Types Of Signal Sources; Floating Signal Sources; Ground-Referenced Signal Sources; Input Configurations - National Instruments DAQ PCI E Series User Manual

Chapter 4 Signal Connections

Types of Signal Sources

Floating Signal Sources

Ground-Referenced Signal Sources

Input Configurations

PCI E Series User Manual
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.
A floating signal source 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 PCI 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.
A ground-referenced signal source is connected in some way to the
building system ground and is, therefore, already connected to a
common ground point with respect to the PCI E Series board, 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 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.
You can configure your PCI 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.
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