AD736
TYPES OF AC MEASUREMENT
The AD736 is capable of measuring ac signals by operating as
either an average responding or a true rms-to-dc converter. 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 will approximate 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 aver-
age 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
dc: an ac signal of 1 volt rms will produce the same amount of
heat in a resistor as a 1 volt dc signal.
Mathematically, the rms value of a voltage is defined (using a
simplified equation) as:
V rms
=
Avg.(V
2
)
tions: input amplifier, full-wave rectifier, rms core, output am-
plifier and bias sections. The FET input amplifier allows
both a high impedance, buffered input (Pin 2) or a low imped-
ance, wide-dynamic-range input (Pin 1). The high impedance
input, with its low input bias current, is well suited for use with
high impedance input attenuators.
The output of the input amplifier drives a full wave precision
rectifier, which in turn, drives the rms core. It is in the core that
the essential rms operations of squaring, averaging and square
rooting are performed, using an external averaging capacitor,
C
AV
. Without C
AV
, the rectified input signal travels through the
core unprocessed, as is done with the average responding con-
nection (Figure 17).
A final subsection, an output amplifier, buffers the output from
the core and also allows optional low-pass filtering to be per-
formed via external capacitor, C
F
, connected across the feed-
back path of the amplifier. In the average responding
connection, this is where all of the averaging is carried out. In
the rms circuit, this additional filtering stage helps reduce any
output ripple which was not removed by the averaging capaci-
tor, C
AV
.
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 will depend upon 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 will have a computational error 11% (of reading)
higher than the true rms value (see Table I).
AD736 THEORY OF OPERATION
As shown by Figure 16, the AD736 has five functional subsec-
Figure 16. AD736 True RMS Circuit
Table I. Error Introduced by an Average Responding Circuit When Measuring Common Waveforms
Crest Factor
(V
PEAK
/V rms)
True rms Value
Average Responding
Circuit Calibrated to
Read rms Value of
Sine Waves Will Read
% of Reading Error*
Using Average
Responding Circuit
Waveform Type
1 Volt Peak
Amplitude
Undistorted
Sine Wave
Symmetrical
Square Wave
Undistorted
Triangle Wave
Gaussian
Noise (98% of
Peaks <1 V)
Rectangular
Pulse Train
SCR Waveforms
50% Duty Cycle
25% Duty Cycle
1.414
0.707 V
0.707 V
0%
1.00
1.73
1.00 V
0.577 V
1.11 V
0.555 V
+11.0%
–3.8%
3
2
10
2
4.7
*%of
Reading Error
=
0.333 V
0.5 V
0.1 V
0.495 V
0.212 V
0.295 V
0.278 V
0.011 V
0.354 V
0.150 V
–11.4%
–44%
–89%
–28%
–30%
Average Responding Value
–
True rmsValue
×
100%
True rmsValue
–6–
REV. C
AD [ ANALOG DEVICES ]
