AD737

Data Sheet

Rev. J | Page 14 of 24

DC ERROR, OUTPUT RIPPLE, AND

AVERAGING ERROR

Figure 24 shows the typical output waveform of the AD737 with

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

the ideal output of VOUT = VIN is never exactly achieved; instead,

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

DC ERROR = EO – EO (IDEAL)

AVERAGE EO = EO

EO

IDEAL

EO

DOUBLE-FREQUENCY

RIPPLE

TIME

Figure 24. 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 the

averaging capacitor used—no amount of post filtering (using a

very large postfiltering capacitor, CF) allows the output voltage

to equal its ideal value. The ac error component, an output

ripple, can be easily removed using a large enough CF.

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

components must be considered when selecting appropriate values

for CAV and CF capacitors. 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 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 time periods between pulses. Figure 10

shows the additional error vs. the crest factor of the AD737 for

various values of CAV.

CALCULATING SETTLING TIME

Figure 18 can be used to closely approximate the time required

for the AD737 to settle when its input level is reduced in amplitude.

The net time required for the rms converter to settle is the

difference between two times extracted from the graph: the

initial time minus the final settling time. As an example, consider

the following conditions: a 33 μF averaging capacitor, an initial

rms input level of 100 mV, and a final (reduced) input level of

1 mV. From Figure 18, the initial settling time (where the

100 mV line intersects the 33 μF line) is approximately 80 ms.

The settling time corresponding to the new or final input level

of 1 mV is approximately 8 seconds. Therefore, the net time for

the circuit to settle to its new value is 8 seconds minus 80 ms,

which is 7.92 seconds.

Note that, because of the inherent smoothness of the decay

characteristic of a capacitor/diode combination, this is the total

settling time to the final value (not the settling time to 1%, 0.1%,

and so on, of the final value). Also, this graph provides the

worst-case settling time because the AD737 settles very quickly

with increasing input levels.

Table 5. Error Introduced by an Average Responding Circuit When Measuring Common Waveforms

Type of Waveform 1 V Peak Amplitude

Crest Factor

(VPEAK/V rms)

True RMS

Value (V)

Reading of an Average Responding Circuit

Calibrated to an RMS Sine Wave Value (V)

Error (%)

Undistorted Sine Wave

1.414

0.707

0.707

0

Symmetrical Square Wave

1.00

1.00

1.11

11.0

Undistorted Triangle Wave

1.73

0.577

0.555

−3.8

Gaussian Noise (98% of Peaks <1 V)

3

0.333

0.295

−11.4

Rectangular

2

0.5

0.278

−44

Pulse Train

10

0.1

0.011

−89

SCR Waveforms

50% Duty Cycle

2

0.495

0.354

−28

25% Duty Cycle

4.7

0.212

0.150

−30