Chapter 4
Theory of Operation
4-4
ni.com
Figure 4-2.
Effect of Input Impedance on Signal Measurements
Although
R
S
does not influence DC measurements, take care when
measuring AC signals since
C
IN
attenuates higher frequencies if
R
S
is too
large. For example:
Common-Mode Rejection Ratio
The ability of a measurement device to reject voltages that are common to
both input terminals is referred to as the common-mode rejection ratio
(CMRR). The CMMR is usually stated in decibels at a given frequency or
over a given frequency band of interest. Common-mode signals can arise
from a variety of sources and can be induced through conductive or
radiated means. One of the most common sources of common-mode
interference is 50 or 60 Hz powerline noise.
The minimum NI PXI-4224 CMRR is 140 dB, which results in a reduction
of CMV by a factor of 10,000,000.
V
S
IG
+
–
+
–
Me
as
ured
Volt
a
ge
V
MEA
S
Input
Imped
a
nce
S
ource
Imped
a
nce
R
IN
R
S
S
ign
a
l
S
ource
C
IN
100
pF
V
MEAS
V
SIG
R
IN
R
S
R
IN
+
--------------------
=
Bandwidth
1
2
π
R
S
C
IN
----------------------
=
