I think a lot of the previous message may be irrelevant. I tested the hardware and it would only register up to 187oC.
I went back to my simulation and EXCEL spreadsheet and found that the response was linear from -78oC to +187oC with a centre at 52oC (my 120 Ohm R_Zero is not quite as accurate as it should be) and a range of 270oC
However: - Chapter 7 (page 22 of document tidu145.pdf says -
Since the current design only uses roughly ±640 mV of the ±1.65 V allowable input range, the system can be used for temperature range roughly 2.5 times larger than what shown without modification
On this basis I (rather stupidly) assumed that It would go up to 250, but, of course the 250 range is still centred at 50.
My ultimate goal is to cover a range from 50 to 300, so I assume that I only need to re-calculate R_Zero, which works out at 147.5. Assuming I could live with the reduced resolution I suppose I could reduce the PGA gain, but would this be advisable?