National Instruments NI 785xR - Manual (Page 29)

Chapter 2 Hardware Overview of the NI 78xxR
R Series Intelligent DAQ User Manual 2-8 ni.com
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 signal types.
Floating Signal Sources
A floating signal source is not 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 connect the ground reference of a floating signal to the
NI 783xR/784xR/785xR 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 to the building system
ground, so it is already connected to a common ground point with respect
to the NI 783xR/784xR/785xR, assuming that the computer is plugged into
the same power system. Instruments or devices with nonisolated outputs
that plug into the building power system are ground referenced signal
sources.
The difference in ground potential between two instruments connected to
the same building power system is typically between 1 and 100 mV. This
difference can be much higher if power distribution circuits are improperly
connected. If a grounded signal source is improperly measured, this
difference might 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
The following sections discuss single-ended and differential measurements
and considerations for measuring both floating and ground-referenced
signal sources.