AD736

Data Sheet

Rev. I | Page 14 of 20

SELECTING PRACTICAL VALUES FOR INPUT

COUPLING (CC), AVERAGING (CAV), AND FILTERING

(CF) CAPACITORS

Table 6 provides practical values of CAV and CF for several

common applications.

The input coupling capacitor, CC, in conjunction with the

8 kΩ internal input scaling resistor, determine the −3 dB

low frequency roll-off. This frequency, FL, is equal to

)

(8000)(

2π

1

Farads

in

C

of

Value

F

C

L =

Note that at FL, 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 CC that sets FL 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 CC.

Table 6. Capacitor Selection Chart

Application

RMS Input Level

Low Frequency

Cutoff (−3 dB)

Max Crest

Factor

CAV

(µF)

CF

(µF)

Settling Time1 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.