Chapter 2 Hardware Overview ofthe NI PXI-7831R
NI PXI-7831R User Manual 2-6 ni.com

Floating Signal Sources

A floating signal source is in no way connected to the building ground
system but instead 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
NIPXI-78 31R AIGND through a bias resistor 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 NI PXI-7831R, 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 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.
Input Modes
You can configure the NI PXI-7831R for one of three input modes—DIFF,
RSE, or NRSE. The following sections discuss the use of single-ended and
differential measurements and considerations for measuring both floating
and ground-referenced signal sources.
Figure 2-4 summarizes the recommended input mode for both types of
signal sources.