AD637

Rev. K | Page 11 of 20

for 1% Averaging Error @ 60 Hz with T = 16.6 ms

Input Waveform

and Period

Absolute Value

Circuit Waveform

and Period

Time Constant

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

0V

T

T

6.8

22

2.67 sec

D

0V

T

T

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

10M

10k

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

INPUT FREQUENCY (Hz)

Figure 15. Frequency Response