AD736

Data Sheet

Rev. I | Page 12 of 20

RAPID SETTLING TIMES VIA THE AVERAGE

RESPONDING CONNECTION

Because the average responding connection shown in Figure 19

does not use the CAV averaging capacitor, its settling time does

not vary with the input signal level. It is determined solely by

the RC time constant of CF and the internal 8 kΩ resistor in the

output amplifier’s feedback path.

+VS

+VS

CF

33µF

CC

10µF

COM

OUTPUT

(OPTIONAL)

POSITIVE SUPPLY

+VS

0.1µF

–VS

0.1µF

COMMON

NEGATIVE SUPPLY

VOUT

8

7

6

5

1

2

3

4

AD736

+

rms

CORE

+

CC

VIN

VIN

FULL

WAVE

RECTIFIER

CF

–VS

–VS

CAV

BIAS

SECTION

INPUT

AMPLIFIER

8kΩ

OUTPUT

AMPLIFIER

8kΩ

Figure 19. AD736 Average Responding Circuit

DC ERROR, OUTPUT RIPPLE, AND AVERAGING

ERROR

Figure 20 shows the typical output waveform of the AD736

with a sine wave input applied. As with all real-world devices,

the ideal output of VOUT = VIN is never achieved exactly. Instead,

the output contains both a dc and an ac error component.

As shown in Figure 20, the dc error is the difference between

the average of the output signal (when all the ripple in the

output is removed by external filtering) and the ideal dc output.

The dc error component is therefore set solely by the value of

the averaging capacitor used. No amount of post filtering (that

is, using a very large CF) allows the output voltage to equal its

ideal value. The ac error component, an output ripple, can be

easily removed by using a large enough post filtering capacitor, CF.

In most cases, the combined magnitudes of both the dc and

ac error components need to be considered when selecting

appropriate values for Capacitor CAV and Capacitor CF. This

combined error, representing the maximum uncertainty of the

measurement, is termed the averaging error and is equal to the

peak value of the output ripple plus the dc error.

DC ERROR = EO – EO (IDEAL)

AVERAGE EO = EO

EO

IDEAL

EO

DOUBLE-FREQUENCY

RIPPLE

TIME

Figure 20. Output Waveform for Sine Wave Input Voltage

As the input frequency increases, both error components

decrease rapidly; if the input frequency doubles, the dc error

and ripple reduce to one quarter and one half of their original

values, respectively, and rapidly become insignificant.

AC MEASUREMENT ACCURACY AND CREST FACTOR

The crest factor of the input waveform is often overlooked when

determining the accuracy of an ac measurement. Crest factor is

defined as the ratio of the peak signal amplitude to the rms

amplitude (crest factor = VPEAK/V rms). Many common waveforms,

such as sine and triangle waves, have relatively low crest factors

(≤2). Other waveforms, such as low duty-cycle pulse trains and

SCR waveforms, have high crest factors. These types of waveforms

require a long averaging time constant (to average out the long

periods between pulses). Figure 8 shows the additional error vs.

the crest factor of the AD736 for various values of CAV.