100 34410A/11A User’s Guide
4 Measurement Tutorial
Noise Rejection
Rejecting Power–Line Noise Voltages
A desirable characteristic of integrating analog–to–digital (A/D) converters
is their ability to reject power–line related noise present with dc input
signals. This is called normal mode noise rejection, or NMR. The
multimeter achieves NMR by measuring the average dc input by
"integrating" it over a fixed period. If you set the integration time to a
whole number of power line cycles (PLCs), these errors (and their
harmonics) will average out to approximately zero.
The multimeter provides four integration selections (1, 2, 10 and 100
PLCs) which achieve NMR. The multimeter measures the power–line
frequency (50 Hz or 60 Hz), and then determines the corresponding
integration time. For a complete listing of NMR, approximate added RMS
noise, reading rate and resolution for each integration setting, see the
Performance vs. Integration Time table on page 124.
You can also program specific apertures, using the INTEGRATION settings, to
reject undesired signal frequency components.
Common Mode Rejection (CMR)
Ideally, a multimeter is completely isolated from earth–referenced circuits.
However, there is finite resistance between the multimeter's input LO
terminal and earth ground, as shown below. This can cause errors when
measuring low voltages which are floating relative to earth ground.
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