Keysight DAQ970A - Page 195

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 res-
istance and capacitance between the 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 internal DMM. As the value of R
S
approaches zero,
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 internal DMM’s integration time to 1 PLC or greater.
Where:
V
f
= Common mode float voltage
R
s
= LO lead resistance
R
i
= Isolation resistance
C
i
= Isolation capacitance
Z
i
= Parallel impedance of R
i
+C
i
Keysight DAQ970A User's Guide 195
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