Data Sheet

AD536A

Rev. G | Page 13 of 15

The primary disadvantage in using a large CAV to remove ripple

is that the settling time for a step change in input level is

increased proportionately. Figure 19 illustrates that the

relationship between CAV and 1% settling time is 115 ms for

each microfarad of CAV. The settling time is twice as great for

decreasing signals as it is for increasing signals. The values in

Figure 19 are for decreasing signals. Settling time also increases

for low signal levels, as shown in Figure 20.

10

100

1k

10k

0.1

1

10

100

0.01

1

100k

INPUT FREQUENCY (Hz)

0.1

1

10

100

0.01

1PERCENT DC ERROR AND PERCENT RIPPLE (PEAK)

VALUES FOR CAV AND

1% SETTLING TIME

FOR STATED % OF READING

AVERAGING ERROR1

ACCURACY ± 20% DUE TO

COMPONENT TOLERANCE

Figure 19. Error/Settling Time Graph for Use with the Standard RMS

Connection (See Figure 13 Through Figure 15)

10m

100m

1

7.5

10.0

5.0

1m

10

rms INPUT LEVEL (V)

1.0

2.5

Figure 20. Settling Time vs. Input Level

A better method to reduce output ripple is the use of a postfilter.

Figure 21 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 22, 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. Therefore, the values of CAV and C2 can be reduced

to permit faster settling times while still providing substantial

ripple reduction.

The two-pole postfilter 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, because the dc error

is dependent on this value and is independent of the postfilter.

For a more detailed explanation of these topics, refer to the RMS to

DC Conversion Application Guide, 2nd Edition.

C2

VIN

CAV

+VS

14

13

12

11

10

9

8

1

2

3

4

5

6

7

AD536A

25kΩ

ABSOLUTE

VALUE

SQUARER/

DIVIDER

CURRENT

MIRROR

–VS

Rx

24kΩ

+

–

C31

Vrms OUT

1FOR SINGLE POLE, SHORT Rx, REMOVE C3.

VIN

NC

–VS

CAV

+VS

NC

NC

NC

dB

COM

BUF OUT

RL

BUF IN

IOUT

BUF

Figure 21. Two-Pole Postfilter

1

1k

100

10k

0.1

10

10

PEAK-TO-PEAK RIPPLE

CAV = 1µF

DC ERROR

CAV = 1µF

(ALL FILTERS)

PEAK-TO-PEAK RIPPLE

CAV = 1µF

C2 = C3 = 2.2µF (TWO-POLE)

Rx = 0Ω

PEAK-TO-PEAK

RIPPLE (ONE POLE)

CAV = 1µF, C2 = 2.2µF

FREQUENCY (Hz)

Figure 22. Performance Features of Various Filter Types

(See Figure 13 to Figure 15 for Standard RMS Connection)