AppendixC CommonQuestions
NI6013/6014 User Man ual C-4 ni.com
FigureC-1. ConfiguringChannels to Acquire in Different Modes in LabVIEW
Toenable multimode scanning in using NI-DAQ functions, call the
AI_Configurefunction for each channel.
Iam seeing crosstalk or ghost voltages when sampling multiple
channels.What does this mean?
Youmaybe experiencing a phenomenon called charge injection,which
occursw hen you sample a series of high-output impedance sources with
am ultiplexer.Multiplexers contain switches, usually made of switched
capacitors.W hen a channel, forexample ACH0, is selected in a
multiplexer,those capacitors accumulate charge. When the next channel,
forexample ACH1, is selected, the accumulated current, or charge, leaks
backwardth rough that channel. If the output impedance of the source
connected to ACH1is high enough, the resulting reading can somewhat
reflect the voltagetrends in ACH0. To circumvent this problem, you must
usea voltage follower that has operational amplifiers (op-amps) with unity
gainfor each high-impedance source before connecting to the DAQ device.
Otherwise,you must decrease the rate at which each channel is sampled.
Anothercommon cause ofchannel crosstalk is due to sampling among
multiple channels at variousgains. In this situation, the settling tim esm ay
increase.For more information on charge injection and sampling channels
atdifferent gains, refer to the Multichannel Scanning Considerations
section of Chapter 3, Hardware O verview.
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