Lots of boards already have 4.7K pullup resistors installed. You are basically making a voltage divider with the pullup connecting to the ADC reference voltage, and the ADC sensor pin connects to the junction between the pullup and the thermistor or PT-100, while the other end of the sensor connects to ground.
On most boards, the circuit is like part of the the schematic on [
reprap.org] but without the R1.
PT-100s increase in resistance from 100 ohms by about 0.4 ohms/C, while a 100K thermistor decreases from 100K ohms at 25C with a more complicated behavior.
