ES636
True RMS-to-DC Converters
current it draws at this mode is less than 1uA.
Choosing the Averaging Time Constant
The ES636 computes the RMS value of AC and DC signals. At low frequencies and DC,
the output tracks the input exactly; at higher frequencies, the average output approaches
the RMS value of the input signal. The actual output differs from the ideal by an
average (or DC) error plus some amount of ripple.
The DC error term is a function of the value of C and the input signal frequency. The
AV
output ripple is inversely proportional to the value of C . Waveforms with high crest
AV
factors, such as a pulse train with low duty cycle, should have an average time constant
chosen to be at least ten times the signal period.
Using a large value of C to remove the output ripple increases the setting time for a
AV
step change in the input signal level. Figure 3 shows the relationship between C and
AV
1 % settling time, where 110ms settling equals 4uF of C . The settling time, or time
AV
for the RMS converter to settle to within a given percent of the change in RMS level, is
set by the averaging time constant, which varies approximately 2:1 between decreasing
and increasing input signals. In addition, the settling time also varies with input signal
levels, increasing as the input signal is reduced, and decreasing as the input is
increased.
External Av. CAP, Cav
Settling Time (sec)
30uF
20uF
0.83
0.55
10uF
0.27
5uF
4uF
0.14
0.11
3uF
0.08
1%
0.5%
0.1%
2uF
0.06
1uF
10
0.03
100
Frequency (Hz)
Figure 4. Errors/Settling Time Graph for Standard Connection
The primary disadvantage in using a large C to remove ripple is that the settling
AV
time for a step change in input level is increased proportionately. A better method to
10
09/02/16
CYRUSTEK [ Cyrustek corporation ]
