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Strange noise probably related to output ground

   It sometimes happen that after some time measuring with my DAQ-2120, the signal appears noisy, like its baseline is changing between two values. Someone told me it's because I'm using the ground of the DAQ device, and this ground is not connected to earth and therefore what I see is the charging and discharging of my AO "ground". If I connect with a cable the AO ground to earth, the noise increases (because of a ground loop?).
   An important thing to note is that I use the same ground for the transducer circuit, the filter and faraday cage; all of them are not connected to any other ground.
   I need to use the AO ground because I need to use the signal from the analog outputs.
   I would like to know what can I do to solve this problem and also to ask: Is the AO ground connected to earth? Does it really matter?
(note: I use a PCI-6036E)
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Errata: where I wrote DAQ-2120, I meant BNC-2120
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Hi Guille,
 
Can you give more details on you problems, for example how are you measuring the signal the output the BNC-2120? Are you measuring with the same board? What are the two values, is it like a changing offset? If you could give a screenshot in MAX if you are meassuring with the same board or another, that will be ver y useful. How are you  connecting to "earth"?
 
Tania Lozoya
LATAM AE
National Instruments

Mensaje editado por Tania Lozoya

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I don't have that problem anymore, and I would like to share with you how I solved it. The plugs from all my devices (the computer with the acquisition card, the current-to-voltage transducer, and the oscilloscope) are not connected to ground, except the low-pass Filter. The signal from all this devices are carried by the inner part of BNC cables, and the ground is carried by the external part of this cables. The Faraday cages are connected to ground through a cable connected to a special connection of the oscilloscope specially designed for that.
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