Chapter 4
Analog Input
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National Instruments Corporation
4-7
Refer to the
Analog Input Terminal Configuration
section for descriptions
of the input modes.
Types of Signal Sources
When configuring the input channels and making signal connections,
first determine whether the signal sources are floating or ground-referenced:
•
Floating Signal Sources
—A floating signal source is not connected in
any way to the building ground system, and instead 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 connect the
ground reference of a floating signal to the AI ground of the device 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 device, assuming that the computer is plugged into the same power
system as the source. Non-isolated 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 the difference can be much higher if power distribution
circuits are improperly connected. If a grounded signal source is
incorrectly measured, this difference can appear as measurement error.
Follow the connection instructions for grounded signal sources to
eliminate this ground potential difference from the measured signal.
Isolated devices have isolated front ends that are isolated from
ground-reference signal sources and are not connected to building
system grounds. Isolated devices require the user to provide a
ground-reference terminal to which its input signals are referenced.
Differential Connections for Ground-Referenced Signal Sources
Figure 4-6 shows how to connect a ground-referenced signal source to a
channel on a non-isolated S Series device.
