AD636

Data Sheet

Rev. E | Page 10 of 16

APPLICATIONS

The input and output signal ranges are a function of the supply

voltages as detailed in the specifications. The AD636 can also be

used in an unbuffered voltage output mode by disconnecting the

input to the buffer. The output then appears unbuffered across

the 10 kΩ resistor. The buffer amplifier can then be used for

other purposes. Further, the AD636 can be used in a current

output mode by disconnecting the 10 kΩ resistor from the ground.

The output current is available at Pin 8 (Pin 10 on the H package)

with a nominal scale of 100 μA per volt rms input, positive out.

STANDARD CONNECTION

The AD636 is simple to connect for the majority of high accuracy

rms measurements, requiring only an external capacitor to set

the averaging time constant. The standard connection is shown

in Figure 9 In this configuration, the AD636 measures the rms

of the ac and dc level present at the input but shows an error for

shown in Figure 13. Therefore, if a 4 μF capacitor is used, the

additional average error at 10 Hz is 0.1%, and at 3 Hz it is 1%.

The accuracy at higher frequencies is according to specification.

If it is desired to reject the dc input, a capacitor is added in

series with the input, as shown in Figure 11; the capacitor must

be nonpolar. If the AD636 is driven with power supplies with a

considerable amount of high frequency ripple, it is advisable to

bypass both supplies to ground with 0.1 μF ceramic discs as near

AD636

14

13

12

11

10

9

8

1

2

3

4

5

6

7

ABSOLUTE

VALUE

SQUARER

DIVIDER

BUF

+

–

CURRENT

MIRROR

10kΩ

10kΩ

(OPTIONAL)

SQUARER

DIVIDER

ABSOLUTE

VALUE

AD636

CURRENT

MIRROR

+

BUF

10kΩ

10kΩ

+

–

1

2

10

9

4

5

6

8

3

7

(OPTIONAL)

–

BUF OUT

BUF IN

COM

+V

erms

–V

dB

+V

NC

NC

NC

COM

BUF IN

BUF OUT

dB

+

–

C

+V

–V

NC

erms

NC = NO CONNECT

Figure 9. Standard RMS Connection

OPTIONAL TRIMS FOR HIGH ACCURACY

If it is desired to improve the accuracy of the AD636, the

external trims shown in Figure 10 can be added. R4 is used to

trim the offset. The scale factor is trimmed by using R1 as

shown. The insertion of R2 allows R1 to either increase or

decrease the scale factor by ±1.5%.

The trimming procedure is as follows:

•

output from Pin 6. Alternatively, R4 can be adjusted to give

•

calibrated ac signal (1 kHz is the optimum frequency); then

trim R1 to give the correct output from Pin 6, that is, 200 mV

dc input should give 200 mV dc output. Of course, a ±200 mV

peak-to-peak sine wave should give a 141.4 mV dc output.

The remaining errors, as given in the specifications, are due to

the nonlinearity.

R2

154Ω

1

2

3

4

5

6

7

AD636

14

13

12

11

10

9

8

ABSOLUTE

VALUE

SQUARER

DIVIDER

10kΩ

10kΩ

CURRENT

MIRROR

–V

SCALE

FACTOR

ADJUST

–

BUF

+

R1

200Ω

±1.5%

R4

500kΩ

OFFSET

ADJUST

R3

470kΩ

+V

NC

NC

NC

COM

erms

NC

dB

BUF OUT

BUF IN

NC = NO CONNECT

+

–

Figure 10. Optional External Gain and Output Offset Trims

SINGLE-SUPPLY CONNECTION

Although the applications illustrated in Figure 9 and Figure 10

assume the use of dual power supplies, three external bias

components connected to the COM pin enable powering the

AD636 with unipolar supplies as low as 5 V. The two resistors

and capacitor network shown connected to Pin 10 in Figure 11

are satisfactory over the same range of voltages permissible with

dual supply operation. Any external bias voltage applied to Pin 10 is

internally reflected to the VIN pin, rendering the same ac operation

as with a dual supply. DC or ac + dc conversion is impractical,

due to the resultant dc level shift at the input. The capacitor

insures that no extraneous signals are coupled into the COM

pin. The values of the resistors are relatively high to minimize

power consumption because only 1 µA of bias current flows

into Pin 10 (Pin 2 on the H package).

Alternately, the COM pin of some CMOS ADCs provides a suitable

artificial ground for the AD636. AC input coupling requires only

Capacitor C2 as shown; a dc return is not necessary because it is

provided internally. C2 is selected for the proper low frequency

break point with the input resistance of 6.7 kΩ; for a cut-off at

10 Hz, C2 should be 3.3 μF. The signal ranges in this connection are