Chapter 4 Signal ConnectionsPCI E Series User Manual4-14 ©National Instruments CorporationTypes of Signal SourcesWhen configuring the input channels and making signal connections,you must first determine whether the signal sources are floating orground-referenced. The following sections describe these two types ofsignals.Floating Signal SourcesA floating signal source is not connected in any way to the buildingground system but, rather, has an isolated ground-reference point. Someexamples of floating signal sources are outputs of transformers,thermocouples, battery-powered devices, optical isolator outputs, andisolation amplifiers. An instrument or device that has an isolated outputis a floating signal source. You must tie the ground reference of afloating signal to your PCI E Series board analog input ground toestablish a local or onboard reference for the signal. Otherwise, themeasured input signal varies as the source floats out of thecommon-mode input range.Ground-Referenced Signal SourcesA ground-referenced signal source is connected in some way to thebuilding system ground and is, therefore, already connected to acommon ground point with respect to the PCI E Series board, assumingthat the computer is plugged into the same power system. Nonisolatedoutputs of instruments and devices that plug into the building powersystem fall into this category.The difference in ground potential between two instruments connectedto the same building power system is typically between 1 and 100 mVbut can be much higher if power distribution circuits are not properlyconnected. If a grounded signal source is improperly measured, thisdifference may appear as an error in the measurement. The connectioninstructions for grounded signal sources are designed to eliminate thisground potential difference from the measured signal.Input ConfigurationsYou can configure your PCI E Series board for one of three inputmodes—NRSE, RSE, or DIFF. The following sections discuss the useof single-ended and differential measurements and considerations formeasuring both floating and ground-referenced signal sources.
