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EK-TM4C123GXL: Hardware problem with ADC generating noise?

Prodigy 120 points

Replies: 19

Views: 5829

Part Number: EK-TM4C123GXL

The non-profit I work for (Smoky Mountain Scientific, smokymtsci.com) develops and sells low cost, open source 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 100 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.  I put the apparatus together on a grounding mat and then plug the usb connector into my computer to flash the microcontroller and do the testing.  I have posted a description of this on the 43oh web site (forum.43oh.com/.../ and have contacted Mouser about it.  Mouser said that I needed to bring the problem to the e2e forum before they could do anything.  Any recommendations?
Thanks,

Jack

  instruments for

 

19 Replies

  • In reply to Jack Summers11:

    Jack Summers11
    ...test protocol involves attaching a test resistor across the electrode leads

    Might the method of such, "Attaching a test resistor across electrode leads" in itself - cause or contribute to your issue?      Is the attachment's connection: secure, guaranteed to be low inductance & resistance, oxidation/contaminant-free and repeatable over time (and between set-ups.)       (note: the build-up of oxidation upon such "connections" predictably introduces  "random and unwanted  measurement variation.")   My firm frequently visits firms where, "insufficient time"  has "justified"  less than: "pristine, robust, and stylized inter-connects" - which too often gives rise to conditions - (just) as you report.

    Beyond this - the presence of  new/added cells, tablets, IOT devices - operated or connecting at random times - all w/the potential to "add noise."     (unaddressed - to this point...)

    As always - your analog front end must strive to best match the input impedance of the MCU's ADC - and in your case - may benefit from (even)  shorter connections - and those routed via,  "shielded" cable.

    It is expected that your use of:  "Additional & Over-Cautions"  may best enable systematic,  "Caution Reduction" - which best identifies the point(s) of maximum system weakness/noise admittance...

  • In reply to Jack Summers11:

    Jack the figures I gave are quoted from the datasheet and should be ones you at least review when you designed your conditioning circuitry. They specify the worst case noise you can expect from the A/D for different configurations. I translated that to effective bits (the dependence is well known and easy to find). You should view noise levels below those figures as fortunate but not to be depended upon. This is a separate issue from accuracy.

    If you don't have a separate reference, then your reference is the analog power supply. This is a fairly common (more so than having a real reference) on microcontroller A/Ds. Your noise will always depend on the analog supply noise, but even more so when using it as the ref. In addition the power supply must be able to support high frequency draws by the A/ D without introducing more noise. As well as noise being introduced from the instabilities of the analog supply, your accuracy is limited by the usually relatively low accuracy of the power supply.

    That leads to a caution about the 20k resistor, that's much too high a source impedance.

    Robert

    TI has some good reference material. If you search using the term bookshelf you should find links to them.
  • In reply to Jack Summers11:

    Jack, capacitors do fail in board production. They can fail in reflow due to thermal shock or high temperature. They can fail during depaneling due to mechanical stress. They can fail during testing due to ESD.

    Worse, these can be intermittent. I've seen caps that appear noisy or intermittent change behaviour dramatically when breathed upon. I've seen these same caps fall apart when desoldered as the invisible stress fracture became apparent. I've also seen them explode from such damage.

    Robert
  • In reply to Robert Adsett72:

    Jack, it may be worth your while showing us your input conditioning circuitry.

    Robert
  • In reply to Robert Adsett72:

    Robert,

    Re: Caps DO Fail...   (I could not import your "quote"...)

    Exactly my (previous) point - rejected by our poster - yet "just as you report" - firm/I have "changed Cap values thru "breathing" and have witnessed their "falling apart" (when ever so gently probed) and when "excitement is required" - we've seen electrolytics LAUNCH - and embed themselves in our (once) pristine dropped ceiling...

    Wishful thinking (all is well) may not always prove, "Accurate Thinking..."

  • In reply to cb1_mobile:

    I've the same issue with quoting here cb1. One of the reasons I mentioned the exploding caps is even people who are aware electrolytics can explode are sometimes surprised to learn that ceramics will do so as well in the right circumstances.

    Robert
  • In reply to Robert Adsett72:

    "Beware the short lived Light Emitting Capacitor"

    Your tag made me chuckle. Thanks for the levity.
  • In reply to Dubnet:

    Make that, chuckle ++;

    Candidate for, "Best TAG ever!"      

    And ... interestingly noted ... such "Light Emitting (i.e. likely Smoking) Capacitor" flows from the ... (drumbeat maestro)... ... Smoky Mountain!      (no one could, "Make this stuff up!")

    (2nd place awarded: via (gentle) admonition to poster - via Robert's, "Every Tag known to man"  - as you've just supplied - "may prove (bit) much!")

    [edit]    Can it (really) be considered, "Indian Giving" when (yet another) TAG vies for 2nd Place?      May I nominate, "Light Emitting Capacitor - Doesn't even Work!"    (accompanied w/ongoing "whine & tear-stains AND - at minimum - three such failed (yet different) caps...?")

    note: "Indian Giving"  is an idiom which describes (some) initial award - and (later) the award's withdrawal...    (often - w/out justification...)

  • In reply to Dubnet:

    Your appreciation is appreciated gentle folk.

    Robert

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