National Instruments PCI-6110E/6111E User Manual

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National Instruments PCI-6110E/6111E User Manual Manual pdf 42 page image

Chapter 4 Signal Connections©National Instruments Corporation4-9PCI-6110E/6111E User ManualTypes of Signal SourcesWhen making signal connections, you must first determine whether thesignal sources are floating or ground-referenced. The followingsections describe these two types of signals.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 the 611X E board analog input ground to establish alocal or onboard reference for the signal. Otherwise, the measured inputsignal varies as the source floats out of the common-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 611X E board, assuming thatthe 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.Differential MeasurementsThe following sections discuss the use of differential (DIFF)measurements and considerations for measuring both floating andground-referenced signal sources.