Agilent Technologies 34901A User Manual page 358

Chapter 8 Tutorial
Measurement Fundamentals
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 (V ) then a current will flow through R
f
V as shown below.
L
Any resulting voltage (V
As the value of R
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).
V
test
R
S
+ V -
L
V
f
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
V
Error (V ) =
L
R
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) will appear as an input to the internal DMM .
L
approaches zero, so does the error. Additionally, if V
S
HI
DMM
LO
≈ C
i
250 pF
R
i
>10 GΩ
+ C
i i
x R
f S
+ Z
S
and create a voltage drop
S
355
f
8