HP 34401A, User Manual

Page: 199 / 236

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 (
PLC
s) of the spurious input, these
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
Integration Time
60 Hz (50 Hz)
NMR
4
1
⁄
2
Fast
4
1
⁄
2
Slow
5 1
⁄
2
Fast
5 1
⁄
2
Slow
6 1
⁄
2
Fast
6 1
⁄
2
Slow
0.02
1
0.2
10
10
100 400
µ
s (400
µ
s)
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
200