Data Sheet

AD637

NOTES

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

2. NIC = NO INTERNAL CONNECTION.

BUFFER

AD637

SQUARER/

DIVIDER

BIAS

SECTION

25kΩ

1

2

3

4

5

6

7

14

13

12

11

10

9

8

CAV

+

ABSOLUTE

VALUE

NIC

SIGNAL

INPUT

NIC

AD548JN

FILTERED

V RMS OUTPUT

1µF

1000pF

6.8MΩ

1MΩ

R

V RMS

4.7kΩ

BUFF

OUT

COMMON

OUTPUT

OFFSET

BUFF IN

CS

DEN

INPUT

dB OUTPUT

VIN

RMS

OUT

–VS

+VS

CAV1

3.3µF

499k

Ω 1%

VIN2

–VS

+VS

100µF

V–

3

2

4

7

6

V+

3.3MΩ

3.3MΩ

1µF

+VS

+VS

–VS

50kΩ

OUTPUT

OFFSET

ADJUST

25kΩ

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. Determine the

averaging time constant by the product of R and CAV1, in this

circuit, 0.5 sec/µF of CAV. This circuit permits a 20:1 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, increase the 20:1 ratio.

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

Use two AD637s for vector summation 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.

If CAV is added to IC1 in this configuration, then the output is

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.

Rev. L | Page 19 of 25