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Part Name  AD736KR  Analog Devices 
Description  Low Cost, Low Power, True RMStoDC Converter 
AD736KR Datasheet PDF : 8 Pages

Applying the AD737
Figure 10. Error vs. RMS Input
Voltage (Pin 2) Using Circuit
of Figure 21
Figure 11. CAV vs. Frequency for
Specified Averaging Error
Figure 12. RMS Input Level vs.
Frequency for Specified Averaging
Error
Figure 13. Pin 2 Input Bias
Current vs. Supply Voltage
Figure 14. Settling Time vs. RMS
Input Level for Various Values of CAV
Figure 15. Pin 2 Input Bias Current
vs. Temperature
CALCULATING SETTLING TIME USING FIGURE 14
The graph of Figure 14 may be used to closely approximate the
time required for the AD737 to settle when its input level is re
duced in amplitude. The net time required for the rms converter
to settle will be the difference between two times extracted from
the graph – the initial time minus the final settling time. As an
example, consider the following conditions: a 33 µF averaging
capacitor, an initial rms input level of 100 mV and a final (re
duced) input level of 1 mV. From Figure 14, the initial settling
time (where the 100 mV line intersects the 33 µF line) is around
80 ms. The settling time corresponding to the new or final input
level of 1 mV is approximately 8 seconds. Therefore, the net
time for the circuit to settle to its new value will be 8 seconds
minus 80 ms which is 7.92 seconds. Note that, because of the
smooth decay characteristic inherent with a capacitor/diode
combination, this is the total settling time to the final value (i.e.,
not the settling time to 1%, 0.1%, etc., of final value). Also, this
graph provides the worst case settling time, since the AD737
will settle very quickly with increasing input levels.
TYPES OF AC MEASUREMENT
The AD737 is capable of measuring ac signals by operating as
either an average responding or a true rmstode 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 lowpass 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 sinewave voltage is 0.636 that of VPEAK;
the corresponding rms value is 0.707 times VPEAK. Therefore,
for sinewave 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.
REV. C
–5–

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