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Chapter 4 Signal Connections
AT-MIO/AI E Series User Manual 4-18
National Instruments Corporation
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 types of
signals.
Floating Signal Sources
A floating signal source is one that is not connected in any way to the
building ground system but, rather, 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 tie the ground
reference of a floating signal to your AT E Series board analog input
ground 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 one that is connected in some way
to the building system ground and is, therefore, already connected to a
common ground point with respect to the AT E Series board, assuming
that the PC 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 not properly
connected. If a grounded signal source is improperly measured, this
difference may appear as an error in the measurement. The connection
instructions for grounded signal sources are designed to eliminate this
ground potential difference from the measured signal.
Input Configurations
You can configure your AT E Series board for one of three input
modes—NRSE, RSE, or DIFF. The following sections discuss the use
of single-ended and differential measurements and considerations for
measuring both floating and ground-referenced signal sources.