Rtd pullup Connection?
Ratiometric voltage measurements are possible - the ADC reference voltage must be used for the pull-up (Ra) for the PT100 like this: -



Ra has to be chosen so that sufficient signal is developed across the PT100 to generate sufficient accuracy (given the ADCs bit resolution) without too much self-heating of the device.

It won't be a direct conversion to temperature because there is a potential divider effect i.e. if you used a current source, PT100 resistance converts directly to voltage. Not so with a ratiometric measurement of a PT100.