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Hi, I have a non-profit (Smoky Mountain Scientific, smokymtsci.com) that develops and sells low cost instruments for science education.  One of our instruments incorporates a Tiva-C launchpad that outputs a pwm voltage and measures voltage and current passed in electrochemical reactions.  I recently purchased a fairly large batch of Tiva launchpads from Mouser and I am having trouble with noise in the analog read channels for about 1/4th of the boards.  We have been building and selling these instruments for a couple of years now and have never had this problem before.    To test the boards, I send a pwm signal through a simple low pass filter (a resistor and a capacitor) then read the output on two channels (A11 and A8).  The attached figures show examples of output from a good (file Board46) and a bad (file Board37) boards as displayed on our user interface.  This seems to be a hardware problem and I dont know how to go about resolving it.  Any recommendations?
Thanks,

Jack 

Board37.png

Board46.png

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Are the good ones and the bad ones using the same power supply? I was getting "wobbly" adc readings due to a cheap power supply. Problem went away when I used a more quality power supply. In my case an iPhone charger. 

 

Cheers

PTB

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Thanks for the suggestion @PTB but I dont think that is the problem.  I am powering these through the USB port on my computer.  I am also using the same micro USB cable and there is no change in the quality of data between trials.  That is, a good board will always give good readings and a bad one will always give bad readings.

Jack

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That was a good suggestion @spirilis, I posted on the E2E site on the recommendation of the folks at Mouser.  Had they not recommended this (and you as well), I would not have done so.  The conversation on the E2E site is very helpful. Thanks,

Jack

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