AD637B AD [Analog Devices], AD637B Datasheet - Page 7

• Manufacturer Part Number: AD637B
• Description: High Precision, Wide-Band RMS-to-DC Converter
• Manufacturer: AD [Analog Devices]

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