REV. B

AD536A

–7–

FREQUENCY RESPONSE

The AD536A utilizes a logarithmic circuit in performing the

implicit rms computation. As with any log circuit, bandwidth is

proportional to signal level. The solid lines in the graph below

represent the frequency response of the AD536A at input levels

from 10 millivolts to 7 volts rms. The dashed lines indicate the

upper frequency limits for 1%, 10%, and 3 dB of reading addi-

tional error. For example, note that a 1 volt rms signal will pro-

duce less than 1% of reading additional error up to 120 kHz. A

10 millivolt signal can be measured with 1% of reading addi-

tional error (100

µV) up to only 5 kHz.

Figure 11. High Frequency Response

AC MEASUREMENT ACCURACY AND CREST FACTOR

Crest factor is often overlooked in determining the accuracy of

an ac measurement. Crest factor is defined as the ratio of the

V rms). Most common waveforms, such as sine and triangle

waves, have relatively low crest factors (<2). Waveforms which

resemble low duty cycle pulse trains, such as those occurring in

switching power supplies and SCR circuits, have high crest

factors. For example, a rectangular pulse train with a 1% duty

cycle has a crest factor of 10 (CF = 1

η ).

Figure 12 is a curve of reading error for the AD536A for a 1 volt

rms input signal with crest factors from 1 to 11. A rectangular

pulse train (pulsewidth 100

µs) was used for this test since it is

the worst-case waveform for rms measurement (all the energy is

contained in the peaks). The duty cycle and peak amplitude

were varied to produce crest factors from 1 to 11 while main-

taining a constant 1 volt rms input amplitude.

Figure 12. Error vs. Crest Factor

Figure 13. AD536A Error vs. Pulsewidth Rectangular

Pulse

Figure 14. AD536A Input and Output Voltage Ranges

vs. Supply