Keysight U2741A Guide D'utilisation page 49

Noise Rejection
Rejecting power-line noise voltages
An important property of integrating analog-to-digital (A/D) converters is their
ability to reject power-line related noise that is present on the DC input signals.
This is called normal mode noise rejection, or NMR. The digital multimeter
achieves NMR by measuring the average DC input by "integrating" it over an
integral number of power line cycles.
Common mode rejection (CMR)
Ideally, a digital multimeter is completely isolated from earth-referenced circuits.
However, there is a finite resistance between the digital multimeter's input LO
terminal and earth ground, as shown below. This can cause errors when
measuring low voltages that are floating in relative to earth ground.
V
test
V
f
Figure 3-1
Keysight U2741A User's Guide
HI
R
s
LO
Common mode source error
V = Float Voltage
f
R = DUT Source Resistance Imbalance
s
R = digital multimeter Isolation Resistance
Ideal
i
Meter
(LO-Earth)
C = digital multimeter Input Capacitance
i
Error (v) =
C R
i
i
Measurement Tutorial
V x R
f s
R + R
s i
3
49