Error in PCI 6120

[vjuno]

Hi. We are using a 4 channel PCI 6120 for acquiring data. The channel AI0 shows a messed signal, even when there is no signal applied. Please find attcahed a graph from the output of AI1 and AI0 channel. Can somebody please help me in this regard.

[Dennis_Knutson]

An input left unconnected will exhibit that behavior and is normal. See the article about ghosting. The article discusses unconnected channels about half-way down.

[Mohammed.Ashraf]

vjuno,

if you are acquiring rapidly from multiple channels there are some precausions you should take care of

make sure that the source impedence seen by each channel is comparable or close to each others , since the multiplexer that multiplexes the channels contains switches, usually made of switched capacitors. When one of the channels, for example channel 0, is selected in a multiplexer, those capacitors accumulate charge. When the next channel, for example channel 1, is selected, the accumulated charge leaks backward through channel 1.

If the output impedance of the source connected to channel 1 is high enough, the resulting reading of channel 1 can be partially affected by the voltage on channel 0. This effect is referred to as ghosting.

you can decrease the scanning rate between channels seeing significantly different impedences ti increase the acquisition accuracy 

 

[vjuno]

Hi Guys. Thqnk you for your prompt reply. Sorry, I think I oculd not explain my problem properly. All the four channels in have almost the same resistance. However, our concern is the large step that comes into play around 3000 acquisiton point. Pleae find attached another picture which might be able to convey the error better.

Thanks once again.

Regards

Vijay 

[Mohammed.Ashraf]

i just want to make sure that you see the same resut when acquiring this channel only, or it is just when you acquire from list of channels ? (problem2.jpg)

also what about the scanning rate of each channel ? i mean you are acquiring from 2 channels simulatenously , can you tell me the scanning rate of each please ?

Regards...

[vjuno]

Hello Mashraf,

We see this problem everytime we use AI0. Either alone or along with other channels. We collect data at the rate of 1 MHz  (warp mode activated using the LabView program available on NI site) .

Regards 

Vijay 

[vjuno]

Mashraf,

We got a new 6120 card yesterday. Kepping everything same, I see normal output on the new card. I think that confirms there is some problem with the old card.

Waiting for your comments.

Vijay 

[Mohammed.Ashraf]

Comparing both DAQs and giving a noticable difference in performans , using the same LabVIEW program, settings,..etc. , all this proves that there might be a hardware issue in the old PCI 6120.

i think you can contact NI for this to fix this issue

Regards...