AD736
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. T he dc
error component is therefore set solely by the value of averaging
capacitor used-no amount of post filtering (i.e., using a very
large CF) will allow the output voltage to equal its ideal value.
T he ac error component, an output ripple, may be easily re-
moved by using a large enough post filtering capacitor, CF.
RMS MEASUREMENT – CH O O SING TH E O P TIMUM
VALUE FO R CAV
Since the external averaging capacitor, CAV, “holds” the recti-
fied input signal during rms computation, its value directly af-
fects the accuracy of the rms measurement, especially at low
frequencies. Furthermore, because the averaging capacitor ap-
pears across a diode in the rms core, the averaging time constant
will increase exponentially as the input signal is reduced. T his
means that as the input level decreases, errors due to nonideal
averaging will reduce while the time it takes for the circuit to
settle to the new rms level will increase. T herefore, lower input
levels allow the circuit to perform better (due to increased aver-
aging) but increase the waiting time between measurements.
Obviously, when selecting CAV, a trade-off between computa-
tional accuracy and settling time is required.
In most cases, the combined magnitudes of both the dc and ac
error components need to be considered when selecting appro-
priate values for capacitors CAV and CF. T his combined error,
representing 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 de-
crease rapidly: if the input frequency doubles, the dc error and
ripple reduce to 1/4 and 1/2 their original values, respectively,
and rapidly become insignificant.
AC MEASUREMENT ACCURACY AND CREST FACTO R
T he 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 am-
plitude (C.F. = VPEAK/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. T hese types of waveforms
require a long averaging time constant (to average out the long
time periods between pulses). Figure 6 shows the additional
error vs. crest factor of the AD736 for various values of CAV
.
SELECTING P RACTICAL VALUES FO R INP UT
CO UP LING (C C), AVERAGING (C AV) AND FILTERING
(CF) CAP ACITO RS
T able II provides practical values of CAV and CF for several
common applications.
Figure 17. AD736 Average Responding Circuit
Table II. AD 737 Capacitor Selection Chart
Application
rm s
Input
Level
Low
Max
CAV
CF
Settling
Tim e*
to 1%
RAP ID SETTLING TIMES VIA TH E AVERAGE
RESP O ND ING CO NNECTIO N (FIGURE 17)
Because the average responding connection does not use the
CAV averaging capacitor, its settling time does not vary with in-
put signal level; it is determined solely by the RC time constant
of CF and the internal 8 kΩ resistor in the output amplifier’s
feedback path.
Frequency Crest
Cutoff
(–3dB)
Factor
General Purpose 0–1 V
rms Computation
20 Hz
200 Hz
5
5
150 µF 10 µF 360 ms
15 µF 1 µF 36 ms
0–200 mV 20 Hz
200 Hz
5
5
33 µF 10 µF 360 ms
3.3 µF 1 µF 36 ms
General Purpose 0–1 V
Average
Responding
20 Hz
200 Hz
None 33 µF 1.2 sec
None 3.3 µF 120 ms
D C ERRO R, O UTP UT RIP P LE, AND AVERAGING
ERRO R
Figure 18 shows the typical output waveform of the AD736 with
a sine-wave input applied. As with all real-world devices, the
ideal output of VOUT = VIN is never exactly achieved; instead,
the output contains both a dc and an ac error component.
0–200 mV 20 Hz
200 Hz
None 33 µF 1.2 sec
None 3.3 µF 120 ms
SCR Waveform
Measurement
0–200 mV 50 Hz
60 Hz
5
5
100 µF 33 µF 1.2 sec
82 µF 27 µF 1.0 sec
0–100 mV 50 Hz
60 Hz
5
5
50 µF 33 µF 1.2 sec
47 µF 27 µF 1.0 sec
Audio
Applications
Speech
Music
0–200 mV 300 Hz
0–100 mV 20 Hz
3
1.5 µF 0.5 µF 18 ms
100 µF 68 µF 2.4 sec
10
*Settling time is specified over the stated rms input level with the input signal increasing
from zero. Settling times will be greater for decreasing amplitude input signals.
Figure 18. Output Waveform for Sine-Wave Input Voltage
REV. C
–7–
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