We suggest using the PGA with a gain of 2.
We base our idea on the following:
PT1000 resistance range is ~670 ohms to 2840 ohms.
We suggest 6800ohms for the Reference resistor - usually connected in series with the PT1000 resistor.
We suggest using an Excitation current value of 150uA.
This will give a reference voltage to the ADC of 1.02V.
The input voltage range will be 100.5mV to 426mV.
There is enough margin here for PT1000 variations and excitation current accuracies and drift.