B
Troubleshooting
This section contains common questions about M Series devices. If your questions are not answered here, refer to the National Instruments KnowledgeBase at ni.com/kb.
Analog Input
I am seeing crosstalk or ghost voltages when sampling multiple channels. What does this mean?
You may be experiencing a phenomenon called charge injection, which occurs when you sample a series of high-output impedance sources with a multiplexer. Multiplexers contain switches, usually made of switched capacitors. When a channel, for example AI 0, is selected in a multiplexer, those capacitors accumulate charge. When the next channel, for example AI 1, is selected, the accumulated current (or charge) leaks backward through channel 1. If the output impedance of the source connected to AI 1 is high enough, the resulting reading can somewhat affect the voltage in AI 0. To circumvent this problem, use a voltage follower that has operational amplifiers (op-amps) with unity gain for each high-impedance source before connecting to an M Series device. Otherwise, you must decrease the sample rate for each channel.
Another common cause of channel crosstalk is due to sampling among multiple channels at various gains. In this situation, the settling times can increase. For more information about charge injection and sampling channels at different gains, refer to the Multichannel Scanning Considerations section of Chapter 4, Analog Input.
I am using my device in differential analog input ground-reference mode and I have connected a differential input signal, but my readings are random and drift rapidly. What is wrong?
In DIFF mode, if the readings from the DAQ device are random and drift rapidly, you should check the ground-reference connections. The signal can be referenced to a level that is considered floating with reference to the device ground reference. Even if you are in DIFF mode, you must still reference the signal to the same ground level as the device reference. There