National Instruments NI 7831R - Manual (Page 25)

Chapter 2 Hardware Overview of the NI 7831R
NI 7831R User Manual 2-6 ni.com
Figure 2-4. NI 7831R Instrumentation Amplifier
The instrumentation amplifier applies common-mode voltage rejection
and presents high input impedance to the AI signals connected to the
NI 7831R. Input multiplexers on the device route signals to the positive and
negative inputs of the instrumentation amplifier. 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 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 7831R. If you have a floating source, 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 of this chapter for more information about these input modes. If you
have a grounded source, do not reference the signal to AIGND. You can
avoid this reference by using DIFF or NRSE input modes.
+
+
–
–
V
m
= [V
in+
– V
in–
]
V
in+
V
in–
V
m
Instrumentation
Amplifier
Measured
Voltage