Chapter 4
Connecting the Signals
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section for more information. If you have a grounded source, do
not reference the signal to AIGND. You can avoid this reference by using
DIFF or NRSE 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 signal types.
Floating Signal Sources
A floating signal source is not connected to the building ground system.
Instead, it has an isolated ground-reference point. An instrument or device
that has an isolated output is a floating signal source. Some floating signal
source examples are the outputs of transformers, thermocouples,
battery-powered devices, optical isolator outputs, and isolation amplifiers.
You must connect the ground reference of a floating signal to the device
AI ground to establish a local or onboard reference for the signal.
Otherwise, the measured input signal varies as the source floats from the
common-mode input range.
Ground-Referenced Signal Sources
A ground-referenced signal source is connected to the building system
ground. Therefore, it is already connected to a common ground point with
respect to the device, assuming that the computer is plugged into the same
power system. Nonisolated outputs of instruments and devices that plug
into the building power system fall into this category.
The difference in ground potential between two instruments connected to
the same building power system is typically between 1 and 100 mV, but can
be much higher if power distribution circuits are improperly connected. If a
ground-referenced signal source is incorrectly measured, this difference
can appear as a measurement error. The connection instructions for
grounded signal sources are designed to eliminate this ground potential
difference from the measured signal.
