Data Sheet

AD637

A preferable ripple reduction method is to use a post conversion

one or two-pole low-pass filter, as shown in Figure 11. Usually

a single-pole filter gives the best overall compromise between

ripple and settling time. Use the two-pole Sallen-Key for more

ripple attenuation.

1

25kΩ

DEN

INPUT

BUFF IN

BUFF

OUT

SQUARER/

DIVIDER

CS

dB

OUTPUT

3 COMMON

ABSOLUTE

VALUE

BIAS

2 NIC

4

OUTPUT

OFFSET

4.7kΩ

+VS

5

25kΩ

6

7

8

9

10

11

12

NIC

13

14

VIN

VIN

CAV

CAV

+

RX

24kΩ

DIVIDER

C2

24kΩ

+

C3

+

RMS

OUT

AD637

FOR A 1 POLE

FILTER SHORT RX

AND REMOVE C3

–

+

–

+

–

+

–VS

–VS

+VS

+VS

NIC = NO INTERNAL CONNECTION

Figure 11. 2-Pole Sallen-Key Filter

Figure 12 shows values of CAV and the corresponding averaging

error as a function of sine wave frequency for the standard rms

connection. The 1% settling time is shown on the right side of

Figure 12.

Figure 13 shows the relationship between the averaging error,

signal frequency settling time, and averaging capacitor value.

Figure 13 is drawn for filter capacitor values of 3.3× the

averaging capacitor value. This ratio sets the magnitude of

the ac and dc errors equal at 50 Hz. As an example, by using a

1 µF averaging capacitor and a 3.3 µF filter capacitor, the ripple

for a 60 Hz input signal is reduced from 5.3% of the reading

using the averaging capacitor alone to 0.15% using the 1-pole

filter. This gives a factor of 30 reduction in ripple, and yet the

settling time only increases by a factor of 3. The values of

Filter Capacitor CAV and Filter Capacitor C2 can be calculated

for the desired value of averaging error and settling time by

using Figure 13.

The symmetry of the input signal also has an effect on the

magnitude of the averaging error. Table 7 gives the practical

component values for various types of 60 Hz input signals.

These capacitor values can be directly scaled for frequencies

other than 60 Hz—that is, for 30 Hz, these values are doubled,

and for 120 Hz they are halved.

Use Figure 14 to determine the required value of CAV, C2, and

C3 for the desired level of ripple and settling time.

INPUT FREQUENCY (Hz)

100

0.01

1

100k

10

100

1k

10k

10

1.0

0.1

100

0.01

10

1.0

0.1

Figure 12. Values for CAV and 1% Settling Time for Stated % of Reading; Averaging

Error (% DC Error + % Ripple (Peak)); Accuracy Includes ±20% Component

Tolerance

INPUT FREQUENCY (Hz)

100

0.01

1

100k

10

100

1k

10k

10

1

0.1

100

0.01

10

1

0.1

Figure 13. Values of CAV, C2, and 1% Settling Time for Stated % of Reading for

1-Pole Post Filter; Averaging Error (% DC Error + % Ripple (Peak) Accuracy

±20% Due to Component Tolerance)

INPUT FREQUENCY (Hz)

100

0.01

1

100k

10

100

1k

10k

10

1

0.1

100

0.01

10

1

0.1

Figure 14. Values of CAV, C2, and C3 and 1% Settling Time for Stated % of

Reading for 2-Pole Sallen-Key Filter; Averaging Error (% DC Error + %Ripple

(Peak) Accuracy ±20% Due to Component Tolerance)

Rev. L | Page 15 of 25