AD536A
The two-pole post-filter uses an active filter stage to provide
even greater ripple reduction without substantially increasing
the settling times over a circuit with a one-pole filter. The values
of CAV, C2, and C3 can then be reduced to allow extremely fast
settling times for a constant amount of ripple. Caution should
be exercised in choosing the value of CAV, since the dc error is
dependent upon this value and is independent of the post filter.
factors, (such as low duty cycle pulse trains), the averaging time
constant should be at least ten times the signal period. For
example, a 100 Hz pulse rate requires a 100 ms time constant,
which corresponds to a 4 µF capacitor (time constant = 25 ms
per µF).
The primary disadvantage in using a large CAV to remove ripple
is that the settling time for a step change in input level is in-
creased proportionately. Figure 5 shows that the relationship
between CAV and 1% settling time is 115 milliseconds for each
microfarad of CAV. The settling time is twice as great for de-
creasing signals as for increasing signals (the values in Figure 5
are for decreasing signals). Settling time also increases for low
signal levels, as shown in Figure 6.
For a more detailed explanation of these topics refer to the
RMS to DC Conversion Application Guide 2nd Edition, available
from Analog Devices.
C3
C2
C3
Figure 7. 2-Pole “Post” Filter
Figure 5. Error/Settling Time Graph for Use with the Stan-
dard rms Connection in Figure 1
Figure 6. Settling Time vs. Input Level
Figure 8. Performance Features of Various Filter Types
A better method for reducing output ripple is the use of a
“post-filter.” Figure 7 shows a suggested circuit. If a single-pole
filter is used (C3 removed, RX shorted), and C2 is approximately
twice the value of CAV, the ripple is reduced as shown in Figure
8 and settling time is increased. For example, with CAV = 1 µF
and C2 = 2.2 µF, the ripple for a 60 Hz input is reduced from
10% of reading to approximately 0.3% of reading. The settling
time, however, is increased by approximately a factor of 3. The
values of CAV and C2, can, therefore, be reduced to permit faster
settling times while still providing substantial ripple reduction.
AD536A PRINCIPLE OF OPERATION
The AD536A embodies an implicit solution of the rms equation
that overcomes the dynamic range as well as other limitations
inherent in a straightforward computation of rms. The actual
computation performed by the AD536A follows the equation:
2
VIN
V rms = Avg.
V rms
REV. B
–5–
ADI [ ADI ]
