Datasheet

AD736 Data Sheet
Rev. I | Page 14 of 20
SELECTING PRACTICAL VALUES FOR INPUT
COUPLING (C
C
), AVERAGING (C
AV
), AND FILTERING
(C
F
) CAPACITORS
Table 6 provides practical values of C
AV
and C
F
for several
common applications.
The input coupling capacitor, C
C
, in conjunction with the
8 kΩ internal input scaling resistor, determine the −3 dB
low frequency roll-off. This frequency, F
L
, is equal to
)(8000)(2π
1
FaradsinCofValue
F
C
L
=
Note that at F
L
, the amplitude error is approximately −30%
(–3 dB) of the reading. To reduce this error to 0.5% of the
reading, choose a value of C
C
that sets F
L
at one-tenth of the
lowest frequency to be measured.
In addition, if the input voltage has more than 100 mV of dc
offset, then the ac-coupling network shown in Figure 27 should
be used in addition to C
C
.
Table 6. Capacitor Selection Chart
Application RMS Input Level
Low Frequency
Cutoff (−3 dB)
Max Crest
Factor
C
AV
(µF)
C
F
(µF) Settling Time
1
to 1%
General-Purpose RMS Computation 0 V to 1 V 20 Hz 5 150 10 360 ms
200 Hz 5 15 1 36 ms
0 mV to 200 mV 20 Hz 5 33 10 360 ms
200 Hz 5 3.3 1 36 ms
General Purpose 0 V to 1 V 20 Hz None 33 1.2 sec
Average 200 Hz None 3.3 120 ms
Responding 0 mV to 200 mV 20 Hz None 33 1.2 sec
200 Hz None 3.3 120 ms
SCR Waveform Measurement 0 mV to 200 mV 50 Hz 5 100 33 1.2 sec
60 Hz 5 82 27 1.0 sec
0 mV to 100 mV 50 Hz 5 50 33 1.2 sec
60 Hz 5 47 27 1.0 sec
Audio Applications
Speech 0 mV to 200 mV 300 Hz 3 1.5 0.5 18 ms
Music 0 mV to 100 mV 20 Hz 10 100 68 2.4 sec
1
Settling time is specified over the stated rms input level with the input signal increasing from zero. Settling times are greater for decreasing amplitude input signals.