Chapter 2
Connecting Signals
2-8
ni.com
Ground-Referenced Signal Connection
Figure 2-6 illustrates the ground-referenced signal connection. In this
configuration, the voltage source being measured is referenced to its own
ground reference that is connected through some conductive path to the
instrument ground reference. For example, the path can be through a
common earth ground or through the power line ground.
Note
This connection is a characteristic of the signal source and no additional wiring
should be added to connect the signal source ground reference to the NI PXI-4204 ground
reference.
To connect a ground-reference signal source to the NI PXI-4204, the signal
(
V
SIG
+) is connected to the NI PXI-4204 channel (AI
X
+). The signal
reference (
V
SIG
–) is connected to the channel reference (AI
X
–). The
signal ground reference (
V
SIG
Ground Reference) is referenced to the
NI PXI-4204 chassis ground reference inherently through some conductive
path that you do not physically connect.
This configuration is very simple to connect, but it also can be susceptible
to capacitive or electrically coupled noise. Also, the difference in ground
potential between the signal source ground and instrument ground creates
an unwanted signal that can introduce an error into the measurement. This
error is presented as a common-mode voltage (CMV) to the differential
front end of the NI PXI-4204, and is reduced by the common-mode
rejection (CMR) of the differential instrumentation amplifier. The
effectiveness of the CMR is affected by the source resistance of the signal
being measured. For more information about the effect of source resistance
on CMR, refer to the
.
