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PICList Thread
'[OT] External A/D jitter'
1998\10\01@110413 by Chris Eddy

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Dave Johnson wrote:

> 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?
>
> 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...
>

Ref both items above, I suggest adding a front end for two reasons.  You would
prefer to low pass filter the incoming signal, esp as in your case it is a
mystery signal.  Also read up on aliasing.  You would also like to drive the
input impedance of the A/D with a low impedance, an impedance that may not be
guaranteed by your target signal.  Third, you would like a chance to protect
the input of your circuit from Johny Danger Fingers.  I would combine an op-amp
buffer with a low pass circuit.  Try the non-inverting circuit, which offers
very high input impedance, with a cap strapped across the feedback resistor.
This cap value, probably OOO 0.1uF, will low pass the signal.  You can add one
more low pass by tacking the target signal into the + pin on the op-amp through
another RC filter.  This will give you two low pass filters combined, not a
perfect filter but far better than none.  THEN you can add a few diodes to the
rails at the + pin, and the R in the RC filter acts as a load when the diodes
conduct on over voltage.  Choose a low current (as you are apt to do with your
low power A/D) rail to rail input rail to rail output amp.

With respect to S/H or T/H, don't do it.  You think ground loops on a 12 bit
A/D was a pain, hold on to your britches.  The up side is that almost every A/D
made in modern day has a built in T/H.  They have saved you all of the
trouble.  S/H circuits are getting somewhat more difficult to locate, Analog
devices does have a few, but the pickins get slim from there.

> 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
>

Tradition indicates that you put the OT in your subject, as you have, and thus
you are safe.  With the ocean of OT that doesn't even relate to electronics,
your OT is not realy that OT.

Best of luck.
Chris Eddy
Pioneer Microsystems

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