Chapter 3
Connecting the Signals
DAQPad-MIO-16XE-50 User Manual
3-10
ni.com
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 DAQPad-MIO-16XE-50 AI ground 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 DAQPad-MIO-16XE-50,
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.
Note
If you power both the DAQPad-MIO-16XE-50 and the PC with a floating power
source (such as a battery) and you are not tying the DAQPad-TB-52 grounding lug to earth
ground, the system is floating with respect to earth ground. In this case, treat all of the
signal sources as floating sources.
Input Configurations
You can configure the DAQPad-MIO-16XE-50 for one of three input
modes—NRSE, RSE, or DIFF. The following sections discuss the use
of single-ended and differential measurements and considerations for
measuring both floating and ground-referenced signal sources. Figure 3-3
summarizes the recommended input configuration for both types of signal
sources.