4 wire RTD (PT100) in a wheatStone bridge
It isn't a good idea to use resistive temperature sensors this way because of the large change  in resistance will delinearize the bridge whether current fed or voltage fed. Devices like strain gauges bearly change their resistance at all and can be ok as a single active element in a bridge but not RTDs. 

So, use the three wire connection and use a constant current feed. This is the most accurate method for RTDs. If you do a double measurement of the two wires feeding the sensor you can determine cable volt drop. If volt drop is minimal or constant use a two wire connection.

Because your RTD  is four wire use an instrumentation amplifier or use it in 2-wire mode or 3-wire mode.

Your current source should be referenced to your ADC ref voltage if using an ADC with an inaccurate reference.