pt100 con pic
That is very true what you said, and I read ANs from ANALOG, but you from your 1st post you indicated that the A/D conversion will be done by a PIC and this microcontroller has only 10-bit resolution.
Voltage drifts of voltage regulators such as 317L (7805, ...) lay much below 10-bit resolution level, so you can safely use them as voltage/current refereces..
Different story with 16-bit, 18-bit or 24-bit A/D converters..
There one have to be very careful what one uses as voltage or current reference source..