Rejecting Power-Line Noise Voltages - HP 34401A User Manual

Chapter 7 Measurement Tutorial

Rejecting Power-Line Noise Voltages

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 (
errors (and their harmonics) will average out to approximately zero.
The HP 34401A provides three A/D integration times 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 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.
Digits NPLCs
4 1 Fast 0.02
2
1 1
4 Slow
2
0.2
5 1 Fast
2
10
5 1 Slow
2
10
1
6 Fast
2
100
6 1 Slow
2
200
s) of the spurious input, these
PLC
Integration Time
NMR
60 Hz (50 Hz)
400 s (400 s) –
16.7 ms (20 ms)
60 dB
3 ms (3 ms)
–
167 ms (200 ms)
60 dB
167 ms (200 ms)
60 dB
1.67 sec (2 sec)
60 dB