Datasheet

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AD736

REV. C

–7–

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 averaging

capacitor used-no amount of post filtering (i.e., using a very

large C

) will allow the output voltage to equal its ideal value.

The ac error component, an output ripple, may be easily re-

moved by using a large enough post filtering capacitor, C

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

error components need to be considered when selecting appro-

priate values for capacitors C

and C

. 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.

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.

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-

plitude (C.F. = V

PEAK

/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

time periods between pulses). Figure 6 shows the additional

error vs. crest factor of the AD736 for various values of C

SELECTING PRACTICAL VALUES FOR INPUT

COUPLING (C

), AVERAGING (C

) AND FILTERING

) CAPACITORS

Table II provides practical values of C

and C

for several

common applications.

Table II. AD737 Capacitor Selection Chart

Application rms Low Max C

Settling

Input Frequency Crest Time*

Level Cutoff Factor to 1%

(–3dB)

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.

RMS MEASUREMENT – CHOOSING THE OPTIMUM

VALUE FOR C

Since the external averaging capacitor, C

, “holds” the recti-

fied input signal during rms computation, its value directly af-

fects the accuracy of the rms measurement, especially at low

frequencies. Furthermore, because the averaging capacitor ap-

pears across a diode in the rms core, the averaging time constant

will increase exponentially as the input signal is reduced. This

means that as the input level decreases, errors due to nonideal

averaging will reduce while the time it takes for the circuit to

settle to the new rms level will increase. Therefore, lower input

levels allow the circuit to perform better (due to increased aver-

aging) but increase the waiting time between measurements.

Obviously, when selecting C

, a trade-off between computa-

tional accuracy and settling time is required.

Figure 17. AD736 Average Responding Circuit

RAPID SETTLING TIMES VIA THE AVERAGE

RESPONDING CONNECTION (FIGURE 17)

Because the average responding connection does not use the

averaging capacitor, its settling time does not vary with in-

put signal level; it is determined solely by the RC time constant

of C

and the internal 8 kΩ resistor in the output amplifier’s

feedback path.

DC ERROR, OUTPUT RIPPLE, AND AVERAGING

ERROR

Figure 18 shows the typical output waveform of the AD736 with

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

ideal output of V

OUT

= V

is never exactly achieved; instead,

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

Figure 18. Output Waveform for Sine-Wave Input Voltage