Measurement Tutorial 7
Keysight 34420A User’s Guide 279
Common Mode Rejection (CMR)
Ideally, a meter is completely isolated from earth-referenced circuits. However,
there is finite resistance and capacitance between the meter’s input LO terminal
and earth ground. If the input terminals are both driven by an earth referenced
signal, V
f
, then a current will flow through R
S
and create a voltage drop V
L
as
shown below.
Any resulting voltage, V
L
, will appear as an input to the meter. As the value of R
S
approaches 0, so does the error. Additionally, if V
f
is at the power line frequency
(50 Hz or 60 Hz), the noise can be greatly reduced by setting the meter’s
integration time (NPLC) to 1 or greater. See page 277.
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