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Chapter 7 Measurement Tutorial
Measurement Techniques and Sources of Error
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
R
and create a voltage drop V
S
V
= Common Mode Float voltage
f
R
= LO Lead Resistance
S
R
= Meter Isolation Resistance
i
C
= Meter Isolation Capacitance
i
Z
= Parallel Impedance of R
i
V
x R
f
S
Error ( V
=
L )
+ Z
R
S
Any resulting voltage, V
value of R
approaches 0, so does the error. Additionally, if V
S
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 252.
254
as shown below.
L
+ C
i
i
, will appear as an input to the meter. As the
L
, then a current will flow through
f
Ideal
Meter
is at the
f
- Quick Links:
- To Measure Resistance
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