AD736JR-REEL-7 データシートの表示（PDF） - Analog Devices

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AD736JR-REEL-7

Low Cost, Low Power, True RMS-to-DC Converter

Analog Devices 

Applying the AD737

Figure 10. Error vs. RMS Input

Voltage (Pin 2) Using Circuit

of Figure 21

Figure 11. CAV vs. Frequency for

Specified Averaging Error

Figure 12. RMS Input Level vs.

Frequency for Specified Averaging

Error

Figure 13. Pin 2 Input Bias

Current vs. Supply Voltage

Figure 14. Settling Time vs. RMS

Input Level for Various Values of CAV

Figure 15. Pin 2 Input Bias Current

vs. Temperature

CALCULATING SETTLING TIME USING FIGURE 14

The graph of Figure 14 may be used to closely approximate the

time required for the AD737 to settle when its input level is re-

duced in amplitude. The net time required for the rms converter

to settle will be 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 (re-

duced) input level of 1 mV. From Figure 14, the initial settling

time (where the 100 mV line intersects the 33 µF line) is around

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 will be 8 seconds

minus 80 ms which is 7.92 seconds. Note that, because of the

smooth decay characteristic inherent with a capacitor/diode

combination, this is the total settling time to the final value (i.e.,

not the settling time to 1%, 0.1%, etc., of final value). Also, this

graph provides the worst case settling time, since the AD737

will settle very quickly with increasing input levels.

TYPES OF AC MEASUREMENT

The AD737 is capable of measuring ac signals by operating as

either an average responding or a true rms-to-de converter. As

its name implies, an average responding converter computes the

average absolute value of an ac (or ac and dc) voltage or current

by full wave rectifying and low-pass filtering the input signal;

this will approximate the average. The resulting output, a dc

“average” level, is then scaled by adding (or reducing) gain; this

scale factor converts the dc average reading to an rms equivalent

value for the waveform being measured. For example, the aver-

age absolute value of a sine-wave voltage is 0.636 that of VPEAK;

the corresponding rms value is 0.707 times VPEAK. Therefore,

for sine-wave voltages, the required scale factor is 1.11 (0.707

divided by 0.636).

In contrast to measuring the “average” value, true rms measure-

ment is a “universal language” among waveforms, allowing the

magnitudes of all types of voltage (or current) waveforms to be

compared to one another and to dc. RMS is a direct measure of

the power or heating value of an ac voltage compared to that of

dc: an ac signal of 1 volt rms will produce the same amount of

heat in a resistor as a 1 volt dc signal.

REV. C

–5–

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