has anybody successfully read out a PT100 or PT1000 temperature sensor on a XBee ADC yet? There is a 12-Bit ADC on the Cortex M3 µC, the XBee has only a 10-Bit ADC at 4 PINs, so the resolution would be 1.25V/1023digit=1.22mV/digit (with 1.25 V_Ref).
To read out a PT1000 sensor it is recommended to connect the supply voltage of the voltage divider with reference resitor and PT1000 to the V_Ref Port at the µC, but this isn't possible at XBee's because V_Ref isn't led out to the PINs. Why isn't V_Ref of the µC led out to the XBee PINs?
So my idea was to read out the actual supply voltage of XBee SX 868 with DIO1_AD1 (three 10K resistors as voltage divider) and the voltage over the PT1000 with DIO3_AD3 (reference resistor with PT1000 as voltage divider). The accuracy is about +/-2 K at 0...100°C, this is a very bad accuracy for a 10-Bit ADC.