National Instruments Data Acquisition Device NI USB-621x User Manual page 68

I/O Connector
Ground-
Referenced
Signal
Source
Common-
Mode
Noise
and Ground
Potential
© National Instruments Corporation
+
V
s
–
Input Multiplexers
AI GND
+
V
cm
USB-6210/6211/6212 Configured in NRSE Mode
–
Figure 4-23. Single-Ended Connections for Ground-Referenced Signal Sources
AI+ and AI– must both remain within ±11 V of AI GND.
To measure a single-ended, ground-referenced signal source, you must use
the NRSE ground-reference setting. Connect the signal to one of
AI <0..31> and connect the signal local ground reference to AI SENSE.
AI SENSE is internally connected to the negative input of the NI-PGIA.
Therefore, the ground point of the signal connects to the negative input of
the NI-PGIA.
Any potential difference between the device ground and the signal ground
appears as a common-mode signal at both the positive and negative inputs
of the NI-PGIA, and this difference is rejected by the amplifier. If the
input circuitry of a device were referenced to ground, as it is in the RSE
ground-reference setting, this difference in ground potentials would appear
as an error in the measured voltage.
Using the DAQ Assistant, you can configure the channels for RSE or NRSE
input modes. Refer to the
Software section for more information about the DAQ Assistant.
AI <0..15>
or AI <16..n>
+
PGIA
–
AI SENSE
(NRSE Configuration)
Configuring AI Ground-Reference Settings in
4-35
Chapter 4
Instrumentation
Amplifier
+
Measured
V
m
Voltage
–
NI USB-621x User Manual
Analog Input