AD637

Rev. K | Page 8 of 20

STANDARD CONNECTION

The AD637 is simple to connect for a majority of rms

measurements. In the standard rms connection shown in Figure 5,

only a single external capacitor is required to set the averaging

time constant. In this configuration, the AD637 computes the

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

of which is dependent on the value of the averaging capacitor,

is present at low frequencies. For example, if the filter capacitor,

3 Hz. To measure ac signals, the AD637 can be ac-coupled by

adding a nonpolar capacitor in series with the input, as shown

in Figure 5.

1

25kΩ

DEN

INPUT

BUFF IN

BUFF

OUT

SQUARER/

DIVIDER

CS

dB OUTPUT

3 COMMON

BIAS

2NC

4

OUTPUT

OFFSET

4.7kΩ

5

25kΩ

6

7

8

9

10

11

12

NC

13

14

NC

(OPTIONAL)

+

AD637

ABSOLUTE

VALUE

Figure 5. Standard RMS Connection

The performance of the AD637 is tolerant of minor variations

in the power supply voltages; however, if the supplies 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

supply voltages, as shown in Figure 6. The output signal can be

used buffered or nonbuffered, depending on the characteristics

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

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

into a 2 kΩ load without degrading the accuracy of the device.

20

15

0

0

10

5

SUPPLY VOLTAGE – DUAL SUPPLY (V)

±3

±5

±10

±15

±18

CHIP SELECT

The AD637 includes a chip select feature that allows the user

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

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

Under these conditions, the output goes into a high impedance

state. In addition to reducing the power consumption,

the outputs of multiple devices can be connected in parallel

to form a wide bandwidth rms multiplexer. Tie Pin 5 high to

disable the chip select.

OPTIONAL TRIMS FOR HIGH ACCURACY

The AD637 includes provisions for trimming out output offset

and scale factor errors resulting in significant reduction in the

maximum total error, as shown in Figure 7. The residual error is

due to a nontrimmable input offset in the absolute value circuit

and the irreducible nonlinearity of the device.

Referring to Figure 8, the trimming process is as follows:

• Offset trim: Ground the input signal (V

IN

) and adjust R1 to

give 0 V output from Pin 9. Alternatively, R1 can be adjusted

• Scale factor trim: Resistor R4 is inserted in series with the

input to lower the range of the scale factor. Connect the

signal, and trim Resistor R3 to give the correct output at Pin 9

(that is, 1 V dc at the input results in a dc output voltage of

l.000 V dc). A 2 V p-p sine wave input yields 0.707 V dc at the

output. Remaining errors are due to the nonlinearity.