AD637

Data Sheet

Rev. L | Page 12 of 25

THEORY OF OPERATION

FILTER/AMPLIFIER

24kΩ

24kΩ

ONE QUADRANT

SQUARER/DIVIDER

BUFFER

AMPLIFIER

Q1

Q2

Q3

Q4

125Ω

6kΩ

6kΩ

12kΩ

24kΩ

A5

A1

A2

ABSOLUTE VALUE VOLTAGE TO

CURRENT CONVERTER

I1

I3

I4

A4

A3

BIAS

Q5

CAV

+VS

RMS

OUT

COMMON

CS

DEN

INPUT

OUTPUT

OFFSET

dB

OUTPUT

AD637

BUFF OUT

BUFF IN

–VS

14

1

13

10

4

6

5

3

7

9

11

8

VIN

Figure 4. Simplified Schematic

The AD637 embodies an implicit solution of the rms equation

that overcomes the inherent limitations of straightforward rms

computation. The actual computation performed by the AD637

follows the equation:



















rms

V

V

Avg

rms

V

IN

2

Figure 4 is a simplified schematic of the AD637, subdivided

into four major sections: absolute value circuit (active rectifier),

squarer/divider, filter circuit, and buffer amplifier. The input

voltage (VIN), which can be ac or dc, is converted to a unipolar

current (I1) by the A1 and A2 absolute value circuit. I1 drives

one input of the squarer/divider, which has the transfer function:

3

1

4

I

I

I

2



The output current of the squarer/divider, I4, drives A4, forming

a low-pass filter with the external averaging capacitor. If the RC

time constant of the filter is much greater than the longest period

of the input signal, then the A4 output is proportional to the

average of I4. The output of this filter amplifier is used by A3

to provide the denominator current I3, which equals Avg I4 and

is returned to the squarer/divider to complete the implicit rms

computation

rms

I

I

I

Avg

I

1

4

1

4

















2

and

VOUT = VIN rms

To compute the absolute value of the input signal, the averaging

capacitor is omitted. However, a small capacitance value at the

averaging capacitor pin is recommended to maintain stability;

5 pF is sufficient for this purpose. The circuit operates identically

to that of the rms configuration, except that I3 is now equal to

I4, giving

4

1

4

I

I

I

2



I4 = |I1|

The denominator current can also be supplied externally by

providing a reference voltage (VREF) to Pin 6. The circuit operates

identically to the rms case, except that I3 is now proportional to

VREF. Therefore,

3

1

4

I

I

Avg

I

2



and

DEN

IN

OUT

V

V

V

2



This is the mean square of the input signal.