Two signals interfering with one another

[Mass5]

Hi,

I have an issue with a pressure trace and a vibration sensor. I have a PCIe-6341 DAQ card. I connect my pressure signal to a channel (say ach0) and I connect the vibration to a different analog channel (say ach7).

In labview I use a DAQ assistant. When I use it to sample only one  of the signals, there is no problem. If I add two channels in the configuration dialog box, the pressure signal becomes superimposed on the vibration signal.

The analog signals going into the DAQ do not show interference (checked with an oscilloscope). I suspect this is happening inside the DAQ card.

Any ideas why this is happening?

Thanks,

Masoud Mashkournia

[enderWiggin10 · Accepted solution]

Hi Masoud,

It appears that you are seeing "ghosting" on your channels.  The 6341 does multiplex through analog inputs.  Therefore, if you are scanning between a high-voltage and a low-voltage, especially when using RSE, you can see ghosting if the signal from the previous channel does not settle out completely or have a path to ground.  Here is a good KnowledgeBase article that describes ways to eliminate this ghosting.

I noticed that you are using RSE configuration on your DAQ Assistant Configuration.  Try using differential input configuration.  This can help to eliminate unwanted common mode voltages.  Also, you can try place an intermediate channel in the scanning that is grounded to AIGND in the DAQ scanning order.  This can help to ground the previous signal.  Most likely you will want to scan in this order: Pressure, Ground, Vibration.

Please let me know if this helps.

Best,  

[Mass5]

That was an excellent suggestion. Works fine now. Thanks!