[sdiy] A/D weirdness

[Karl Ekdahl]

So right now both the input and reference is buffered with an opamp follower (TLC2254), the +3.3V has a 100uF cap to GND on it. I measured in+, in- (tied to GND), +3.3V and REF (with my Extech 82CM which i'm told is very accurate) and can't see any fluctuations beyond +-0.1mV at worst... An interesting thing is that probing the circuit doesn't actually make the A/D jump, just a tiny bit when probing the +in - but no change in stability.Am i not to rely on these measurements? The only thing i can think of right now would be to ditch what my meter says and go ahead and experiment with adding tiny capacitors around the circuit... ?

Thanks

Karl



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Skickat: torsdag, 22 december 2011 13:30
Ämne: Re: [sdiy] A/D weirdness

Am 22.12.2011 18:35, schrieb Karl Ekdahl:
> So i'm stumped, any help greatly appreciated.

Well, that would only be 4mV of noise on the voltage reference or supply noise that is getting through with an equivalent of that.  I can't find the PSRR figures in the datasheet, but there is mention of the part being very susceptible to supply noise and I'd take that seriously.  It is not easy to clean up the supply and reference of a 16bit A/D to actually get the performance that is on the label... anyway when you say that your reference is shared with other circuitry, that seems to imply a good deal of wiring that could be picking up noise.  Voltage references should never be distributed like that if possible.  If you absolutely need to share a reference, put it closest to the part that needs the cleanest reference.

-- Achim.

(on the road :-)
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