© National Instruments Corporation B-1 NI USB-621x User ManualBTroubleshootingThis section contains common questions about M Series devices. If yourquestions are not answered here, refer to the National InstrumentsKnowledgeBase at ni.com/kb.Analog InputI am seeing crosstalk or ghost voltages when sampling multiplechannels. What does this mean?You may be experiencing a phenomenon called charge injection, whichoccurs when you sample a series of high-output impedance sources with amultiplexer. Multiplexers contain switches, usually made of switchedcapacitors. When a channel, for example AI 0, is selected in a multiplexer,those capacitors accumulate charge. When the next channel, for exampleAI 1, is selected, the accumulated current (or charge) leaks backwardthrough channel 1. If the output impedance of the source connected to AI 1is high enough, the resulting reading can somewhat affect the voltage inAI 0. To circumvent this problem, use a voltage follower that hasoperational amplifiers (op-amps) with unity gain for each high-impedancesource before connecting to an M Series device. Otherwise, you mustdecrease the sample rate for each channel.Another common cause of channel crosstalk is due to sampling amongmultiple channels at various gains. In this situation, the settling times canincrease. For more information about charge injection and samplingchannels at different gains, refer to the Multichannel ScanningConsiderations section of Chapter 4, Analog Input.I am using my device in differential analog input ground-referencemode and I have connected a differential input signal, but my readingsare random and drift rapidly. What is wrong?In DIFF mode, if the readings from the DAQ device are random and driftrapidly, you should check the ground-reference connections. The signal canbe referenced to a level that is considered floating with reference to thedevice ground reference. Even if you are in DIFF mode, you must stillreference the signal to the same ground level as the device reference. There
