National Instruments NI 784xR User Manual

Also see for NI 781xR: User manual User manual

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National Instruments NI 784xR User Manual Manual pdf 34 page image

Chapter 2 Hardware Overview of the NI 78xxR© National Instruments Corporation 2-13 R Series Intelligent DAQ User Manualline to AIGND, connect it to AIGND through a resistor that is about100 times the equivalent source impedance. The resistor puts the signalpath nearly in balance. About the same amount of noise couples onto bothconnections, which yields better rejection of electrostatically couplednoise. Also, this input mode does not load down the source, other than thevery high-input impedance of the instrumentation amplifier.You can fully balance the signal path by connecting another resistor of thesame value between the positive input and AIGND, as shown in Figure 2-7.This fully balanced input mode offers slightly better noise rejection but hasthe disadvantage of loading down the source with the series combination(sum) of the two resistors. If, for example, the source impedance is 2 kΩand each of the two resistors is 100 kΩ, the resistors load down the sourcewith 200 kΩ and produce a –1% gain error.Both inputs of the instrumentation amplifier require a DC path to groundfor the instrumentation amplifier to work. If the source is AC coupled(capacitively coupled), the instrumentation amplifier needs a resistorbetween the positive input and AIGND. If the source has low-impedance,choose a resistor that is large enough not to significantly load the source butsmall enough not to produce significant input offset voltage as a result ofinput bias current, typically 100 kΩ to 1 MΩ. In this case, connect thenegative input directly to AIGND. If the source has high output impedance,balance the signal path as previously described using the same valueresistor on both the positive and negative inputs. Loading down the sourcecauses some gain error.Single-Ended Connection ConsiderationsWhen an NI 783xR/784xR/785xR AI signal is referenced to a ground thatcan be shared with other input signals, it forms a single-ended connection.The input signal connects to the positive input of the instrumentationamplifier and the ground connects to the negative input of theinstrumentation amplifier.You can use single-ended input connections for any input signal that meetsthe following conditions:• The input signal is high-level (>1 V).• The leads connecting the signal to the NI 783xR/784xR/785xR are lessthan 3 m (10 ft).• The input signal can share a common reference point with othersignals.