AD737

REV. C

–7–

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 am-

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

time periods between pulses). Figure 6 shows the additional er-

SELECTING PRACTICAL VALUES FOR INPUT

common applications.

Table II. AD737 Capacitor Selection Chart

Application

rms

Low

Max

Settling

Input

Frequency Crest

Time*

Level

Cutoff

Factor

to 1%

(–3 dB)

General Purpose

0–1 V

20 Hz

5

150

µF 10 µF 360 ms

rms Computation

200 Hz

5

15

µF1 µF 36 ms

0–200 mV 20 Hz

5

33

µF 10 µF 360 ms

200 Hz

5

3.3

µF1 µF 36 ms

General Purpose

0–1 V

20 Hz

None

33

µF 1.2 sec

Average

200 Hz

None

3.3

µF 120 ms

Responding

0–200 mV 20 Hz

None

33

µF 1.2 sec

200 Hz

None

3.3

µF 120 ms

SCR Waveform

0–200 mV 50 Hz

5

100

µF 33 µF 1.2 sec

Measurement

60 Hz

5

82

µF 27 µF 1.0 sec

0–100 mV 50 Hz

5

50

µF 33 µF 1.2 sec

60 Hz

5

47

µF 27 µF 1.0 sec

Audio

Applications

Speech

0–200 mV 300 Hz

3

1.5

µF 0.5 µF 18 ms

Music

0–100 mV 20 Hz

10

100

µF 68 µF 2.4 sec

* Settling time is specified over the stated rms input level with the input signal increasing

from zero. Settling times will be greater for decreasing amplitude input signals.

Ω

internal input scaling resistor, determine the –3 dB low fre-

F

1

2

π(8,000)(TheValue of C

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

quency to be measured.

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

offset, than the ac coupling network at Pin 2 should be used in

RAPID SETTLING TIMES VIA THE AVERAGE

RESPONDING CONNECTION (FIGURE 17)

Because the average responding connection does not use an av-

eraging capacitor, its settling time does not vary with input sig-

and the internal 8 k

Ω output scaling resistor.

Figure 17. AD737 Average Responding Circuit

DC ERROR, OUTPUT RIPPLE, AND AVERAGING

ERROR

Figure 18 shows the typical output waveform of the AD737 with

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

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

Figure 18. Output Waveform for Sine-Wave Input Voltage

As shown, the dc error is the difference between the average of

the output signal (when all the ripple in the output has been

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

error component is therefore set solely by the value of averag-

ing capacitor used–no amount of post filtering (i.e., using a

value. The ac error component, an output ripple, may be easily

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

components need to be considered when selecting appropriate values

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. As the input frequency increases, both error components de-

crease rapidly: if the input frequency doubles, the dc error and ripple

reduce to 1/4 and 1/2 their original values, respectively, and rapidly

become insignificant.