AD637

Rev. K | Page 14 of 20

BUFFER

AD637

SQUARER/DIVIDER

BIAS

SECTION

FILTER

25kΩ

1

2

3

4

5

6

7

14

13

12

11

10

9

8

+

ABSOLUTE

VALUE

NC

SIGNAL

INPUT

NC

AD548JN

FILTERED

V RMS OUTPUT

1µF

1000pF

6.8MΩ

1MΩ

R

3.3MΩ

V RMS

4.7kΩ

BUFF

OUT

COMMON

OUTPUT

OFFSET

BUFF IN

CS

DEN

INPUT

dB OUTPUT

NOTES

1. VALUES CHOSEN TO GIVE 0.1% AVERAGING ERROR @ 1Hz.

2. NC = NO CONNECT.

3.3µF

499kΩ 1%

100µF

25kΩ

V–

3

2

4

7

6

V+

3.3MΩ

1µF

50kΩ

OUTPUT

OFFSET

ADJUST

Figure 21. AD637 as a Low Frequency RMS Converter

LOW FREQUENCY MEASUREMENTS

If the frequencies of the signals to be measured are below 10 Hz,

the value of the averaging capacitor required to deliver even 1%

averaging error in the standard rms connection becomes

extremely large. Figure 21 shows an alternative method of

obtaining low frequency rms measurements. The averaging

reduction in the value of the averaging capacitor, permitting the

use of high quality tantalum capacitors. It is suggested that the

2-pole, Sallen-Key filter shown in Figure 21 be used to obtain a

low ripple level and minimize the value of the averaging

capacitor.

If the frequency of interest is below 1 Hz, or if the value of the

averaging capacitor is still too large, the 20:1 ratio can be

increased. This is accomplished by increasing the value of R.

If this is done, it is suggested that a low input current, low offset

voltage amplifier, such as the AD548, be used instead of the

internal buffer amplifier. This is necessary to minimize the

offset error introduced by the combination of amplifier input

currents and the larger resistance.

VECTOR SUMMATION

Vector summation can be accomplished through the use of two

AD637s, as shown in Figure 22. Here, the averaging capacitors

are omitted (nominal 100 pF capacitors are used to ensure

stability of the filter amplifier), and the outputs are summed as

shown. The output of the circuit is

2

2

Y

X

OUT

V

V

V

+

=

This concept can be expanded to include additional terms by

feeding the signal from Pin 9 of each additional AD637 through

a 10 kΩ resistor to the summing junction of the AD711 and

tying all of the denominator inputs (Pin 6) together.

2

2

Y

X

V

V

+

If the averaging capacitor is included on both IC1 and IC2, the

output is

2

2

Y

X

V

V

+

This circuit has a dynamic range of 10 V to 10 mV and is

limited only by the 0.5 mV offset voltage of the AD637.

The useful bandwidth is 100 kHz.