Re: Do I need to connect all Vref in a bank together?

"Jan Panteltje" <[EMAIL PROTECTED]> wrote in message
news:[EMAIL PROTECTED]
> hehe, I am using Vref as video input, trying to use the thing as AD.
> On the other input of the comparator is an r2r ladder.

Hi Jan,

All this discussion about how tough it is to do stuff
with Vref just gave me an idea...

How about doing the SA A/D just a little differently...
Instead of putting video on comparator Vref and DAC output
on Vin, why not ADD video+DAC (analogue summation) and
apply it to Vin, and leave Vref at a constant level?
(Yes, I realise there are some AC coupling issues to
worry about.  But you already know all about clamping
on sync tips, so that should not be too hard.)
Obviously the successive approximation will "go
the wrong way" and finally give you an inverted result,
but that's easy to deal with. And your input comparator
is always working about the same Vref so you can apply
lots of bypass caps.  Analogue addition is trivially
achieved by connecting the "tail" of the R2R ladder to
video instead of ground.  Make sure you bypass Vref to
the same ground that's used by the DAC output buffers.

Just a thought.
--
Jonathan Bromley, Consultant

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