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STM32F373-EVAL board,SDADC used with PT100 problem

Question asked by tx.txmsszd on Jul 18, 2016
Latest reply on Jul 26, 2016 by tx.txmsszd

In STM32F373-EVAL board,SDADC is used to measure temperature with PT100 as sensor.The circuit is showed below.
These two pictures are screenshots of tutorials of STM32F373-EVAL board.The document says the gain of op amp A1 is 1.SINCE "the resistor bridge, R1 and R2, connected to VDD is equivalent to the VDD/2 generator where R1/2 = R internal resistor".Due to my poor analog circuit experience,I just can not understand this point.

Besides, In the derivation procedure  in snapshot 2, the refreence voltage(Vref) is VDD/2(Equation 5).How it get this point?

Best Regards.