Table of Contents
Advertisement
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
approaches zero, so does the error. Additionally, if V
S
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).
HI
V
test
R
S
LO
+ 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
x R
f
S
Error (V
) =
L
R
Z
S
and create a voltage drop
S
) will appear as an input to the internal DMM .
L
DMM
C
i
250 pF
R
i
>10 G
+ C
i
i
f
8
355
Advertisement
Table of Contents
