I built an altimeter with a PIC14000 which I have in bench test now.  I am
using the 16 bit A/D of the PIC14000 to measure voltage from an air pressure
sensor.  It is used to control an autopilot, and the PIC runs the autopilot
servo too.

Everything works great with one big exception.... the PIC is extraordinarily
sensitive to light!  The logic all functions fine and is unaffected by
light, but the A/D measurement is strongly influenced by ambient light on
the chip.  I'm using a JW part now, with tape over the window.  But even
with tape over the window, I can effect the A/D count by an order of
magnitude just by shining a flashlight on the PIC from across the room.  I
have replaced the little red sticker tape that came with the part from
Microchip with double layers of black tape, which improves the A/D
stability, but only to about 6 or 7 bits, nowhere close to the 16 bit
resolution.  If I control ambient light carefully, it still moves around
randomly in the least significant 9 or 10 bits.

I have noticed that the chip measures its own internal reference with good
accuracy and stability and is nearly insensitive to ambient light (stable to
about 14 bits).  This much works great, but the measurement of external
voltages at the pins is hugely affected by light???? (Yes I have checked
that the voltages at the pins are stable).

I am hoping that the OTP version without a window will not have these
problems.  But I am concerned that I will not even be able to get a good
test done with the JW part.  I haven't yet found a way to get more than
about 7 bits of stability (much less accuracy) out of this JW part, which is
totally inadequate for an altimeter or autopilot application.

I have heard of EPROMS having light sensitivity, but the program and data
memory in this PIC seems to work just fine.  Only the A/D is affected.  Has
anyone else seen this light sensitivity problem?  Has anyone used this part
successfully with stability of 12 or more bits?  Any ideas out there?

                                        Dewey Davis