Data Sheet

AD737

Rev. J | Page 15 of 24

APPLICATIONS INFORMATION

RMS MEASUREMENT—CHOOSING AN OPTIMUM

VALUE FOR CAV

Because the external averaging capacitor, CAV, holds the rec-

tified input signal during rms computation, its value directly

affects the accuracy of the rms measurement, especially at low

frequencies. Furthermore, because the averaging capacitor is

connected across a diode in the rms core, the averaging time

constant (τAV) increases exponentially as the input signal

decreases. It follows that decreasing the input signal decreases

errors due to nonideal averaging but increases the settling time

approaching the decreased rms-computed dc value. Thus,

diminishing input values allow the circuit to perform better

(due to increased averaging) while increasing the waiting time

between measurements. A trade-off must be made between

computational accuracy and settling time when selecting CAV.

RAPID SETTLING TIMES VIA THE AVERAGE

RESPONDING CONNECTION

Because the average responding connection shown in Figure 25

does not use an averaging capacitor, its settling time does not

vary with input signal level; it is determined solely by the RC

time constant of CF and the internal 8 kΩ output scaling

resistor.

POSITIVE SUPPLY

+VS

0.1µF

–VS

0.1µF

COMMON

NEGATIVE SUPPLY

VOUT

CC

VIN

CF

33µF

COM

OUTPUT

AD737

BIAS

SECTION

INPUT

AMPLIFIER

8kΩ

8kΩ

POWER

DOWN

–VS

+VS

+

CAV

1

2

3

4

8

7

6

5

FULL-WAVE

RECTIFIER

RMS

CORE

Figure 25. AD737 Average Responding Circuit

Selectable Average or RMS Conversion

For some applications, it is desirable to be able to select between

rms-value-to-dc conversion and average-value-to-dc conversion.

If CAV is disconnected from the root-mean core, the AD737 full-

wave rectifier is a highly accurate absolute value circuit. A CMOS

switch whose gate is controlled by a logic level selects between

average and rms values.

VINRMS

–2.5V

1

8

7

6

5

4

3

2

CC

VIN

COM

+VS

OUT

CAV

–VS

33µF

33µF

AD737

VOUTDC

+2.5V

1MΩ

rms

AVG

NTR4501NT1

ASSUMED TO

BE A LOGIC

SOURCE

Figure 26. CMOS Switch is Used to Select RMS or Average Responding Modes

SELECTING PRACTICAL VALUES FOR CAPACITORS

Table 6 provides practical values of CAV and CF for several

common applications.

The input coupling capacitor, CC, in conjunction with the 8 kΩ

internal input scaling resistor, determines the −3 dB low frequency

roll-off. This frequency, FL, is equal to

(

)

Farads

in

C

F

C

L

×

×

π

=

8000

2

1

(1)

Note that, at FL, the amplitude error is approximately −30%

(−3 dB) of reading. To reduce this error to 0.5% of reading,

choose a value of CC that sets FL at one-tenth of the lowest

frequency to be measured.

In addition, if the input voltage has more than 100 mV of dc

offset, the ac coupling network at Pin 2 is required in addition

to Capacitor CC.

SCALING INPUT AND OUTPUT VOLTAGES

The AD737 is an extremely flexible device. With minimal

external circuitry, it can be powered with single- or dual-

polarity power supplies, and input and output voltages are

independently scalable to accommodate nonmatching

input/output devices. This section describes a few such

applications.

Extending or Scaling the Input Range

For low supply voltage applications, the maximum peak voltage

to the device is extended by simply applying the input voltage to

Pin 1 across the internal 8 kΩ input resistor. The AD737 input

circuit functions quasi-differentially, with a high impedance

FET input at Pin 2 (noninverting) and a low impedance input at

Pin 1 (inverting, see Figure 25). The internal 8 kΩ resistor behaves

as a voltage-to-current converter connected to the summing

node of a feedback loop around the input amplifier. Because the

feedback loop acts to servo the summing node voltage to match

the voltage at Pin 2, the maximum peak input voltage increases

until the internal circuit runs out of headroom, approximately

double for a symmetrical dual supply.