User`s guide

Measurement Tutorial 3
34450A User’s Guide 77
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.
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.
Figure 3-1 Common Mode Rejection (CMR)
V
f
R
s
R
i
C
i
Ideal
Meter
V
f
=FloatVoltage
R
s
=DUTSourceResistance
Imbalance
R
i
= Multimeter Isolation Resistance
(LO-Earth)
C
i
= Multimeter Input Capacitance:
V
f
×R
s
Error (v) =
R
s
+R
i
HI
LO
V
test