Common Mode Rejection (Cmr); Noise Caused By Magnetic Loops - HP 34401A User Manual

Chapter 7 Measurement Tutorial

Common Mode Rejection (CMR)

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.
HI
V
test
R
s
LO
V
f

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 = float voltage
f
R = DUT source resistance
s
Ideal
imbalance
Meter
R = multimeter isolation
i
resistance (LO-Earth)
C = multimeter input
i
capacitance:
200 pF (LO-Earth)
C R >10 G
i i
Error v
V x R
f s
R R
s i
7
201