Re: PT100 and ATMEGA
10mA is very high it will cause the PT100 to heat up.
It depends on. It's no problem with a Pt100 mounted with good thermal conductivity to a heater block, but a problem e.g. with
an air thermometer. I would also use 1 - 2 mA.
I also think, that a ratiometric circuit, as used in the above Maxim application, is better suited for an ADC with a supply voltage
reference. The accuracy only depends on resistance ratios in this case. Using an OP scaling amplifier has the additional avantage
of utilizing the full range of a low resolution 10 bit ADC.
Instead of an analog compensation of Pt characteristic, a polynomial approximation can be used. It doesn't need a
lot of memory, but some arithmetic effort. It's pretty easy with float and a bit tricky with fixed point, but feasible though.
The equivalent to the shown circuit would be a 2nd order (quadratic) polynomial. For wide range Pt100 and thermocouple
characteristics, a higher order is recommended. Polynomial fitting is a feature e.g. of spreadsheet calculators as MS Excel.