National Instruments PCI-6023E - Manual (Page 38)

Chapter 4 Signal Connections
© National Instruments Corporation 4-9 PCI-6023E/6024E/6025E User Manual
Floating Signal Sources
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 isolators, 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 board’s
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 connected in some way to the
building system ground and is, therefore, already connected to a common
ground point with respect to the 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.
Analog Input Modes
You can configure your board for one of three input modes: nonreferenced
single ended (NRSE), referenced single ended (RSE), and differential
(DIFF). With the different configurations, you can use the PGIA in
different ways. Figure 4-3 shows a diagram of your board’s PGIA.
PCI.book Page 9 Wednesday, September 16, 1998 9:09 AM