National Instruments NI PXI-7831R - Manual (Page 26)

Chapter 2 Hardware Overview of the NI PXI-7831R
© National Instruments Corporation 2-5 NI PXI-7831R User Manual
Figure 2-3. NI PXI-7831R Instrumentation Amplifier
The instrumentation amplifier applies common-mode voltage rejection
and presents high input impedance to the AI signals connected to the
NI PXI-7831R. Signals are routed to the positive and negative inputs of
the instrumentation amplifier through input multiplexers on the device.
The instrumentation amplifier converts two input signals to a signal that is
the difference between the two input signals. The amplifier output voltage
is referenced to the device ground. The NI PXI-7831R ADC measures this
output voltage when it performs A/D conversions.
You must reference all signals to ground either at the source device or at the
NI PXI-7831R. If you have a floating source, you should reference the
signal to ground by using RSE input mode or the DIFF input mode with
bias resistors. Refer to the
Differential Connections for Nonreferenced or
Floating Signal Sources section for more information about these input
modes. If you have a grounded source, you should not reference the signal
to AIGND. You can avoid this reference by using DIFF or NRSE input
modes.
Types of Signal Sources
When configuring the input channels and making signal connections,
you must first determine whether the signal sources are floating or ground
referenced. The following sections describe these two signal types.
+
+
–
–
V
m
= [V
in+
– V
in–
]
V
in+
V
in–
V
m
Instrumentation
Amplifier
Measured
Voltage