A temperature sensor with about 8 bits resolution digital outputs is needed.
A circuit as simple as possible is appreciated. For example,chopper amp
may be avoided in a PTAT bias generator if necessary.
Offset are going to be an issue. A good layout with large W and L' could not get offset lower than 4 to 5 mV.
If your post processor can do some calibaration ,it could help.
As for the architecture a PTAT reference followed by a "Reliable ADC, slow speed" would be a good bet