National Instruments USB-621 Series - Using Non-Referenced Single-Ended Connections for Ground-Referenced Signal Sources

Chapter 4 Analog Input
© National Instruments Corporation 4-35 NI USB-621x User Manual
Using Non-Referenced Single-Ended Connections
for Ground-Referenced Signal Sources
Figure 4-23 shows how to connect ground-reference signal sources to the
USB-6210/6211/6212 device in non-referenced single-ended mode.
Figure 4-23. Single-Ended Connections for Ground-Referenced Signal Sources
(NRSE Configuration)
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 non-referenced single-ended 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
inputcircuitry of a device were referenced to ground, as it is in the
referenced single-ended ground-reference setting, this difference in ground
potentials would appear as an error in the measured voltage.
USB-6210/6211/6212 Configured in NRSE Mode
Common-
Mode
Noise
and Ground
Potential
Ground-
Referenced
Signal
Source
PGIA
Input Multiplexers
–
+
–
+
V
s
V
cm
I/O Connector
AI GND
AI SENSE
AI <0..15>
or AI <16..n>
–
+
Instrumentation
Amplifier
Measured
Voltage
V
m
–
+