Common Mode Rejection (CMR)
Ideally, a meter is completely isolated from earth-referenced circuits.
However, there is finite resistance and capacitance between the meter’s
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 meter. As the
value of R
S
approaches 0, 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 meter’s integration time (NPLC) to 1 or greater.
V
f
=
Common Mode Float voltage
R
S
=
LO
Lead Resistance
R
i
=
Meter Isolation Resistance
C
i
=
Meter Isolation Capacitance
Z
i
=
Parallel Impedance of R
i
+
C
i
Error
(
V
L
)
=
V
f
x R
S
R
S
+
Z
Ideal
Meter
Chapter 7 Measurement Tutorial
Measurement Techniques and Sources of Error
254
Содержание 34420A
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Страница 13: ...1 Quick Start ...
Страница 31: ...2 Front Panel Operation ...
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Страница 245: ...6 Chapter 6 Application Programs Microsoft Excel Macro Example 245 ...
Страница 246: ...Chapter 6 Application Programs Microsoft Excel Macro Example 246 ...
Страница 247: ...7 Measurement Tutorial ...
Страница 274: ...8 Specifications ...
Страница 279: ...Dimensions Chapter 8 Specifications 34420A Nano Volt Micro Ohm Meter 280 ...
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