piclist 1998\10\01\102916a >
Thread: External A/D jitter
face BY : Dave Johnson email (remove spam text)

Chris Eddy wrote:

>I have been very fortunate to learn a few great tricks from old pros.  The
>answer to your jitter may have alot to do with your ground system.
Yes, after a little more prowling through my trusty "Art of Electronics"
book, I'm convinced that I've made lots of classic layout and grounding
blunders :-) But I'm learning fast. Some careful reworking of the layout
should fix it, or at least improve the situation markedly. I'll find out
this morning...

Matt Bonner wrote:
>What comprises the rest of your analog
>circuit?  You can hardly ever expect N noise-free bits out of an N-bit
>A/D converter (even without any other analog interface circuitry).
The analog circuit is simply an unspecified "thing" to which I must
provide 5V and Gnd, and it gives me an analog signal between 0 and 5V to
read. Of course I've been testing with one particular "thing", but I need
to make the circuit work with a whole variety of them, which will vary in
their electrical characteristics (some consume more power, etc.). Because
of that, I'm considering putting a sample and hold or something similar
between the "thing" and the converter, does that sound like a good idea?
I am loathe to increase the component count beyond what's absolutely
necessary (the finished circuit needs to be as tiny and low power as
possible), but this sounds like it might fall into that category.

>What conversion method does the ADS7822P use?  How many stable bit are
>you getting?
It's a classic successive approximation converter. The reason I chose it
is for it's VERY low power consumption (supply current well under 10uA at
1 kHz sample rate, shutdown current something like 50 nA). Right now I'm
only getting something like 7 stable bits, the lower 4 or so are jumping
all over, but as I said my initial, un-thinking circuit layout is really
bad, so I'll work on that...

Bob Blick wrote:
>The most troubles I've had with
>A/D converters nowadays has been related to high impedance analog signals.
>By high impedance I mean 1000 ohms or more. seriously. Any noise that
>occurs during the conversion process is deadly to accuracy.
Youch. I don't even know what the impedance of the signal is, and it will
vary between "things." That sample and hold, or some sort of buffer with
known characteristics, is sounding better all the time...

Thanks very much to all for the responses, I'm fairly new at this but I'm
loving it, and learning fast.

BTW, is it OK to hit the PICList with these sorts of more general
electronics questions? I figure if I'm grappling with these questions
others are too. Or is there some other mailing list that might be better?
(I'm not terribly fond of newsgroups anymore...)

Dave Johnson


See also: www.piclist.com/techref/io/atod.htm?key=a%2Fd
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