Does 9205 module is having crosstalk or ghosting problem

[GiridharYadav]

Dear Sir,

Recently I have procured 9205 AI module in which I have found the cross talk between the channels. 

In fact I have connected the signal to be acquired to the channel 0, and the remaining channels were unconnected. But I had configured the remaining channels for future use. Even though the remaining channels were floating, I am able to see the voltage change as it is in the channel 0. I had tried the same application in 9201, It is working good without any problem. Here my doubt is whether all 9205 modules behave in the same way or I have got the fault piece. In this regard I request you to confirm me whether 9205 is having cross talk problem or not and also let me know the diffreence between 9201  and 9205, which states that 9205 is having the problem.

Thanks and Regards

Giridhar

[JamesBeams]

Giridhar,

What you are seeing is normal behavior for the 9205 given you mention the other channels you are sampling are unconnected.  I have seen the same behavior on 9205's as well when I left sampled channels open.  This is because the module is scanned and makes use of an analog mux.  If you switch to a channel that is unconnected then the inputs are left floating and the ADC will read whatever charge is leftover since it has nowhere to dissipate.  If you don't wish to see this behavior then I suggest you temporarily short the inputs of the first unused channel in your scanlist and the rest should return what you expect (close to 0V).

-James

[GiridharYadav]

Dear James,

But the 9201 module is not having this problem? Can you tell me why this 9201 is not having this problem?

Giridhar

[Seth_B.]

There are lots of possible reasons that a 9205 would show this behavior when a 9201 wouldn't.  The 9205 has better resolution, is higher channel count, isolated, and has a different internal architecture for the analog circuitry.  It's not really a problem in that this is what you should expect to see on any multiplexed module when reading open channels.  The 9201 is just being nice this time and not showing the behavior as strongly.  As James suggested, if you need to read 0V on unused channels, they should be tied to COM.

Regards,

[GiridharYadav]

Dear Seth,

If the channel what I am using becomes open (due to some loose connection), then there is a chance that it may cross talk with another channel. So in that case there is a chance of mal function in the process.

How can this problem can be sorted.


Thanks and Regards
Giridhar

[Seth_B.]

This is unfortunately a standard trait of multiplexed modules.  Options for handling ghosting are explained here: How Do I Eliminate Ghosting From My Measurements?

Regards,