Chapter4 ConnectingSignals
©NationalInstruments Corporation 4-7 NI6013/6014 User Manual
Types of Signal Sources
When making signal connections, you must first determine whether the
signalsources are floating or ground-referenced. The following sections
describe these two types of signals.
Floating Signal Sources
Afloating signal source is not connected in any way to the buil ding ground
systembut, rather, has an isolated ground-reference point. Som e examples
offloat ing signal sources are outputs of transformers, thermocouples,
battery-powereddevices, optical isolator outputs, and isolation amplifiers.
An instrument or device that has an isolated output is a floating signal
source.You m ust tie the ground reference of a floating signal to the
NI6 013/6014 AIGND 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 in some way to the
building system ground and is, therefore, already connected to a common
groundpoint with respect to the NI 6013/6014,assuming that the computer
isplugged into the same power system. Nonisolated outputsof instruments
anddevices that plug into the building power system fall into this category.
The differencein ground potential between two instruments connected
to the same buildingpower system is typically between 1 and 100 mV,
butit can be much higher if power distribution circuits are improperly
connected. If a grounded signal source is improperly measured, this
differencemay appear as a measurement error. The connection instructions
forgrounded signal sou rces are designed to eliminate this ground potential
differencefrom the measured signal.
Analog Input Modes
You can use the NI 6013/6014 PGIA in different ways, depending on
whether you configure the NI 6013/6014 for NRSE or DIFF mode.
Figure 4-2 shows a diagram of the device PGIA.