AD637

REV. E

–6–

BUFFER

AD637

SQUARER/DIVIDER

BIAS

SECTION

FILTER

25k

25k

1

2

3

4

5

6

7

14

13

12

11

10

9

8

+

ABSOLUTE

VALUE

RMS

OUTPUT

BUFFER

OUTPUT

ANALOG COM

OUTPUT

OFFSET

+

C2

24k

BUFFER INPUT

NC

CHIP

SELECT

DENOMINATOR

INPUT

dB

24k

FOR 1 POLE

FILTER, SHORT

REMOVE C3

SIGNAL

INPUT

+

C3

NC

Figure 8. Two Pole Sallen-Key Filter

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

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

the graph.

Figure 9b shows the relationship between averaging error, signal

frequency settling time and averaging capacitor value. This

graph is drawn for filter capacitor values of 3.3 times the averag-

ing 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 averag-

ing capacitor and a 3.3

µF filter capacitor, the ripple for a 60 Hz

input signal will be reduced from 5.3% of reading using the

averaging capacitor alone to 0.15% using the single pole filter.

This gives a factor of thirty reduction in ripple and yet the set-

tling time would only increase by a factor of three. The values of

value of averaging error and settling time by using Figure 9b.

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

nitude of the averaging error. Table I gives practical component

values for various types of 60 Hz input signals. These capacitor

values can be directly scaled for frequencies other than 60 Hz,

i.e., for 30 Hz double these values, for 120 Hz they are halved.

For applications that are extremely sensitive to ripple, the two pole

configuration is suggested. This configuration will minimize

capacitor values and settling time while maximizing performance.

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

1% SETTLING TIME

FOR STATED % OF READING

AVERAGING ERROR*

ACCURACY

2% DUE TO

COMPONENT TOLERANCE

* %dc ERROR + %RIPPLE (Peak)

100

0.01

10

1.0

0.1

Figure 9a.

INPUT FREQUENCY – Hz

100

0.01

1

100k

10

100

1k

10k

10

1.0

0.1

1% SETTLING TIME FOR

STATED % OF READING

AVERAGING ERROR*

FOR 1 POLE POST FILTER

* %dc ERROR + % PEAK RIPPLE

ACCURACY

20% DUE TO

COMPONENT TOLERANCE

100

0.01

10

1.0

0.1

Figure 9b.

INPUT FREQUENCY – Hz

100

0.01

1

100k

10

100

1k

10k

10

1.0

0.1

AND 1% SETTLING TIME FOR

STATED % OF READING

AVERAGING ERROR*

2 POLL SALLEN-KEY FILTER

* %dc ERROR + % PEAK RIPPLE

ACCURACY

20% DUE TO

COMPONENT TOLERANCE

100

0.01

10

1.0

0.1

Figure 9c.