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 multimeter 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 (PLCs) of the spurious input, these
errors (and their harmonics) will average out to approximately zero.
The Agilent 34401A provides three A/D integration ti mes to reject
power-line frequency noise (and power-line frequency harmonics). When
you apply power to the multimeter, it measures the power-line
frequency (50 Hz or 60 Hz), and then determines the proper integra tion
time. The table below shows the noise rejection achieved with various
configurations. For better reso lution and increased noise reje ction,
select a longer integration time.
Digits NPLCs Integrat ion Time
60 Hz (50 Hz) NMR
41⁄2 Fast
41⁄2 Slow
51⁄2 Fast
51⁄2 Slow
61⁄2 Fast
61⁄2 Slow
0.02
1
0.2
10
10
100
400 µs (400 µs)
16.7 ms (20 ms)
3 ms (3 ms)
167 ms (200 ms)
167 ms (200 ms)
1.67 sec (2 sec)
–
60 dB
–
60 dB
60 dB
60 dB
Chapter 7 Measurement Tutorial
Rejecting Power-Line Noise Voltages
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