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Chapter 7 Measurement Tutorial
Measurement Techniques and Sources of Error
Rejecting Power-Line Noise Voltages
A desirable characteristic of integrating analog-to-digital (A/D)
converters is their ability to reject spurious signals. Integrating
techniques reject power-line related noise present with dc signals on the
input. This is called normal mode rejection or
rejection is achieved when the meter measures the average of the input
by "integrating" it over a fixed period. If you set the integration time to a
whole number of power line cycles (
errors (and their harmonics) will average out to approximately zero.
When you apply power to the meter, it measures the power-line
frequency (50 Hz or 60 Hz), and uses this measurement to determine the
integration time. The table below shows the noise rejection achieved with
various configurations. For better resolution and increased noise
rejection, select a longer integration time.
Integration Time
NPLC's
60 Hz
0.02
334 s
.2
3 ms
1
16.7 ms (20 ms)
2
33.4 ms (40 ms)
10
167 ms (200 ms)
20
334 ms (400 ms)
100
1.67 s
200
3.34 s
252
s) of the spurious input, these
PLC
NMR
(50 Hz)
none
(400 s)
none
(4 ms)
60 dB
90 dB
95 dB
100 dB
(2 s)
105 dB
(4 s)
110 dB
. Normal mode noise
NMR
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