National Instruments NI 783xR Benutzerhandbuch

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.

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 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.

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, 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.

The following sections discuss 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.