Chapter 2 NI 4050 Operation
NI 4050 User Manual 2-6 www.ni.com
Figure 2-3. Common Mode Measurement Effects
Using the equation in Figure 2-3, you can calculate the voltage error due to
the common mode voltage. If you are measuring signals in the presence of
large common mode voltages, consult Appendix A, Specifications, to
calculate the additional error to your system.
Effective Common Mode Rejection
Effective common mode rejection is the sum of the CMRR and the NMRR
at a given frequency. It is the effective rejection on a given noise signal that
is applied to both input leads as it gets rejected first by the CMR capability
of the instrument then again by its NMR capability. This specification is
most useful at the powerline frequency where most of the noise resides and
is only valid for DC measurements.
AC Voltage
In the AC voltage ranges, the NI 4050 measures the AC-coupled RMS
value of a signal. The RMS value of a signal is a fundamental measurement
of the magnitude of an AC signal. The RMS value of an AC signal can be
defined mathematically as the square root of the average of the square of
the signal.
In practical terms, the RMS value of an AC signal is the DC value required
to produce an equivalent amount of heat in the same resistive load. The
NI 4050 first AC-couples the measured signal to remove any DC
HI
Source
Voltage
V
s
Common
Voltage
V
c
+
–
LO
Input
Measured
Voltage
V
m
VΩ
+
–
+
–
V
error
V
2
------
V
c
+
10
CMRR
–
20
--------------------
×=
s
V
m
V
s
V
error
+=
