Chapter 4 Signal ConnectionsPCI-6110E/6111E User Manual4-12 ©National Instruments CorporationFigure 4-4. Differential Input Connections for Nonreferenced SignalsFigure 4-4 shows a bias resistor connected between ACH0 – and thefloating signal source ground. If you do not use the resistor and thesource is truly floating, the source is not likely to remain within thecommon-mode signal range of the PGIA, and the PGIA will saturate,causing erroneous readings. You must reference the source to therespective channel ground.Common-Mode Signal Rejection ConsiderationsFigure 4-3 shows connections for signal sources that are alreadyreferenced to some ground point with respect to the 611X E board. Inthis case, the PGIA can reject any voltage caused by ground potentialdifferences between the signal source and the board. In addition, withdifferential input connections, the PGIA can reject common-mode noisepickup in the leads connecting the signal sources to the board. ThePGIA can reject common-mode signals as long as V+ in and V- in (inputsignals) are both within ±11 V of the channel ground, for gain ≥ 1. Forgain <1, the input signals, for ACHO +, can be within ±42 V of thechannel ground.+-+FloatingSignalSourceInstrumentationAmplifierV mMeasuredVoltage-VS-+I/O ConnectorACH0GNDBiasCurrentReturnPathsACH0-ACH0+ACH0 Connections ShownPGIA1MΩ100pf10nfBiasResistor(see text)
