AD536A

Data Sheet

Rev. E | Page 12 of 16

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AD536A

25kΩ

25kΩ

ABSOLUTE

VALUE

SQUARER/

DIVIDER

CURRENT

MIRROR

R4

50kΩ

OFFSET

ADJUST

R3

750kΩ

R2

365Ω

BUF

NC

NC

NC

NC

dB

COM

BUF OUT

BUF IN

SCALE

FACTOR

ADJUST

R1

500Ω

Figure 16. Optional External Gain and Output Offset Trims

SINGLE-SUPPLY OPERATION

Refer to Figure 17 for single supply-rail configurations between

5 V and 36 V. When powered from a single supply, the input

stage (VIN pin) is internally biased at a voltage between ground

and the supply, and the input signal ac coupled. Biasing the

device between the supply and ground is simply a matter of

connecting the COM pin to an external resistor divider and

bypassing to ground. The resistor values are large, minimizing

power consumption, as the COM pin current is only 5 μA.

Note that the 10 kΩ and 20 kΩ resistors connected to the COM pin

(Figure 17) are asymmetrical, that is, the voltage at the COM pin is

1/3 of the supply. This ratio of input bias to supply is optimum

for the precision rectifier (aka absolute value circuit) input

circuit employed for rectifying ac input waveforms and ensures

full input symmetry for low signal voltages.

Capacitor C2 is required for AC input coupling, however an

external dc return is unnecessary because biasing occurs

internally. SelectC2 for the desired low frequency breakpoint

using an input resistance of 16.7 kΩ for the 1/ωRC calculation;

C2 = 1 μF for a cutoff at 10 Hz. Figure 11 and Figure 12 show

the input and output signal ranges for dual and single supply

configurations, respectively. The load resistor, RL, provides a

path to sink output sink current when an input signal is

disconnected.

14

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12

11

10

9

8

1

2

3

4

5

6

7

AD536A

25kΩ

ABSOLUTE

VALUE

SQUARER/

DIVIDER

CURRENT

MIRROR

C2

1µF

NONPOLARIZED

0.1µF

20kΩ

10kΩ

0.1µF

10k

Ω

TO

1k

Ω

NC

NC

NC

NC

dB

COM

BUF OUT

BUF IN

BUF

Figure 17. Single-Supply Connection

CHOOSING THE AVERAGING TIME CONSTANT

The AD536A computes the rms of both ac and dc signals. If the

input is a slowly varying dc signal, the output of the AD536A

tracks the input exactly.

At higher frequencies, the average output of the AD536A

approaches the rms value of the input signal. The actual output

of the AD536A differs from the ideal output by a dc (or average)

error and some amount of ripple, as shown in Figure 18.

DOUBLE FREQUENCY

RIPPLE

TIME

Figure 18. Typical Output Waveform for Sinusoidal Input

The dc error is dependent on the input signal frequency and

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 a

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 10 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).