[sdiy] A/D weirdness

[John Speth]

My 2 cents: 

If your ref is good (stability, impedance, etc), then I'd start looking
at my grounds.

Make sure your inputs are true differential if that's your design
intent.  If they aren't you could be inviting noise through the inputs.
You might want to characterize the output change as a function of time
for a DC input.  That might give you a clue of what's wrong.

The DS says the part is sensitive to power supply noise.  Check that.
Add more bypassing.

The part has an input sample window during which the voltage is latched
for conversion.  I think you'll get good results if you make sure your
input is stable during that interval.

JJS

> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl [mailto:synth-diy-
> bounces at dropmix.xs4all.nl] On Behalf Of Karl Ekdahl
> Sent: Thursday, December 22, 2011 9:35 AM
> To: synth-diy at dropmix.xs4all.nl
> Subject: [sdiy] A/D weirdness
> 
> 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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