AD637

REV. E

–4–

the AD637 can be ac coupled through the addition of a non-

polar capacitor in series with the input as shown in Figure 2.

BUFFER

AD637

ABSOLUTE

VALUE

SQUARER/DIVIDER

BIAS

SECTION

FILTER

25k

25k

1

2

3

4

5

6

7

14

13

12

11

10

9

8

NC

NC

OPTIONAL

AC COUPLING

CAPACITOR

Figure 2. Standard RMS Connection

The performance of the AD637 is tolerant of minor variations in

the power supply voltages, however, if the supplies being used

exhibit a considerable amount of high frequency ripple it is

advisable to bypass both supplies to ground through a 0.1

µF

ceramic disc capacitor placed as close to the device as possible.

The output signal range of the AD637 is a function of the sup-

ply voltages, as shown in Figure 3. The output signal can be

used buffered or nonbuffered depending on the characteristics

of the load. If no buffer is needed, tie buffer input (Pin 1) to

common. The output of the AD637 is capable of driving 5 mA

into a 2 k

Ω load without degrading the accuracy of the device.

SUPPLY VOLTAGE – DUAL SUPPLY – Volts

20

15

0

0

18

5

10

10

5

15

3

CHIP SELECT

The AD637 includes a chip select feature which allows the user

to decrease the quiescent current of the device from 2.2 mA to

350

µA. This is done by driving the CS, Pin 5, to below 0.2 V

dc. Under these conditions, the output will go into a high im-

pedance state. In addition to lowering power consumption, this

feature permits bussing the outputs of a number of AD637s to

form a wide bandwidth rms multiplexer. If the chip select is not

being used, Pin 5 should be tied high.

FUNCTIONAL DESCRIPTION

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

V rms

= Avg

V

IN

2

V rms

















Figure 1 is a simplified schematic of the AD637, it is subdivided

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

square/divider, filter circuit and buffer amplifier. The input volt-

I1 by the active rectifier A1, A2. I1 drives one input of the

squarer divider which has the transfer function

2

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

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

the RC time constant of the filter is much greater than the long-

est period of the input signal than A4s output will be propor-

tional 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.

2

















= I

and

If the averaging capacitor is omitted, the AD637 will compute the

absolute value of the input signal. A nominal 5 pF capacitor should

be used to insure stability. The circuit operates identically to that of

the rms configuration except that I3 is now equal to I4 giving

2

The denominator current can also be supplied externally by pro-

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

2

and

2

This is the mean square of the input signal.

STANDARD CONNECTION

The AD637 is simple to connect for a majority of rms measure-

ments. In the standard rms connection shown in Figure 2, only

a single external capacitor is required to set the averaging time

constant. In this configuration, the AD637 will compute the

true rms of any input signal. An averaging error, the magnitude

of which will be dependent on the value of the averaging capaci-

tor, will be present at low frequencies. For example, if the filter

µF this error will be 0.1% at 10 Hz and in-

creases to 1% at 3 Hz. If it is desired to measure only ac signals,