AD536A Datasheet, PDF (7/8 Pages) Analog Devices – Integrated Circuit True RMS-to-DC Converter

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.
AD536A
Figure 12. Error vs. Crest Factor
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
peak signal amplitude to the rms value of the signal (CF = V_P/
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 13. AD536A Error vs. Pulsewidth Rectangular
Pulse
REV. B
Figure 14. AD536A Input and Output Voltage Ranges
vs. Supply
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