AD636

REV. B

–4–

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 capaci-

tor C2 as shown; a dc return is not necessary as it is provided

internally. C2 is selected for the proper low frequency break

point with the input resistance of 6.7 k

Ω; for a cut-off at 10 Hz,

C2 should be 3.3

µF. The signal ranges in this connection are

slightly more restricted than in the dual supply connection. The

1

2

3

4

5

6

7

AD636

14

13

12

11

10

9

8

ABSOLUTE

VALUE

SQUARER

DIVIDER

10k

10k

CURRENT

MIRROR

–+

20k

C2

3.3 F

NONPOLARIZED

10k

to 1k

39k

0.1 F

0.1 F

BUF

Figure 3. Single Supply Connection

CHOOSING THE AVERAGING TIME CONSTANT

The AD636 will compute the rms of both ac and dc signals. If

the input is a slowly-varying dc voltage, the output of the AD636

will track the input exactly. At higher frequencies, the average

output of the AD636 will approach the rms value of the input

signal. The actual output of the AD636 will differ from the ideal

output by a dc (or average) error and some amount of ripple, as

demonstrated in Figure 4.

DOUBLE-FREQUENCY

RIPPLE

IDEAL

TIME

Figure 4. Typical Output Waveform for Sinusoidal Input

The dc error is dependent on the input signal frequency and the

frequency using the standard rms connection.

The ac component of the output signal is the ripple. There are

two ways to reduce the ripple. The first method involves using

reduction in ripple. When measuring waveforms with high crest

factors, (such as low duty cycle pulse trains), the averaging time

constant should be at least ten times the signal period. For

example, a 100 Hz pulse rate requires a 100 ms time constant,

which corresponds to a 4

µF capacitor (time constant = 25 ms

per

µF).

APPLYING THE AD636

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 k

Ω 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 k

Ω 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.

OPTIONAL TRIMS FOR HIGH ACCURACY

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

nal trims shown in Figure 2 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:

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

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

then trim R1 to give the correct output from Pin 6, i.e.,

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 specifica-

tions, are due to the nonlinearity.

1

2

3

4

5

6

7

AD636

14

13

12

11

10

9

8

ABSOLUTE

VALUE

SQUARER

DIVIDER

10k

10k

CURRENT

MIRROR

SCALE

FACTOR

ADJUST

R1

200

1.5%

–+

R4

500k

OFFSET

ADJUST

R3

470k

R2

154

BUF

Figure 2. Optional External Gain and Output Offset Trims

SINGLE SUPPLY CONNECTION

The applications in Figures 1 and 2 assume the use of dual

power supplies. The AD636 can also be used with only a single

positive supply down to +5 volts, as shown in Figure 3. Figure 3

is optimized for use with a 9 volt battery. The major limitation

of this connection is that only ac signals can be measured since

the input stage must be biased off ground for proper operation.

This biasing is done at Pin 10; thus it is critical that no extrane-

ous signals be coupled into this point. Biasing can be accom-

The values of the resistors can be increased in the interest of

lowered power consumption, since only 1 microamp of current