MAX6691 is designed for Thermistors. Maxim has not tested MAX6691 for Pt100 sensors.
MAX6691
Production
The MAX6691 four-channel thermistor temperature-to-pulse-width converter measures the temperatures of up
to four thermistors and converts them to a series...
Datasheet
MAX6691 on Analog.com
MAX6691 is designed for Thermistors. Maxim has not tested MAX6691 for Pt100 sensors.