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Very noisy analog inputs


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#1 cafewalter

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Posted 02 July 2015 - 12:01 AM

I have a rather ancient U12, serial # 100006854.  I haven't used it in a few years.  I dug it out today, hooked it up to a computer running LJLogger, and noticed that the analog inputs seem very noisy.  If I short the input to ground, I get approximately 0V, but if I connect to (say) a charged LiPo battery, I see a signal jumping anywhere from 2.5 to 4.2V.  A multimeter in parallel with the same battery shows a steady 4.15V.

 

Is my old U12 just toast?  Or is there a different way I can troubleshoot it?



#2 LabJack Support

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Posted 02 July 2015 - 09:02 AM

A DMM is not very useful to compare noise, as it often smooths readings over 1 second or so, whereas the U12 is acquiring each reading in less than 1 ms.  I would  expect a battery to provide a steady low-noise source, though.  Perhaps try with a normal alkaline AA, D, or 9V from AI0 to GND?

 

Try testing with no connections except USB and a jumper from AO0 to AI0.  Then use LJlogger to vary AO0 from 0.5 to 4.5 volts, and see how much noise you see on the AI0 readings.  You should see just 1 or 2 counts of noise, where each count is 5mV.

 

Also, run LJtest with nothing connected to the U12 except for USB.



#3 cafewalter

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Posted 02 July 2015 - 10:37 PM

Thanks for the quick response!  It passes ljtest, and powered by its own analog out, the measurement is stable (and agrees with the DMM).  Perhaps I had a bad connection somewhere, when I was testing the battery.

 

All is well.




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