>Sounds like you may not be giving the A/D converter time to settle. Try
>selecting the channel then delay for a few cycles before starting the A/D
>conversion.

Well I took a look at it again, and I did find a mistake, I was not giving
as much time as I had computed, so I increased it to what was required, and
I still have problems.

According to the equations in the PIC17C7X data sheet, section 13.1
(1995/1996 data book):

Sampling time = Amp Settling time + Holding cap. charge time + temp coeff.
              = 5us - 51.2pF( 1K + Rss + Rs) ln (1/511) + (Temp - 25) *
.05us/ deg C

Rs = 2K (at most)
Rss = 7K (at 5V)
Temp = 125 deg C(for automotive part)

so plugging into this equation:
sampling time = 5us + 3.2us + 5us = 13.2us

I noticed the crosstalk when the sampling time was 22us, so I tried a bunch
of parts to see where the crosstalk stopped.  I had delays of 22, 43, 65,
105, 210, and 360 us, and the crosstalk was evident up to the 43 us
sampling time, and not evident for the 65us and up sampling times.  I used
a heat gun to warm up the 65us sampling PIC, and saw no degredation in the
accuracy.  Thankfully I can afford to lose the extra clock cycles, but
this is awfully frustrating.  Should I not trust the equations in the data
sheet?  I have to resort to 4x the reccomended sampling time to make sure
the conversion is correct.


Matt Bennett




-----------------------------
Matt Bennett                |
mjb@hazmat.com              |
http://www.hazmat.com/~mjb/ |