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 (see page 103 for a discussion of integration time).
Where:
Vf = Common mode float voltage
Rs = LO lead resistance
Ri = Isolation resistance
Ci = Isolation capacitance
Zi = Parallel impedance of Ri + Ci
Error (VL) = Vf x RS
RS + Z
Vtest
Vf
HI
LO
RS
DMM
≈ Ci
250 pF
+ VL -
Ri
>10 GΩ
8
Chapter 8 Tutorial
Measurement Fundamentals
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