National Instruments Data Acquisition Device E Series - Manual (Page 55)

Chapter 2 Analog Input
E Series User Manual 2-22 ni.com
Types of Signal Sources
When configuring the input channels and making signal connections, first
determine whether the signal sources are floating or ground-referenced.
Floating Signal Sources
A floating signal source is not connected to the building ground system, but
has an isolated ground-reference point. Some examples of floating signal
sources are outputs of transformers, thermocouples, battery-powered
devices, optical isolators, 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 AI ground of the device to
establish a local or onboard reference for the signal. Otherwise, the
measured input signal varies as the source floats outside 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 device, assuming that the computer is plugged into the same power
system as the source. Non-isolated 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 the
difference can be much higher if power distribution circuits are improperly
connected. If a grounded signal source is incorrectly measured, this
difference can appear as measurement error. Follow the connection
instructions for grounded signal sources to eliminate this ground potential
difference from the measured signal.
Differential Connection Considerations
A DIFF connection is one in which the AI signal has its own reference
signal or signal return path. These connections are available when the
selected channel is configured in DIFF input mode. The input signal is
connected to the positive input of the PGIA, and its reference signal, or
return, is connected to the negative input of the PGIA.
When you configure a channel for DIFF input, each signal uses
two multiplexer inputs one for the signal and one for its reference signal.