National Instruments PCI-1200 - Manual (Page 29)

Chapter 3 Signal Connections
© National Instruments Corporation 3-7 PCI-1200 User Manual
grounded source, you must use a nonreferenced input connection at the
PCI-1200.
Types of Signal Sources
When configuring the input mode of the PCI-1200 and making signal
connections, first determine whether the signal source is floating or ground
referenced. These two types of signals are described as follows.
Floating Signal Sources
A floating signal source is not connected in any way to the building ground
system but 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.
Tie the ground reference of a floating signal to the PCI-1200 analog input
ground to establish a local or onboard reference for the signal. Otherwise,
the measured input signal varies or appears to float. An instrument or
device that supplies an isolated output falls into the floating signal source
category.
Ground-Referenced Signal Sources
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-1200, 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.
The connection instructions that follow for grounded signal sources
eliminate this ground potential difference from the measured signal.
Note If you power both the PCI-1200 and your computer with a floating power source
(such as a battery), your system may be floating with respect to earth ground. In
this case, treat all of your signal sources as floating sources.
Input Configurations
You can configure the PCI-1200 for one of three input modes—RSE,
NRSE, or DIFF. The following sections discuss the use of single-ended and
differential measurements, and considerations for measuring both floating