National Instruments AT-MIO-16X - Manual (Page 43)

Chapter 2 Configuration and Installation
© National Instruments Corporation 2-21 AT-MIO-16X User Manual
Types of Signal Sources
When configuring the input mode of the AT-MIO-16X and making
signal connections, you must first determine whether the signal source
is floating or ground-referenced. These two types of signals are
described in the following sections.
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 provides
an isolated output falls into the floating signal source category. The
ground reference of a floating signal must be tied to the AT-MIO-16X
analog input ground in order 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-MIO-16X board,
assuming that the PC AT 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 mV and
100 mV but can be much higher if power distribution circuits are not
properly connected. If grounded signal source is improperly measured,
this difference may show up as an error in the measurement. The
following connection instructions for grounded signal sources are
designed to eliminate this ground potential difference from the
measured signal.
Input Configurations
The AT-MIO-16X can be configured 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.