multichannel measurement

[Dimo]

I want to use NI DAQ input device (NI USB-6289) to capture multiple channels, e.g. Dev1/ai0:3.

Smpling rate is not an issue, the problem is my input sources are weak (high impedance) what shouldn't be a problem, and it is not a problem when I use single channel measurement.

In multichannel configuration there is a problem- the next channel's mesurement result depends on the previous channel voltage, e.g.

ch0 input 0V, ch 1 measures 1.246V (expected 1.25)

ch0 input 2V, ch 1 measures 1.255v etc.

I suspect the mux switch time is too fast and there is not enough time the input to settle.

Q: Is there any setting to control the mux switch time (or time before sample).

Also there is a difference in the results if I use faster sampling clock (inputs are DC), I suspect the same issue.

[Dimo]

OK, doing RTFM (read that ... manual) and with the help of:

http://forums.ni.com/t5/LabVIEW/Does-NI-DAQmx-8-5-choose-the-Convert-Clock-rate-incorrectly/m-p/7339...

I found the property named DelayFromSamplClock.Delay 

which does the job... 

And ... the problem is still here. Even if I make the delay longer, the ghost signal is stronger.... for me this is more like an input multiplexer leakage.

In single channel mode the result is correct.

[John_P1]

Delay from Sample Clock sets the amount of time between the sample clock pulse and the actual sampling of the first channel in the scan list (the first line to go through the multiplexer).

To change the switching rate, you need to use the Convert Clock Rate property (see How Do I Increase Interchannel Delay Using NI-DAQmx or Traditional NI-DAQ (Legacy)?)

You're absolutely correct that the issue is most likely that there is not enough time for the measurement to settle between channels.  The behavior is sometimes referred to as ghosting.

Best Regards,