Data Sheet

AD737

Rev. J | Page 13 of 24

THEORY OF OPERATION

The AD737 has four functional subsections: an input amplifier,

a full-wave rectifier, an rms core, and a bias section (see Figure 23).

The FET input amplifier allows a high impedance, buffered

input at Pin 2 or a low impedance, wide dynamic range input at

Pin 1. The high impedance input, with its low input bias

current, is ideal for use with high impedance input attenuators.

The input signal can be either dc-coupled or ac-coupled to the

input amplifier. Unlike other rms converters, the AD737

permits both direct and indirect ac coupling of the inputs. AC

coupling is provided by placing a series capacitor between the

input signal and Pin 2 (or Pin 1) for direct coupling and

between Pin 1 and ground (while driving Pin 2) for indirect

coupling.

RMS

TRANSLINEAR

CORE

8

COM

+VS

7

6

OUTPUT

5

CAV

CURRENT

MODE

ABSOLUTE

VALUE

1

2

3

POWER

DOWN

4

CA

33µF

AC

CC = 10µF

CF

10µF

(OPTIONAL

LPF)

VIN

–VS

–VS

+VS

VIN

CC

+

OPTIONAL RETURN PATH

8kΩ

+

+

DC

BIAS

SECTION

FET

OP AMP

IB < 10pA

8kΩ

0.1µF

0.1µF

COMMON

POSITIVE SUPPLY

NEGATIVE SUPPLY

Figure 23. AD737 True RMS Circuit (Test Circuit)

The output of the input amplifier drives a full-wave precision

rectifier, which, in turn, drives the rms core. It is the core that

provides the essential rms operations of squaring, averaging,

and square rooting, using an external averaging capacitor, CAV.

Without CAV, the rectified input signal passes through the core

unprocessed, as is done with the average responding connection

(see Figure 25). In the average responding mode, averaging is

carried out by an RC post filter consisting of an 8 kΩ internal

scale factor resistor connected between Pin 6 and Pin 8 and an

external averaging capacitor, CF. In the rms circuit, this addi-

tional filtering stage reduces any output ripple that was not

removed by the averaging capacitor.

Finally, the bias subsection permits a power-down function.

This reduces the idle current of the AD737 from 160 µA to

30 µA. This feature is selected by connecting Pin 3 to Pin 7 (+VS).

TYPES OF AC MEASUREMENT

The AD737 measures ac signals either by operating as an

average responding converter or by operating as a true rms-to-dc

converter. As its name implies, an average responding converter

computes the average absolute value of an ac (or ac and dc)

voltage or current by full-wave rectifying and low-pass filtering

the input signal; this approximates the average. The resulting

output, a dc average level, is then scaled by adding (or reducing)

gain; this scale factor converts the dc average reading to an rms

equivalent value for the waveform being measured. For example,

the average absolute value of a sine wave voltage is 0.636 times

that of VPEAK; the corresponding rms value is 0.707 times VPEAK.

Therefore, for sine wave voltages, the required scale factor is

1.11 (0.707 divided by 0.636).

In contrast to measuring the average value, true rms measure-

ment is a universal language among waveforms, allowing the

magnitudes of all types of voltage (or current) waveforms to be

compared to one another and to dc. RMS is a direct measure of

the power or heating value of an ac voltage compared to that of

a dc voltage; an ac signal of 1 V rms produces the same amount

of heat in a resistor as a 1 V dc signal.

Mathematically, the rms value of a voltage is defined (using a

simplified equation) as

)

( 2

V

Avg

V rms =

This involves squaring the signal, taking the average, and then

obtaining the square root. True rms converters are smart recti-

fiers; they provide an accurate rms reading regardless of the

type of waveform being measured. However, average

responding converters can exhibit very high errors when their

input signals deviate from their pre-calibrated waveform; the

magnitude of the error depends on the type of waveform being

measured. As an example, if an average responding converter is

calibrated to measure the rms value of sine wave voltages and

then is used to measure either symmetrical square waves or dc

voltages, the converter has a computational error 11% (of

reading) higher than the true rms value (see Table 5).

The transfer function for the AD737 is

)

(

2

IN

OUT

V

Avg

V

−

=