AD637
Table I. Practical Values of CAV and C2 for Various Input
Waveforms
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
Recommended C and C2
AV
Values for 1% Averaging
Error@60Hz with T = 16.6ms
Minimum
Absolute Value
Circuit Waveform
and Period
Input Waveform
and Period
1%
Settling
Time
AV
Recommended Recommended
Time
Constant
Standard
Value CAV
Standard
Value C2
1/2T
T
1/2T
0.47F
0.82F
1.5F
2.7F
181ms
325ms
A
0V
Symmetrical Sine Wave
T
η
cycle has a crest factor of 10 (C.F. = 1
).
T
T
B
C
D
100s
T
T
= DUTY CYCLE =
0V
Sine Wave with dc Offset
Vp
e0
CF = 1/
0
T
T
e
IN
(rms) = 1 Volt rms
100F
10(T – T )
2
T
2
T
6.8F
5.6F
22F
18F
2.67sec
2.17sec
2
0V
10
Pulse Train Waveform
T
C
AV
= 22F
T
T
2
10(T – 2T )
2
T
2
0V
1.0
0.1
CF = 10
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.
CF = 3
0.01
1
10
100
1000
PULSEWIDTH – s
10
Figure 11. AD637 Error vs. Pulsewidth Rectangular Pulse
7V RMS INPUT
2V RMS INPUT
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
1V RMS INPUT
1
1%
10%
؎3dB
100mV RMS INPUT
0.1
+1.5
+1.0
+0.5
0
0.01
10mV RMS INPUT
1k
10k
100k
1M
10M
INPUT FREQUENCY – Hz
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.
+0.5
POSITIVE INPUT PULSE
C
= 22F
AV
–1.0
–1.5
1
2
3
4
5
6
7
8
9
10
11
CREST FACTOR
Figure 12. Additional Error vs. Crest Factor
REV. E
–7–