AD737
CALCULATING SETTLING TIME USING TPC 14
absolute value of a sine wave voltage is 0.636 that of VPEAK; the
corresponding rms value is 0.707 times VPEAK. Therefore, for
sine wave voltages, the required scale factor is 1.11 (0.707
divided by 0.636).
TPC 14 may be used to closely approximate the time required for
the AD737 to settle when its input level is reduced in amplitude.
The net time required for the rms converter to settle is 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 (reduced) input level of 1 mV.
From TPC 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 is 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%, and so on, of the final value). Also, this graph provides the
worst-case settling time, since the AD737 settles very quickly
with increasing input levels.
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
a dc voltage; an ac signal of 1 V rms produces the same amount
of heat in a resistor as a 1 V dc signal.
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 rectifiers;
they provide an accurate rms reading regardless of the type of
waveform being measured. However, average responding convert-
ers can exhibit very high errors when their input signals deviate
from their precalibrated waveform; the magnitude of the error
depends on 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).
TYPES OF AC MEASUREMENT
The AD737 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 approximates 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 average
Table I. Error Introduced by an Average Responding Circuit When Measuring Common Waveforms
Average Responding
Waveform Type
1 V Peak
Amplitude
Circuit Calibrated to
Read RMS Value of
Sine Waves Will Read
% of Reading Error
Using Average
Responding Circuit
Crest Factor
(VPEAK/V rms)
True RMS Value
Undistorted Sine Wave
Symmetrical Square Wave
Undistorted Triangle Wave
Gaussian Noise
1.414
1.00
1.73
0.707 V
1.00 V
0.577 V
0.707 V
1.11 V
0.555 V
0%
11.0%
–3.8%
(98% of Peaks <1 V)
Rectangular
Pulse Train
3
2
10
0.333 V
0.5 V
0.1 V
0.295 V
0.278 V
0.011 V
–11.4%
–44%
–89%
SCR Waveforms
50% Duty Cycle
25% Duty Cycle
2
4.7
0.495 V
0.212 V
0.354 V
0.150 V
–28%
–30%
REV. D
–7–
ADI [ ADI ]
