National Instruments AT E Series - Manual (Page 64)

Chapter 4 Connecting Signals
© National Instruments Corporation 4-17 AT E Series User Manual
You must reference all signals to ground either at the source device or at the
device. If you have a floating source, you should reference the signal to
ground by using the RSE input mode or the DIFF input configuration with
bias resistors. See the
Differential Connections for Nonreferenced or
Floating Signal Sources section later in this chapter. If you have a grounded
source, you should not reference the signal to AIGND. You can avoid this
reference by using DIFF or NRSE input configurations.
Types of Signal Sources
When configuring the input channels and 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 one that 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 AT E Series device AIGND 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 one that is connected in some way
to the building system ground and is, therefore, already connected to a
common ground point with respect to the AT E Series device, assuming
that the PC 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.