reading multi chan differential voltage w NI 9209: all channels read similar voltages

[Cham35]

Hello everyone,

I used to use simple NI USB-6009 to acquire voltage signals from sensors. Fairly simple, never failed me.

Now needing more than 4 analog voltage inputs in differential mode, I have switched to a NI 9209 module (connected to a 1-slot cDAQ-9171 chassis) with a screw terminal accessory on top of it.

According to guidelines, I have connected (for testing) a 9V battery to chan 0 in differential mode (AI0 - AI8 which correspond to pin 1 and 20) (see https://www.ni.com/pdf/manuals/376909d.pdf page 11). And I locate chan 2 in differential mode (AI2 - AI10 which correspond to pin 3 and 22)

no problem reading 1 channel at a time with NI MAX. When I connect the 9V battery to chan 0, clear signal . When I connect it to any other channels without reading it, no signal on chan 0. Perfect.

Then I make a very simple VI to read 2 channels at the same time to test if I can read 9V on chan 0 and 0V on chan 2 and vice versa.

And here you can see the problem: I do read about 9V stable on chan 0, but there is an awful parasitic signal on chan 2. And if I connect the 9V battery to the chan 2, the exact inverse happens. If I connect the 9V battery to chan 1 that is not read in the VI, nothing: 0V on both channels with a clear signal.

So it is not just a shortcut or bad shielding in between the channels, I think it is a multiplexing problem.

What do you think?

Have I missed something in the VI that would prevent from such problem?

Thanks for your help. I have tried several things, google a lot, but I have not found a solution to that.

Regards

Hicham

[santo_13]

You should not leave any AI floating. No signal does not mean a 0V and the lines being high impedance is very much affected by any charge nearby. Ground any unused AI.

It can be considered a problem, if you connected 9V to one channel and ground the other channel, still see this parasitic.