5 mV offset on DAC0

[Gene Civillico]

hello -

I need some help removing a 5 mV offset from the DAC0 signal of a BNC-2110 which is connected to a PCI-MIO-16E-4 board.  I have run the E-series self-calibration within labview and the problem is still there.  I realize there is a possibility of a ground loop, so I also need help locating the "main ground" being used by this board so I can tie it as closely as possible to the ground of my measuring probe (sometimes an oscilloscope, sometimes a second nearby PC running acquisition software.

thanks in advance for any help,

Gene

[ChrisR.]

Hi, Gene. You might try measuring the output with only a handheld DMM to see if ground loops could be part of the problem. If the offset is still there with nothing but the DMM hooked up, try running the self-calibration in that condition.

5mV is only about a 1LSB offset for the 16E-4. Depending on how the calibration algorithm works, it might be trying to optimize accuracy over the full range of the DAC, which could leave a little offset around 0. If there's a little nonlinearity around 0, you might find that the offset goes away when you output a small negative voltage such as -4.88mV.

If it turns out for some reason that you're stuck with the offset, and it turns out to be consistent and repeatable, you could of course subtract it out in software.

Chris

[Gene Civillico]

hey Chris -

Thanks for the quick reply!  That was a great suggestion - when I test the center/shield of DAC0 with a DMM (DC mode, right?), I don't see the offset.  I've also since discovered that I can also change the size of the offset by changing which receptacle the oscilloscope is plugged into.  So it's probably a ground loop between the PC and my various measuring devices.

Unfortunately, that's not my biggest problem anymore, since I seem to have altered my labview installation while trying to get the most up-to-date diagnostic tools, and now my application doesn't run (new thread here: http://forums.ni.com/ni/board/message?board.id=250&thread.id=35361).  Your expertise is most welcome on this one as well!

Gene