[sdiy] A/D weirdness

[Karl Ekdahl]

I figured adding smaller bypass to the supply was a next logical step, tried a bunch but no difference :( I also added small bypasses on the REF and +IN but to no avail, the +IN even messed up the sampled values so i had to remove it.


I did however notice something very interesting; my assumption that some erratic behavior came from over-clocking the A/D seems to be false. I just measured the number of cycles if bypassing the delays in my routine and i got 8 cycles per period which should mean i'm way good as my processor is running 16MHz (8 cycles at 4 clocks per cycle for a PIC would mean 16 / (4 * 8) = 0.5MHz right?) so that can't be it.

After removing this delay and making a "history" of converted values i've noticed a pattern: out of 10 stored values (sampled at the same voltage), about 4-8 of them will be the same, some will be +/- 200 bytes, and one might be really off. Also, as im testing this by sampling the voltage from a CV keyboard and i've noticed that some keys (voltages) will almost always sample perfectly while others might have a more erratic sample history - it's repeatable so there's another pattern, i have no idea what it means tho... 


Granted with this information i could just sample 10 times and take the value that occurs most often, but that seems like cheating - i'd rather fix whatever is being broken.


KArl



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Skickat: torsdag, 22 december 2011 14:28
Ämne: Re: [sdiy] A/D weirdness

Try bypassing larger electrolytics with smaller caps, like 0.01uF or 0.047uF. This helps with higher frequency noise. Small tip, I know, but the lower effective series resistance helps.

AA

On Thu, 22 Dec 2011 17:35:08 +0000 (GMT), Karl Ekdahl wrote:
> Hi list, got some trouble with an A/D converter and wanted you guyses
> thoughts.
> 
> I'm working with a ADS8326 16-bit super nice converter but i keep
> getting erratic results, the data i get back fluctuates ~+-100 bytes
> (so app. 7 bits) which is clearly way out of range for this converter.
> While testing i've been tying both inputs to GND to try and minimize
> the variables, i've tried both buffered and unbuffered signals. The
> only way i've gotten non-erratic results so far was to tie the
> "ref"-input to GND so i figured it might be the voltage reference.
> However whatever i've tried (buffered through opamp, unbuffered etc.)
> i get the same results - also this reference (a REF5050 divided to
> 2.5V as the ADS8326 is running on 3.3V) is driving a D/A that works
> just great so i guess this is not the problem. I did for a while have
> some issues with not getting the lower bits in due to accidentally
> over-clocking the A/D, however i put in a delay that fixed that issue.
> I tried putting in various delays in the code but to no avail.
> 
> So i'm stumped, any help greatly appreciated.
> 
> Karl
> 
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