> > Does anyone have an idea of how i can increase the sensitivity of
> > a 10 bit A/D
> > on a 12f675 ?
>
>
> If you draw a straight line graph, the formula is Y=MX+C where M is the
> gradient and C is the offset.  What you are asking for (I think) is to get
a
> 14 bit number out of a voltage from 0-5V.  This could be done with some
very
> careful opamp signal conditioning.  You need to raise the gradient of the
> line, but at +5v input, this would give you too many volts at your A-D....
> Ok, so you could have a comparator checking the voltage at the A-D and
when
> it reaches +5v, it could put an offset back into the amp section bringing
it
> back to 0v.  With that same comparator, you could then tell the PIC there
is
> another MSB to add.  You could then calibrate the device to compensate for
> ant offset.

This is a possible solution BUT fraught with practical problems. It depends
for success on the ability to accurately amplify the signal by (in this
case) 2 with rather better than half a bit accuracy (not impossible),
measure the unamplified level to better than half a bit (not too hard) and
add a -5v (or whatever) offset with better than 1 bit accuracy.

The same "trick" could be used to split the input range into say 4 portions
to get 2 extra bits. The requisite accuracies increase by a factor of 2 in
each case.

What this is effectively doing is adding a 1 or 2 bit "flash converter" to
the front end to control a series of accurately defined offsets. The
complexity is so great that it would be far easier and cheaper to use a more
capable converter instead. Unless there are overwhelming reasons to do this
(eg the converter is inside my processor and I can't use an external one)
then this method is probably excessively hard. That said, a PIC supplied PWM
signal may make a good source of accurately controlled offset.



    RM

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