Datasheet

AD637
Rev. K | Page 11 of 20
Table 5. Practical Values of C
AV
and C2 for Various Input Waveforms
Recommended Standard Values for C
AV
and C2
for 1% Averaging Error @ 60 Hz with T = 16.6 ms
Input Waveform
and Period
Absolute Value
Circuit Waveform
and Period
Minimum R × C
AV
Time Constant
C
AV
(μF) C2 (μF)
1% Settling
Time
Symmetrical Sine Wave
A
0V
T
1/2T
1/2T 0.47 1.5 181 ms
Sine Wave with dc Offset
B
0V
T
T
T 0.82 2.7 325 ms
Pulse Train Waveform
C
T
2
0V
T
T
2
T
10 (T − T
2
) 6.8 22 2.67 sec
D
T
2
0V
T
T
2
T
10 (T − 2T
2
) 5.6 18 2.17 sec
FREQUENCY RESPONSE
The frequency response of the AD637 at various signal levels is
shown in Figure 15. The dashed lines show the upper frequency
limits for 1%, 10%, and ±3 dB of additional error. For example,
note that for 1% additional error with a 2 V rms input, the
highest frequency allowable is 200 kHz. A 200 mV signal can
be measured with 1% error at signal frequencies up to 100 kHz.
To take full advantage of the wide bandwidth of the AD637,
care must be taken in the selection of the input buffer amplifier.
To ensure that the input signal is accurately presented to the
converter, the input buffer must have a −3 dB bandwidth that is
wider than that of the AD637. Note the importance of slew rate
in this application. For example, the minimum slew rate required
for a 1 V rms, 5 MHz, sine wave input signal is 44 V/μs. The user is
cautioned that this is the minimum rising or falling slew rate
and that care must be exercised in the selection of the buffer
amplifier, because some amplifiers exhibit a two-to-one
difference between rising and falling slew rates. The AD845 is
recommended as a precision input buffer.
10
1k 10M10k
V
OUT
(V)
100k 1M
1
0.1
0.01
1%
±3dB
10%
7V RMS INPUT
2V RMS INPUT
1V RMS INPUT
100mV RMS INPUT
10mV RMS INPUT
00788-015
INPUT FREQUENCY (Hz)
Figure 15. Frequency Response