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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
NMR
. Normal mode noise
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 (
PLC
s) of the spurious input, these
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.
NPLC’s
Integration Time
60 Hz (50 Hz)
NMR
0.02
334
µ
s (400
µ
s)
none
.2 3 ms (4 ms) none
1 16.7 ms (20 ms) 60 dB
2 33.4 ms (40 ms) 90 dB
10 167 ms (200 ms) 95 dB
20 334 ms (400 ms) 100 dB
100 1.67 s (2 s) 105 dB
200
3.34 s (4 s)
110 dB
Chapter 7 Measurement Tutorial
Measurement Techniques and Sources of Error
252