REV. E

–7–

AD637

Waveforms

Input Waveform

and Period

Absolute Value

Circuit Waveform

and Period

Minimum

R

Time

Constant

Values for 1% Averaging

Error@60Hz with T = 16.6ms

Recommended

Standard

Recommended

Standard

Value C2

1%

Settling

Time

Symmetrical Sine Wave

Sine Wave with dc Offset

Pulse Train Waveform

1/2T

T

A

B

C

D

181ms

325ms

2.67sec

2.17sec

1.5 F

2.7 F

22 F

18 F

6.8 F

0.82 F

0.47 F

5.6 F

1/2T

0V

0V

0V

0V

T

T

T

T

T

T

T

FREQUENCY RESPONSE

The frequency response of the AD637 at various signal levels is

shown in Figure 10. 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 high-

est frequency allowable is 200 kHz. A 200 mV signal can be

measured with 1% error at signal frequencies up to 100 kHz.

INPUT FREQUENCY – Hz

10

1k

10M

10k

100k

1M

1

0.1

0.01

1V RMS INPUT

2V RMS INPUT

7V RMS INPUT

100mV RMS INPUT

10mV RMS INPUT

1%

3dB

10%

Figure 10. Frequency Response

To take full advantage of the wide bandwidth of the AD637 care

must be taken in the selection of the input buffer amplifier. To

insure that the input signal is accurately presented to the con-

verter, the input buffer must have a –3 dB bandwidth that is

wider than that of the AD637. A point that should not be over-

looked is 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 as some amplifi-

ers exhibit a two-to-one difference between rising and falling slew

rates. The AD845 is recommended as a precision input buffer.

AC MEASUREMENT ACCURACY AND CREST FACTOR

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 (C.F. = Vp/

V rms). Most common waveforms, such as sine and triangle

waves, have relatively low crest factors (

≤2). Waveforms which

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 (C.F. = 1

η ).

PULSEWIDTH –

s

10

1.0

0.01

1

1000

10

100

0.1

CAV = 22 F

CF = 10

CF = 3

0

100 F

Vp

T

= DUTY CYCLE =

100 s

T

CF = 1/

Figure 11. AD637 Error vs. Pulsewidth Rectangular Pulse

Figure 12 is a curve of additional reading error for the AD637

for a 1 volt rms input signal with crest factors from 1 to 11. A

rectangular pulse train (pulsewidth 100

µs) was used for this test

since it is the worst-case waveform for rms measurement (all

CREST FACTOR

+1.5

0

–1.5

111

2

3456

7

8

9

10

+1.0

+0.5

+0.5

–1.0

POSITIVE INPUT PULSE

Figure 12. Additional Error vs. Crest Factor