Datasheet

Data Sheet AD636
Rev. E | Page 9 of 16
Figure 4 shows the value of R
EXTERNAL
for a particular ratio of
V
PEAK
to −V
S
for several values of R
LOAD
. The addition of
R
EXTERNAL
increases the quiescent current of the buffer amplifier
by an amount equal to R
EXT
/−V
S
. Nominal buffer quiescent
current with no R
EXTERNAL
is 30 µA at −V
S
= −5 V.
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
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 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.