National Instruments NI PXI-7831R - Manual (Page 34)

Chapter 2 Hardware Overview of the NI PXI-7831R
© National Instruments Corporation 2-13 NI PXI-7831R User Manual
amplifier, and the signal local ground reference is connected to the negative
input of the instrumentation amplifier. The ground point of the signal
should, therefore, be connected to AISENSE. Any potential difference
between the NI PXI-7831R ground and the signal ground appears as a
common-mode signal at both the positive and negative inputs of the
instrumentation amplifier, and this difference is rejected by the amplifier.
If the input circuitry of a NI PXI-7831R were referenced to ground, in this
situation as in RSE input mode, this difference in ground potentials would
appear as an error in the measured voltage.
Figure 2-8 shows how to connect a grounded signal source to a channel on
the NI PXI-7831R configured for NRSE input mode.
Figure 2-8. Single-Ended Input Connections for Ground-Referenced Signals
Common-Mode Signal Rejection Considerations
Figures 2-5 and 2-8 show connections for signal sources that are already
referenced to some ground point with respect to the NI PXI-7831R.
In these cases, the instrumentation amplifier can reject any voltage caused
by ground potential differences between the signal source and the device.
In addition, with differential input connections, the instrumentation
amplifier can reject common-mode noise pickup in the leads connecting the
signal sources to the device. The instrumentation amplifier can reject
NRSE Input Mode Selected
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+
–
+
–
+
–
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V
cm
V
s
Ground-
Referenced
Signal
Source
Common-
Mode
Noise and
Ground
Potential
I/O Connector
AISENSE
AIGND
V
m
Measured
Voltage
Instrumentation
Amplifier
x8 Channels
AI+
AI–