FYI, the most complicated stuff to solve is the pcb design when talking F103 ADC.
With 2 layers pcb you most probably will get +/- 20 noise, whatever ADC supply config (including well made decoupling) you may create.
A quieter readings are, imho, possible only with 4 layers and pretty careful design, especially the stuff around analog and digital Vdd and Gnd routing.