EK-TM4C123GXL: Hardware problem with ADC generating noise?

Jack,

The "non-profit" mention is honorable, and from your description, the project seems quite interesting!

However, they do publish these terms for a reason...

Document link here: www.ti.com/.../sszz027m.pdf

The actual suggestion here, while not that sweet, is to design a board for the purpose, with proper analog signals care.

Regards

Bruno

Jack Summers11 said:

the noise at the ADC would be ~17 mV. So, 0.017*4095/3.3 ~ 21 counts. That may not seem like a whole lot,

21 counts isn't a lot. Two points from the data sheet to note

First total error over range

Datasheet said:

ET Total unadjusted error, over full input range  ±30.0 LSB

Second for noise, single ended

Datasheet said:

SNRS Signal-to-noise-ratio, Single-ended input, 60  dB

Even the typical noise figure is just 65 db (~10.5bits). Differential input gets as high as 72dB typical (~11.7 bits)

Then you have to consider whether your reference is stable.

Robert

Jack,
Being trained as a chemist, you do show some good knowledge of signal analysis already!
Don't get too desperate, I've been educated as a mechanical engineer (that degree after which wives assume you will be able to fix a modern car if it suddenly stalls on the road shoulder)... And yet, here I am talking (reasonably correct) EE matters.
It won't be less than a good year to be able to design by yourself a schematic/circuit from scratch with proper quality. But the very Launchpad manual has all its schematics and even board layout (I believe the latter can be downloaded somewhere). They are built in Eagle (lots of design software out there...)
Regards
Bruno

Might you have presented a "key" clue - which thus far - sits/kicked (roadside)    [clearly awaiting M.E. Bruno]    while bruised & abandoned?

Jack Summers11 said:

... having trouble with noise ... for about 1/4th of the boards ... AND building/selling these for a couple of years - have never had this problem!

So - "What's changed" - either on (your) end or this vendor's?     For now - we must accept your report as "true & properly reflective of reality" - yet that proves (not always) the case...

These questions spring from that "key clue:"

• How sure are you that this "issue" had NOT occurred at a similar frequency - in the past?      (i.e. were your "past" tests (really) able to detect - and alarm - such issues?)
• What's changed on your end?      Input analog circuitry, test protocol, added or altered product software, test set-up, board handling (throughout all venues of your production/test protocol)
• What's changed on the vendor's board end?     Ideally - even a "non-profit" - should maintain a series of, "Golden Boards" (pristine boards - which passed all tests - and then are safely placed w/in "Quarantine" so they may (later) be "compared" against boards "failing or w/issues."
• Should you have your own "good" boards - should you not rigorously "A-B" test/inspect/compare/contrast those w/your new boards - which fail?
• Similarly - cannot you compare "good" vendor boards - just as listed above - w/those vendor boards which have "issues?"
• Has your "test set-up" continued - unchanged - between past dates of "success" - and today's "issues?"     Might the "C" of your R-C (LPF) have degraded - changed value?
• Beyond your (mention) of the R-C (LPF) all here are, "in the dark" as to the extent of your (unique) processing electronics - which pass signals to the vendor's LPad.     May I suggest that the vendor's board is less likely to "deviate" than is yours?     (due to the volume & controls placed upon vendor's production - yours stand "unknown.")

It should be both simple & quick to - one at a time - swap boards "In-Out" - and "lock the issue down" to:

• your unique boards  (most likely)
• vendor's boards   (unlikely)
• both your & vendor's boards   (most unlikely)

Identifying the "source" of your problem is Job #1.     Once identified - corrective action may commence.      (btw - how kind of Mouser to "shunt you here" for rescue.      Should they not - at minimum -  "offer dinner" to Robert, Bruno & myself before "ordering up" our combined, time/effort?)

One more item of note (I cannot (now) "edit" the above post) - might your issue reflect the "reality" of "More & Stronger" RF signals in your near vicinity? Cell-phones, Tablets, IOT - even a (stronger) local radio or TV station - may induce such noise!

Again - the value of proper, "A-B" Testing - will best shine great light upon the "reality" of your issue...

And...yet another (obvious) item I earlier missed: "Reverse the board's connected to Ports 37 & 46 - even the difference between Ports must be held, "suspect."

Thanks for the input Robert.  I am not sure that I follow what you are saying.  Total error has to include both systematic and random error, and systematic error would not contribute to the problem shown in the figure.  Not sure what is meant by single ended noise.  I am not using an external reference and assume that the internal references in the launchpads should be similar.     

That notwithstanding, you have pushed me down what may be a profitable direction.  To check the noise on the ADCs I made a voltage divider with two 20 KOhm resistors, plugged one end into VCC, the other into GND and tested A8 and A11 from the center of the voltage divider.  I modified an Energia sketch to read the two ADCs and output to the serial port every few ms. After a few seconds I stopped the scrolling text and copied the values into excel.  I used excel to determined the mean and standard deviation of the data.  To my surprise, all of the boards I tested (both those that performed well and those that did not) gave data with a standard deviation of ~13 counts.       From this, I surmise that the problem is not with the ADCs. 

I changed the test circuit back to the low pass filter and tested the effects of changing the pwm clock speed and the delay between readings.  So far, I have not found the factor that differentiates the two sets of boards. 

To complicate matters further, some of the boards that originally tested bad are now showing the desired behavior (perhaps the software did not load properly the first time?). 

I dont know what is going on.  I will keep testing and let you know what i find.

Thanks again.

Jack

Jack Summers11 said:

...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...

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..."

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...)