Appendix C
Common Questions
C-4
ni.com
Figure C-1.
Configuring Channels to Acquire in Different Modes in LabVIEW
To enable multimode scanning in using NI-DAQ functions, call the
AI_Configure function for each channel.
I am seeing crosstalk or ghost voltages when sampling multiple
channels. What does this mean?
You maybe 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 ACH0, is selected in a
multiplexer, those capacitors accumulate charge. When the next channel,
for example ACH1, is selected, the accumulated current, or charge, leaks
backward through that channel. If the output impedance of the source
connected to ACH1 is high enough, the resulting reading can somewhat
reflect the voltage trends in ACH0. To circumvent this problem, you must
use a voltage follower that has operational amplifiers (op-amps) with unity
gain for each high-impedance source before connecting to the DAQ device.
Otherwise, you must decrease the rate at which each channel is sampled.
Another common cause of channel crosstalk is due to sampling among
multiple channels at various gains. In this situation, the settling times may
increase. For more information on charge injection and sampling channels
at different gains, refer to the
Multichannel Scanning Considerations
section of Chapter 3,
Hardware Overview
.
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