Page five of the following whitepaper gives a circuit suitable for 3-wire RTDs:
http://ww1.microchip.com/downloads/en/appnotes/00687c.pdf
It looks straight forward enough (though I don’t claim to understand it). Is that the sort of thing you had in mind?
They have suggested a 12bit ADC and note that this gives a resolution of 0.22 degrees C per LSB over the -200 to 600 degree range.
I note the ADC in the ATMega is only 10bit, so the resolution would be less (around 1 degree?).
The whitepaper mentions that with proper calibration, such sensors/circuits can achieve 0.01 degree accuracy, is the plan to include a high resolution ADC to take advantage of some of this accuracy? (assuming the calibration problem can be solved).
The following post links to a few ADCs, though 100k SPS is certainly overkill for anything I could imagine using a pid for:
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1287074482
The MPC342x (first link on that post) certainly looks interesting, 18bit resolution, i2c connection for around $4. Is this considered an acceptable price for components on an input shield? The 3.75 SPS would be plenty for any application I’d consider.
