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