Data Sheet

AD636

Rev. E | Page 9 of 16

FREQUENCY RESPONSE

The AD636 uses a logarithmic circuit to perform the implicit rms

computation. As with any log circuit, bandwidth is proportional to

signal level. The solid lines in Figure 5 represent the frequency

response of the AD636 at input levels from 1 mV to 1 V rms.

The dashed lines indicate the upper frequency limits for 1%,

10%, and ±3 dB of reading additional error. For example, note

that a 1 V rms signal produces less than 1% of reading additional

error up to 220 kHz. A 10 mV signal can be measured with 1%

of reading additional error (100 µV) up to 14 kHz.

AC MEASUREMENT ACCURACY AND CREST

FACTOR (CF)

Crest factor is often overlooked in determining the accuracy of

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

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

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

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 6 is a curve of reading error for the AD636 for a

200 mV rms input signal with crest factors from 1 to 7. A

rectangular pulse train (pulse width 200 μs) was used for this

test because 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 7

while maintaining a constant 200 mV rms input amplitude.