Chapter 4 Analog Input© National Instruments Corporation 4-21 M Series User ManualConnecting Ground-Referenced Signal SourcesWhat Are Ground-Referenced Signal Sources?A ground-referenced signal source is a signal source connected to thebuilding system ground. It is already connected to a common ground pointwith respect to the device, assuming that the computer is plugged into thesame power system as the source. Non-isolated outputs of instruments anddevices that plug into the building power system fall into this category.The difference in ground potential between two instruments connected tothe same building power system is typically between 1 and 100 mV, but thedifference can be much higher if power distribution circuits are improperlyconnected. If a grounded signal source is incorrectly measured, thisdifference can appear as measurement error. Follow the connectioninstructions for grounded signal sources to eliminate this ground potentialdifference from the measured signal.When to Use Differential Connections with Ground-Referenced SignalSourcesUse DIFF input connections for any channel that meets any of the followingconditions:• The input signal is low level (less than 1 V).• The leads connecting the signal to the device are greater than 3 m(10 ft).• The input signal requires a separate ground-reference point or returnsignal.• The signal leads travel through noisy environments.• Two analog input channels, AI+ and AI–, are available.DIFF signal connections reduce noise pickup and increase common-modenoise rejection. DIFF signal connections also allow input signals to floatwithin the common-mode limits of the NI-PGIA.Refer to the Using Differential Connections for Ground-Referenced SignalSources section for more information about differential connections.
