5962-8963701CA(2015) View Datasheet(PDF) - Analog Devices
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Description
Manufacturer
5962-8963701CA Datasheet PDF : 25 Pages
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Data Sheet
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 (CF = VP/V rms).
Most common waveforms, such as sine and triangle waves, have
relatively low crest factors (≤2). Waveforms that 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 (CF = 1 ).
T
Vp
0
e0
100µs
η
=
DUTY
CYCLE
=
100µs
T
CF = 1/ η
eIN(RMS) = 1 V RMS
Figure 16. Duty Cycle Timing
10
CAV = 22µF
1
CF = 10
0.1
CF = 3
0.01
1
10
100
PULSE WIDTH (µs)
1000
Figure 17. AD637 Error vs. Pulse Width Rectangular Pulse
Figure 18 is a curve of additional reading error for the AD637
for a 1 V rms input signal with crest factors from 1 to 11.
A rectangular pulse train (pulse width 100 μs) is used for this
test because it is the worst-case waveform for rms measurement
(all the energy is contained in the peaks). The duty cycle and
peak amplitude were varied to produce crest factors from l to
10 while maintaining a constant 1 V rms input amplitude.
AD637
1.5
1.0
0.5
0
–0.5
POSITIVE INPUT PULSE
CAV = 22µF
–1.0
–1.5
1
2 3 4 5 6 7 8 9 10 11
CREST FACTOR
Figure 18. Additional Error vs. Crest Factor
2.0
1.8
1.6
1.4
1.2
CF = 10
1.0
0.8
CF = 7
0.6
0.4
0.2
0
0
CF = 3
0.5
1.0
1.5
2.0
VIN (V RMS)
Figure 19. Error vs. RMS Input Level for Three Common Crest Factors
CONNECTION FOR dB OUTPUT
Another feature of the AD637 is the logarithmic, or decibel,
output. The internal circuit that computes dB works well over
a 60 dB range. Figure 20 shows the dB measurement connection.
The user selects the 0 dB level by setting R1 for the proper 0 dB
reference current, which is set to cancel the log output current
from the squarer/divider circuit at the desired 0 dB point. The
external op amp is used to provide a more convenient scale and to
allow compensation of the 0.33%/°C temperature drift of the
dB circuit. The temperature resistor, R3, as shown in Figure 20,
is available from Precision Resistor Co., Inc., in Largo, Fla.
(Model PT146).
Rev. L | Page 17 of 25
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