National Instruments NI 6014 - Manual (Page 33)

Chapter 4 Connecting Signals
© National Instruments Corporation 4-7 NI 6013/6014 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
NI 6013/6014 AIGND to establish a local or onboard reference for the
signal. Otherwise, the measured input signal varies as the source floats
outside 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 NI 6013/6014, 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 it can be much higher if power distribution circuits are improperly
connected. If a grounded signal source is improperly measured, this
difference may appear as a measurement error. The connection instructions
for grounded signal sources are designed to eliminate this ground potential
difference from the measured signal.
Analog Input Modes
You can use the NI 6013/6014 PGIA in different ways, depending on
whether you configure the NI 6013/6014 for NRSE or DIFF mode.
Figure 4-2 shows a diagram of the device PGIA.