National Instruments E Series - Manual (Page 48)

Chapter 4 Signal Connections
DAQCard E Series User Manual 4-12  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
output, 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 DAQCard 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 DAQCard, assuming that
the computer is plugged into the same power system. Nonisolated
output of instruments and devices that plug into the building power
system falls 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 DAQCard 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.