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.
Noise Caused by Magnetic Loops
If you are making measurements near magnetic fields, you should take
the necessary precautions to avoid inducing voltages in the measurement
connections. You should be especially careful when working near
conductors carrying large currents. Use twisted-pair connections to the
multimeter to reduce the noise pickup loop area, or dress the test leads
as close together as possible. Loose or vibrating test leads will also
induce error voltages. Make sure your test leads are tied down securely
when operating near magnetic fields. Whenever possible, use magnetic
shielding materials or physical separation to reduce problem magnetic
field sources.
V
f
R
s
R
i
>10 GΩ
HI
LO
V
test
Ideal
Meter
V
f
= float voltage
R
s
= DUT source resistance
imbalance
R
i
= multimeter isolation
resistance (LO-Earth)
C
i
= multimeter input
capacitance:
≈200 pF (LO-Earth)
Error ( v ) =
V
f
x R
s
R
s
+ R
i
C
i
7
Chapter 7 Measurement Tutorial
Common Mode Rejection (CMR)
201
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