A PT100 is a commonly used RTD. Compared to NTC (the most common) thermistors work, the PT100 is “PTC” so the change in resistance with temperature is “other way around”. (PT100's also have a very different curve than NTCs handled by Thermistor class.)

“PTC” means “positive temperature coefficient” so electrical resistance increases with raising temperature. They cannot be used as-is, you must use an amplifier circuit to raise the voltage change so they become usable by SmoothieBoard.

Currently Smoothie only supports E3D's PT100 amplifier board. Other PT100 amplifiers may be supported later (i.e. MAX31865 via SPI).

The PT100 E3D amplifier needs to be powered from the AVCC and AGND header pins on the SmoothieBoard, only newer boards have this header.

If you do not have AVCC/AGND pins, you can use 3.3V and GND. However this may introduce noise in the ADC system and may affect all temperature readings from analog sources (i.e. thermistors and PT100).

DO NOT power from 5v or you will kill the port.

You wire a PT100 almost the same way you would a thermistor, but you need to specify to Smoothie it is a PT100 and where you connected the amplifier signal output: 

temperature_control.hotend.enable              true
temperature_control.hotend.sensor              pt100_e3d
temperature_control.hotend.e3d_amplifier_pin   1.30        # must be a free ADC pin, not a temperature input
When using a PT100 sensor type you do not need to set values for the thermistor kind or thermistor_pin. If you do, these values will be ignored.
You must use a spare ADC pin to connect the amplifier signal; you cannot use a thermistor input due to the pull-up that is already on the board. On most boards that means either 1.30 or 1.31, depending on what is more convenient.