Chapter 4 Analog Input© National Instruments Corporation 4-15 NI 6232/6233 User ManualWith this type of connection, the PGIA rejects both the common-modenoise in the signal and the ground potential difference between the signalsource and the device ground, shown as V cm in the figure. Refer to theNI 6232/6233 Specifications for the usable range of V cm.Common-Mode Signal Rejection ConsiderationsFor signal sources that are already referenced to some ground point withrespect to the device, the PGIA can reject any voltage caused by groundpotential differences between the signal source and the device. In addition,with DIFF input connections, the PGIA can reject common-mode noisepickup in the leads connecting the signal sources to the device. The PGIAcan reject common-mode signals as long as AI+ and AI– (input signals) areboth within ±11 V of AI GND.Differential Input BiasingFigure 4-4 shows AI GND connected to the negative lead of the signalsource. If you do not connect AI GND, the source is not likely to remainwithin the common-mode signal range of the PGIA due to the floatingsource or the isolation barrier. The PGIA then saturates, causing erroneousreadings.You must reference the source to AI GND. The easiest way to make thisreference is to connect the positive side of the signal to the positive input ofthe PGIA and connect the negative side of the signal to AI GND as well asto the negative input of the PGIA, without using resistors.Differential Connections for Non-Referenced or Floating Signal SourcesFigure 4-5 shows how to connect a floating signal source to a channelconfigured in DIFF mode.
