I'm using a STM32F303 (landing page: http://www.st.com/en/microcontrollers/stm32f303k8.html) for one of my project. I have PA0 configured as a GPIO output with push pull used to turn ON/OFF a BJT. I also have PA3 configured to use the ADC1 channel 4 module.
When I set PA0 high, the ADC readings increase by about 1.8V. When it is low, the readings are correct. Why does this happen? Externally, these two pins aren't connected directly to each other.