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When no connections are made to my PCI 6221 why do I get noise and voltage in NIMAX??

Using NIMAX I select Ai0 and hit start, I then get a voltage reading and alot of noise. I discovered this after painstakingly troubleshooting what I thought was noise coming in from my LVDT. Then I happened to run NIMAX without the LVDT powered up and I noticed the readings...thank you for any anticipated help from a person at his wits end! I can post a screenshot if it would help?

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Message 1 of 18

Hi there.


I think we could do with a little more information to help you out.


What is the size of the noise you are seeing? Are we talking single micro volts? or tens of? or hundereds of micro volts?

What sort of enviroment is the card working in? Are you surrounded by equipment/machinary or other devices. In an office or on a factory floor?


In your post, do you mean to say that your problem is now solved having turned off the LVDT?


If you could post a graph that would be great. And dont forget to label if the image was taken with or without the LVDT running.


Thanks, I look forward to being to helping you out!

Applications Engineer
National Instruments
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Message 2 of 18

Hi Rhys,


I will endeavour to answer your questions as best I can, I am fairly new to DAQ setups. I am in a factory but not around machinery.


I have been trying different things this morning and here is where I am:-


I have the analog outputs from the LVDT connected directly to the card on Pins 1 for the +output and pin 2 for -output. I connected it straight to the card to see if it was the cable picking up the noise.


Graph2 that is attached is what i see with the LVDT powered on. I have tried adding resistors to ground but it has little effect.


I am really getting to the end of what I know to try and any suggetions would be greatly appreciated.

Thank you


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Message 3 of 18

Sorry pic didnt upload first time.....

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Message 4 of 18

What connector do you have for the 6221? If it's the 37 pin? Pins 1 and 2 are not correct - they are ai0 and ai9. For a differential measurement, you want ai0 and ai8. If you have the 68 pin connector, pins 1 and 2 make even less sense.


Open pins float and noise and a voltage reading is normal. If you want to test the DAQ card, ground the input.

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Message 5 of 18

I have the 37 pin. I will try pin 3. Thank you for your reply.

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Message 6 of 18

And how do you have the acquisition configured - RSE, Differential, etc.?

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Message 7 of 18

Sorry I meant pin 20 which is Ai8

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Message 8 of 18

I believe I need to have Ai8 configured as Differential but in it is "greyed out" in NiMAX and will not let me select it? Ai0 is configured as Differential.

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Message 9 of 18

You don't do anything with ai8. When you select differential and ai0, ai8 is automatically routed to the a/d and it's selection is disabled.

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Message 10 of 18