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Sources of error in DC voltage measurements
Common Mode rejection
Ideally, the internal DMM is completely isolated from earth-referenced circuits. However, there is finite resistance
and capacitance between the input LO terminal and earth ground. If the input terminals are both driven by an earth-
referenced signal (Vf) then a current will flow through RS and create a voltage drop VL as shown below:
Any resulting voltage (VL) will appear as an input to the internal DMM. As the value of RS approaches zero, so does
the error. Additionally, if Vf is at the power line frequency (50 Hz or 60 Hz), the noise can be greatly reduced by
setting the internal DMM's integration time to 1 PLC or greater.
Where:
V
= Common mode float voltage
f
R
= LO lead resistance
s
R
= Isolation resistance
i
C
= Isolation capacitance
i
Z
= Parallel impedance of R
i
Keysight DAQ970A/DAQ973A User's Guide
+C
i
i
4 Measurement Tutorials
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