Common Mode Rejection (CMR) Ideally, a multimeter is completely isolated from earth-referenced circuits.
However, there is finite resistance between the multi met er ’ s inpu t LO
terminal and earth ground as shown below. This can cause erro rs 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 secure ly
when operating near magnetic fields. Whenever possible, u se magn et ic
shielding materials or physical separation to reduce problem magnetic
field sources.
Vf
Rs
Ri >10 GΩ
HI
LO
Vtest Ideal
Meter
Vf = float voltage
Rs = DUT source resistance
imbalance
Ri = multimeter isolation
resistance (LO-Earth)
Ci = multimeter input
capacitance:
≈200 pF (LO-Earth)
Error ( v ) = Vf x Rs
Rs + Ri
Ci
7
Chapter 7 Measurement Tutorial
Common Mode Rejection (CMR)
201