Why does labview forget the connections to the virtual channels in MAX?

Question

I create a bunch of virtual channels in MAX - some with double assignment to a single AI port.
Context - a digital output controls a MUX to switch the inputs into the AI ports of my USB-6361(BNC) DAQmx; thereby allowing me to double up on the number of AI ports I can access (switching every 5ms).
I can run the code without any troubles. I have now completed my hardware set up and and am now testing the integration with the code. This is where things become a little unstuck.
Problem: some ports, over time, start displaying erroneous data that is not correct. It is not until I delete the virtual channel and re-create it in MAX, does everything settle and work. This 'bad' data is seen in the MAX test panels tab and virtual channels tab.
What is going on - should I bite the bullet and program all port assignments (channels) within my code?
Am i mistakenly killing a link between MAX and the DAQmx?

Answer 1

natashw already told you that it's expected behaviour when you leave the analog input open. There are two aspects to this:
1) The analog input amplifier is a high impedance operation amplifier. It's minimal stray capacitance is high enough that it gets charged through very small leakage currents in the amplifier input stage, but the high input impedance doesn't allow those capacitance to discharge quick enough to stay at a defined voltage. So your input is usually very slowly floating to one of the power supply rails depending what transistor side has a slightly higher leakage.
2) You only have one analog to digital converter. To get multiple channels there is a multiplexer than connects the different input channels to this single ADC. When the multiplexer switches between from a connected signal to an unconnected signal those stray capacitance at the ADC amplifier input has been charged to a certain voltage from the connected channel. With only the internal impedance of the amplifier to discharge this capacitance after the multiplexer switched to an unconnected input, the ADC will simply see the voltage that the stray capacitance has been charged to.
Having a low impedance amplifier input would solve that problem but create many more problems that would affect the measurement accuracy significantly.
Rolf Kalbermatter
CIT Engineering Netherlands
a division of Test & Measurement Solutions