National Instruments NI 784xR - Manual (Page 28)

Chapter 2 Hardware Overview of the NI 78xxR
© National Instruments Corporation 2-7 R Series Intelligent DAQ User Manual
The instrumentation amplifier applies common-mode voltage rejection
and presents high input impedance to the AI signals connected to the
NI 783xR/784xR/785xR. 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 783xR/784xR/785xR 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 783xR/784xR/785xR. 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.
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.
Floating Signal Sources
A floating signal source is not connected to the building ground system but
instead has an isolated ground-reference point. Some examples of floating
signal sources are outputs of transformers, thermocouples, battery-powered
devices, optical isolator outputs, and isolation amplifiers. An instrument or
device that has an isolated output is a floating signal source. You must
connect the ground reference of a floating signal to the
NI 783xR/784xR/785xR AIGND through a bias resistor to establish a local
or onboard reference for the signal. Otherwise, the measured input signal
varies as the source floats out of the common-mode input range.
Ground-Referenced Signal Sources
A ground-referenced signal source is connected to the building system
ground, so it is already connected to a common ground point with respect
to the NI 783xR/784xR/785xR, assuming that the computer is plugged into
the same power system. Instruments or devices with nonisolated outputs
that plug into the building power system are ground referenced signal
sources.