Had this problem a while ago and just didn't care because the
application wasn't too important, but I'm coming up on a project in
which temperature measurements will need to be within a few degrees...

For this test, I am using an LM35 (Centigrade temp sensor, 10mV/C
output), and a 16F870's 10bit ADC, and spitting the value out onto an
LCD.

I consistantly get around 10C (8-10 in the range I can test) less than
the actual output (by probing).

Say, for instance if I probe and get .334mV, that is 33.4C, but my
output is around 22C.  That's just not right.

I'm wondering if the built-in ADC's are just simply horrible?  Or am I
doing something wrong?

My scaling is as follows:

0-5V for the voltage range (Vdd as reference)
0-1024 for the input value range (10bit ADC)

so it's simply (adcvalue * 5) / 1024 -> actual voltage, correct?

The output does of course rise when I heat the sensor, and drop back
down as it cools, it's definately getting *some* value from the ADC...

I don't know of any offset register in the pic as there are only 4
registers for the ADC in general, and none mention an offset i would
need to set up.

Now, I realize I could use a more accorate reference, a lower voltage
for the reference to gain resolution, but right now I just want to get
this working and move from there, and honestly, it should be much, much
more accurate than this.

Ideas? Pitfalls?

--
Nick Veys | nick@veys.com | www.veys.com

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