Differential Connections For Ground-Referenced Signal Sources - National Instruments X Series User Manual

Chapter 4 Analog Input
higher if power distribution circuits are improperly connected. If a grounded signal source
is incorrectly measured, this difference can appear as measurement error. Follow the
connection instructions for grounded signal sources to eliminate this ground potential
difference from the measured signal.
Isolated devices have isolated front ends that are isolated from ground-reference signal
sources and are not connected to building system grounds. Isolated devices require the user
to provide a ground-reference terminal to which its input signals are referenced.

Differential Connections for Ground-Referenced Signal Sources

Figure 4-26 shows how to connect a ground-referenced signal source to a channel on an
Simultaneous MIO X Series device.
Figure 4-26. Differential Connection for Ground-Referenced Signals on
Ground-
Referenced
V
Signal
Source
Common-
Mode Noise
V
and Ground
Potential
I/O Connector
Note (NI USB-6346/6356/6366 BNC Devices)
source on X Series USB BNC devices, move the switch under the BNC connector to
the GS position.
With these types of connections, the instrumentation amplifier rejects both the common-mode
noise in the signal and the ground potential difference between the signal source and the device
ground, shown as Vcm in Figure 4-26.
Common-Mode Signal Rejection Considerations
The instrumentation amplifier can reject any voltage caused by ground potential differences
between the signal source and the device. In addition, the instrumentation amplifier can reject
common-mode noise pickup in the leads connecting the signal sources to the device. The
instrumentation amplifier can reject common-mode signals as long as V+in and V-in (input
signals) are both within the working voltage range of the device.
4-42 | ni.com
Simultaneous MIO X Series Devices
Simultaneous X Series Device
AI 0 +
+
s
–
AI 0 –
+
cm
–
AI 0 GND
AI 0 Connections Shown
Instrumentation
Amplifier
+
+
Measured
–
V
m
–
To measure a floating signal
Voltage