National Instruments Isolated Analog Input Device NI PXI-4224 - Manual (Page 31)

Chapter 4 Theory of Operation
NI PXI-4224 User Manual 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
SIG
+
–
+
–
Measured
Voltage
V
MEAS
Input
Impedance
Source
Impedance
R
IN
R
S
Signal Source
C
IN
100
pF
V
MEAS
V
SIG
R
IN
R
S
R
IN
+
-------------------- =
Bandwidth
1
2 πR
S
C
IN
---------------------- =