AD737
REV. C–6–
Waveform Type Crest Factor True rms Value Average Responding % of Reading Error*
1 Volt Peak (V
PEAK
/V rms) Circuit Calibrated to Using Average
Amplitude Read rms Value of Responding Circuit
Sine Waves Will Read
Undistorted 1.414 0.707 V 0.707 V 0%
Sine Wave
Symmetrical
Square Wave 1.00 1.00 V 1.11 V +11.0%
Undistorted
Triangle Wave 1.73 0.577 V 0.555 V –3.8%
Gaussian
Noise (98% of
Peaks <1 V) 3 0.333 V 0.295 V –11.4%
Rectangular 2 0.5 V 0.278 V –44%
Pulse Train 10 0.1 V 0.011 V –89%
SCR Waveforms
50% Duty Cycle 2 0.495 V 0.354 V –28%
25% Duty Cycle 4.7 0.212 V 0.150 V –30%
input (Pin 1). The high impedance input, with its low input
bias current, is well suited for use with high impedance input
attenuators. The input signal may be either dc 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.
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, the bias section, permits a “power down”
function. This reduces the idle current of the AD737 from 160
µA down to a mere 30 µA. This feature is selected by tying Pin
3 to the +V
S
terminal. In the average responding connection, all
of the averaging is carried out by an RC post filter consisting of
an 8 kΩ internal scale-factor resistor connected between Pins 6
and 8 and an external averaging capacitor, C
F
. In the rms cir-
cuit, this additional filtering stage helps reduce any output
ripple which was not removed by the averaging capacitor, C
AV
.
RMS MEASUREMENT – CHOOSING THE OPTIMUM
VALUE FOR C
AV
Since the external averaging capacitor, C
AV
, “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 con-
stant will increase exponentially as the input signal is reduced.
This means that as the input level decreases, errors due to
nonideal averaging will reduce while the time it takes for the cir-
cuit to settle to the new rms level will increase. Therefore, lower
input levels allow the circuit to perform better (due to increased
averaging) but increase the waiting time between measure-
ments. Obviously, when selecting C
AV
, a trade-off between
computational accuracy and settling time is required.
Mathematically, the rms value of a voltage is defined (using a
simplified equation) as:
V rms =Avg.(V
2
)
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 de voltages, the
converter will have a computational error 11% (of reading)
higher than the true rms value (see Table I).
AD737 THEORY OF OPERATION
As shown by Figure 16, the AD737 has four functional subsec-
tions: input amplifier, full-wave rectifier, rms core and bias sec-
tions. The FET input amplifier allows both a high impedance,
buffered input (Pin 2) or a low impedance, wide-dynamic-range
FULL
WAVE
RECTIFIER
INPUT
AMPLIFIER
RMS
CORE
8kVAD737
BIAS
SECTION
8kV
8
7
6
5
1
2
3
4
COM
+VS
OUTPUT
CAV
–VS
POWER
DOWN
VIN
CC
0.1mF
0.1mF–VS
+VS
POSITIVE SUPPLY
COMMON
NEGATIVE SUPPLY
CAV
33mF
CF
10mF
(OPTIONAL)
CC
10mF
(OPTIONAL
VOUT
Figure 16. AD737 True RMS Circuit
Table I. Error Introduced by an Average Responding Circuit When Measuring Common Waveforms