STM32F030 analogue input pin impedance (does value change during conversion period?)
You've misread the datasheet. RAIN is the maximum external impedance that can be connected to the ADC pin in order to keep error below 1/4 LSb on a calibrated ADC.

The impedance of the S+H circuit is RADC, listed below that. The impedance of the circuitry beyond the S+H circuit is not given, but based on a RAIN of 50kohm with an error of 1/4LSb with a 12-bit ADC, I do not expect it to be any less than approximately 820Mohm.

See figure 23 for details.