Measurement Tutorial 7
Keysight 34420A User’s Guide 277
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 (PLCs) 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
NMR
60 Hz (50 Hz)
0.02 334 ms (400 ms) 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
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