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Chapter 4 Signal Connections
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National Instruments Corporation 4-9 PCI-6110E/6111E User Manual
Types of Signal Sources
When 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 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 the 611
X
E 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.
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 611
X
E 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.
Differential Measurements
The following sections discuss the use of differential (DIFF)
measurements and considerations for measuring both floating and
ground-referenced signal sources.
PCI_E.book Page 9 Thursday, June 25, 1998 12:55 PM