Well, it looks like I've learned a little about decoupling caps.  A .1u at the
MAX improved things, but a .1u at the INA was like pure magic.  Thanks for
everyone's help.  I guess the irony is that I've decided on the MPX chip which
negates the need for an amp, but I think the lesson here was almost worth the
pain.  I must use my scope more!  I am curious if anyone could explain what kind
of cap would be best for decoupling (electrolytic, etc.)  Most drawings I see
seem to freely interchange polarity sensitive and insensitive caps for
decoupling...
   Does anyone know a source for the MPX4115ASX?  I tried Arrow, Newark, Digikey
and FutureActive.  Some stock some of the other MPX's but not this one.  (I sure
can pick 'em, eh?)
   Also, anyone know where I can get a 16C774?  Everyone seems backordered...
   And lastly, for those interested, I called Microchip, and the 16F628 should be
available in samples in April and to our doors in June.


>    Greg I had forgot about the SDX series and that would be a better choice
> over the SCX. For a model rocket altimiter, you might want to check out
> Lucas NovaSensor's NPP-301-100A which is a 15 PSI absolute sensor in a SO-8
> package. It's low cost and the temperature compensation typically consists
> of two resistors. There is no pressure port (just a hole in the package) so
> the chip will fit in a very small space and it weighs only 0.10 grams. Lucas
> sent an application kit and I made a simple barometer. Though not as
> accurate as the SCX/SDX or NovaSensor's NPC-1220 series, the results were
> still very good. They have two related application notes:
>
>       Basic Temperature Compensation for the NPP-301
>       Designing an Altimeter using the NPP-301
>
>    I also checked out Motorola's MPX4115 series that David Fansler
> mentioned. This seems ideal if the cost is right. You give it +5V and you
> get a conditioned voltage output from ~0.5V to 4.8V.
>
>    Back to your current `woes', I'd recommend selecting the sensor you are
> going to use in the altimiter, breadboard the circuit and test it with a
> DVM. Once your are satisfied with the results, then go on to the A/D
> interface. It sounds like you are going around in circles with the basic SX
> element considering the calibration and compensation issues. As far as why
> the circuit stops working with the PIC hooked up, I've been following this
> but I don't have a clue at this point...
>
>    - Tom
>
>       http://www.novasensor.com/
>       http://mot-sps.com/products/sensors/
>
> At 09:35 AM 2/2/00 -0700, Greg Hartung wrote:
> >   I am using an SX15A, which is an absolute, uncompensated.  This is just
> for
> >testing, I am sure I will go with a compensated DIP package like the SDX15A2
> >(the SCX are way too many $).  So for now, I'm not worried about temperature
> >compensation, but this is all for a rocket altimeter so it will become
> important
> >later.
>
> ------------------------------------------------------------------------
> Tom Handley
> New Age Communications
> Since '75 before "New Age" and no one around here is waiting for UFOs ;-)