Data Sheet

AD536A

Rev. E | Page 13 of 16

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

increased proportionately. Figure 19 illustrates that the

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

1% SETTLING TIME

FOR STATED % OF READING

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

= 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

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

settling times for a constant amount of ripple. Caution should

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, available online

from Analog Devices, Inc., at www.analog.com.

C2

14

13

12

11

10

9

8

1

2

3

4

5

6

7

AD536A

25kΩ

ABSOLUTE

VALUE

SQUARER/

DIVIDER

CURRENT

MIRROR

Rx

24kΩ

+

–

NC

NC

NC

NC

dB

COM

BUF OUT

BUF IN

BUF

Figure 21. Two-Pole Postfilter

1

1k

100

10k

0.1

10

10

PEAK-TO-PEAK RIPPLE

DC ERROR

(ALL FILTERS)

PEAK-TO-PEAK RIPPLE

C2 = C3 = 2.2µF (TWO-POLE)

Rx = 0Ω

PEAK-TO-PEAK

RIPPLE (ONE POLE)

FREQUENCY (Hz)

Figure 22. Performance Features of Various Filter Types

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