the ad589 voltage reference, and a ? a77 6 low power operational amplifier (see figure 18) . this meter offers excellent bandwidth and superior high and low level accuracy while consuming minimal power from a standard 9 v transistor radio battery. in this circuit, the built - in buffer ampli fier of the ad636 is used as a bootstrapped input stage increasing the normal 6.7 k input z to an input impedance of approximate ly 10 10 . circuit descr iption the input volt age, v in , is ac - coupled by c4 while r8, together with d1 and d2, provide high input voltage protection. th e buffers output, pin 6, is ac - coupled to the rms converters input (pin 1) by capacitor c2. resistor r9 is connected between th e buffers output, a class a output stage, and the negative output swing. resistor r1 is the amplifiers bootstrapping resistor. with this circuit, single - supply o peration i s made possible by setting ground at a point between the positive and negative side s of the battery. this is accomplished by sending 250 a from the positive battery terminal through r2, then through the 1.2 v ad589 band gap reference, and finally back to the negative side of the battery via r10. this sets ground at 1.2 v + 3.18 v (250 a 12.7 k) = 4.4 v below the positive battery terminal and 5.0 v (250 a 20 k) above the negative battery terminal. bypass c apacitors , c3 and c5 , keep both sides of the battery at a low ac impedance to ground. the ad589 band gap reference establishes the 1.2 v regulated reference voltage , which together with r3 and trimming p otentiometer r4 , set s the 0 db reference current , i ref . performance data 0 db reference range = 0 dbm (770 mv) to ?20 dbm (77 mv) rms 0 dbm = 1 mw in 600 input range (a t i ref = 7 70 mv) = 50 dbm input impedance = approximately 10 10 ? v supply operating range = +5 v dc to +20 v dc i quiescent = 1. 8 ma typical accuracy with 1 khz sine wave and 9 v dc supply: 0 db to ?40 dbm 0.1 dbm 0 dbm to ?50 dbm 0.15 dbm +10 dbm to ?50 dbm 0. 5 dbm
data sheet ad636 rev. e | page 13 of 16 frequency response 3 db m input 0 dbm = 5 hz to 380 khz ?10 dbm = 5 hz to 370 khz ?20 dbm = 5 hz to 240 khz ?30 dbm = 5 hz to 100 khz ?40 dbm = 5 hz to 45 khz ? 50 dbm = 5 hz to 17 khz calibration first , calibrate the 0 db reference level by applying a 1 khz sine wave from an audio oscillator at the desired 0 db amplitude. this can be anywhere from 0 dbm (770 mv rms ? 2.2 v p - p) to ?20 dbm ( 77 mv rms ? 220 mv p - p ). adjust the i ref cal ibration trimmer for a zero indication on the analog meter. then, ca librate the meter scale factor or gain. apply an input signal ?40 db below the set 0 db reference and adjust the scale factor calibration trimmer for a 40 a reading on the analog meter. the temperature compensation resistors for this circuit can be purcha sed from micro - ohm corporation, 1088 hamilton rd . , duarte, ca 91010, part #type 401f, 2 k ,1% + 3500 ppm/c. r2 900k? 1 2 3 4 5 6 7 ad636 14 13 12 1 1 10 9 8 buf + ? + ? + + ? +v dd ref hi ref lo com hi lo + ? + on off lxd 7543 lin db lin db lin db 200mv 2v 20v 200v com v in r1 9m? r3 90k? r4 10k? c3 0.02f r5 47k? 1w 10% d1 1n4148 c4 2.2f r6 1m? absolute v alue squarer divider current mirror 6.8f r7 20k? d4 1n4148 10k? 10k? c7 6.8f d2 1n4148 r8 2.49k? +v s r9 100k? 0db set r10 20k? d3 1.2v ad589 lin scale r15 1m? c6 0.01f ?v s ?v ss analog in 3-1/2 digit 7106 type a/d converter +v dd ?v ss 9v ba tte r y 1f r1 1 10k? r12 1k? r13 500? r14 10k? db scale 3-1/2 digit lcd displ a y 00787-018 v in nc ?v s c a v db buf out buf in +v s nc nc nc com r l i out nc = no connect figure 17 . portable, high - z input, rms dpm and db meter circuit al l resis t ors 1/4w 1% me t al film unless othe r wise s ta ted except *which is 2k? +3500ppm 1% tc resis t or. 1 2 3 4 5 6 7 ad636 14 13 12 1 1 10 9 8 buf + ? + ? + ? a776 + ? + + + + on/off 9v +1.2v ad589j 250 a 100 a + +4.2v ?4.8v d1 1n6263 signa l input c4 0.1f r8 47k? 1w d2 1n6263 c1 3.3f r1 1m? c2 6.8f 10k? 10k? r9 10k? absolute v alue squarer divider current mirror r2 12.7k? c3 10f c5 10f r10 20k? c6 0.1f *r7 2k? r6 100? r3 5k? r4 500k? i ref adjust r1 1 820k? 5% 0?50 a r5 10k? scale f ac t or adjust 2 3 4 8 7 6 00787-019 nc = no connect v in nc ?v s c a v db buf out buf in +v s nc nc nc com r l i out figure 18 . low power, h i g h input impedance db mete r
ad636 data sheet rev. e | page 14 of 16 outline dimensions 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. 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 19 . 14 - lead side - brazed ceramic dual in - line package [sbdip] (d - 14) dimensions shown in inches and (millimeters) 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. dimensions per jedec s t andards mo-006-af 0.500 (12.70) min 0.185 (4.70) 0.165 (4.19) reference plane 0.050 (1.27) max 0.040 (1.02) max 0.335 (8.51) 0.305 (7.75) 0.370 (9.40) 0.335 (8.51) 0.021 (0.53) 0.016 (0.40) 1 0.034 (0.86) 0.025 (0.64) 0.045 (1.14) 0.025 (0.65) 0.160 (4.06) 0. 1 10 (2.79) 6 2 8 7 5 4 3 0. 1 15 (2.92) bsc 9 10 0.230 (5.84) bsc base & se a ting plane 36 bsc 022306- a figure 20 . 10 - pin metal header pa ckage [to - 100] (h- 10) dimensions shown in inches and (millimeters) ordering guide model 1 temperature range package description package option ad636jdz 0c to +70c 14- lead sbdip d -14 AD636KDZ 0c to +70c 14- lead sbdip d -14 ad636jh 0c to +70c 10- pin to -100 h -10 ad636jhz 0c to +70c 10- pin to -100 h -10 ad636kh 0c to +70c 10- pin to -100 h -10 ad636khz 0c to +70c 10- pin to -100 h -10 1 z = rohs - complia nt part.
data sheet ad636 rev. e | page 15 of 16 notes
ad636 data sheet rev. e | page 16 of 16 notes ?2013 analog devices, inc. all rights reserved. trademarks and registered trademarks are the prop erty of their respective owners. d00787-0-5/13(e)

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