User`s guide

277
Chapter 7 Tutorial
Measurement Fundamentals
4
7
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
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 (see page 120 for a discussion of integration time).
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
Error (V
L
) =
DMM
R
i
>10 G
C
i
250 pF
HI
V
test
V
f
R
s
LO
+ V
L
-
V
f
R
s

R
s
Z+
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34970A Refresh UG.book Page 277 Wednesday, February 17, 2010 12:34 PM