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From: jpopelish on 6 Jul 2005 14:07


MarkMc wrote:
> Ah, I see. The calibration I had in mind was to pull things in to line
> with the look-up tables.

A couple suggestions. Excite the RTD with a current proportional to
the PIC supply, scaled down to about 1 mA. That would involve
replacing the precision 2.5 volt reference with a 2 to 1 voltage
divider from the 5 volt supply. Now, since the excitation will be
proportional to the supply, if you use the supply as the positive
reference voltage for the A/D, the signal will also be proportional to
the A/D reference. This cancels any scaling errors for changes in the
supply and eliminates the need for a precision component.

Use the two point calibration method (which eliminates the need for .1%
or better standard resistors and compensates for tolerances in your
RTD) and apply this formula to the result.
temperature in degrees C = (counts-Czero)*100/C100
where Czero is the A/D result for ice bath, and C100 is A/D result with
boiling bath. If you do this at sea level, this linear approximation
for the RTD averages about .235 degree error over the range of 0 to 100
degrees C. If done at higher elevations, you can change the constant
100 factor to something lower that represents the local boiling
temperature. If you have precision resistors available, they can be
substituted for the RTD during calibration. Store the calibration
constants in EEPROM.

From: jpopelish on 6 Jul 2005 14:37
jpopelish(a)rica.net wrote:
(snip)
> If you do this at sea level, this linear approximation
> for the RTD averages about .235 degree error over the range of 0 to 100
> degrees C.

I found an error in my math. The average error in the linear
approximation over this range with a best fit is .097 degrees. If you
force the errors to zero at the end points (a sub optimal fit, overall)
the worst error occurs at about the mid point temperature and is about
-.4 degrees. If you fudge the endpoints to read .25 degree high, the
midpoint error is reduced to about -.15 degree.

From: Jeff Thon on 7 Jul 2005 14:56

Look at these.

http://ww1.microchip.com/downloads/en/AppNotes/00687b.pdf
http://ww1.microchip.com/downloads/en/AppNotes/00895a.pdf


"MarkMc" <mmcnospam(a)yahoo.co.uk> wrote in message
news:1120581097.596235.247300(a)f14g2000cwb.googlegroups.com...
> Can anybody give me any hints ideas or circuits to allow me to read the
> temperature from a Pt100 RTD probe in to a PIC microcontroller?
>
> Regards,
> Mark
>


From: MarkMc on 8 Jul 2005 14:42
Thanks, these look like really good reading.

Regards,
Mark

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