Chapter4 SignalConnections
©NationalInstruments Corporation 4-9 6023E/6024E/6025EUser Manual
Floating Signal Sources
Afloating signal source is not connected in any way to the buil ding 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 isolationamplifiers. A n
instrumentor dev icet hat has an isolated output is a floating signal source.
You must tie the ground reference of a floating signal to the analog input
ground of your device to establish a local or onboard reference for the
signal.Otherwise, the measured input signal varies as the sou rcefl oats out
oft he 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 device, assuming that the computer is
plugged into the same power system. Non-isolated outputs of instruments
anddevices that plug into the building power system fall into this category.
The differencein ground potential between two instruments connected to
thesame building power system is typically between 1 and 100 mV,butcan
be much higher if powerdistribution circuits are not properly connected.
Ifa grounded s ignal source is improperly measured, this differencecan
appearas an error int he measurement. The connectioninstructions for
grounded signal sources are designed to eliminate this ground potential
differencefrom the measured signal.
Analog Input Modes
You cancon figure your device for one of three input
modes—nonreferenced single ended (NRSE), referenced single ended
(RSE), and differential (DIFF). With the different configurations, you can
use the PGIA in different ways. Figure 4-3 shows a diagram of the PGIA
ofy our device.