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DAC offset on PCI-6014 or PCI-6024 analog outputs when connected to intrumentation

This is a followup on a previous question. We have installed a PCI-6014 which shows a variable offset on the analog output, typically ~50mV, whenever the output is connected to a grounded instrument (scope, etc) with a standard coaxial cable (via a BNC-2090 or BNC-2110).

Thus, if we set the output to 0 with DaqMX, and measure the output directly with a voltmeter (and no other connection), the output is within specs. But, if we then connect the output to some grounded instrument, or simply touch the analog output BNC shield to chassis ground, the output jumps to around 50mV. Various maneuveurs can reduce this - for example, if we disconnect the LCD display (!), the output goes down to several mV (still not within the spec of a few hundred microvolts).

These observations hold even with the PCI-6014 installed in another computer (different make and model), and with similar tests using a PCI-6024.

I have the feeling that we are doing something basically wrong, but what?

Thanks in advance for any ideas.
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Hello,

It seems to be a common mode voltage or ground loop problem.
When you are sourcing an output signal from your DAQ board, the signal is already grounded, it means that it has already a reference. If you measure this output with a measurement device, which has also a ground, the both ground potential are not necessary equal. This difference is the common mode voltage and influences your measurement (offset).

Please check the following links to have more detail about grounded signal.

http://zone.ni.com/devzone/conceptd.nsf/webmain/8D907374C5E867F186256C640059F30C?opendocument

http://digital.ni.com/worldwide/france.nsf/webtechsupp/0E6A8D5999CEE1AF8025683C0056A4B1?OpenDocument&node=163080_f

Best regards,

Alexandre D

National Instruments
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