ADC values fluctuating

[PAcha.1]

The project I am working on presently need to capture dc voltage with precision of 1mv. I am using STM32f446RE development board. As mentioned in the datasheet STM32F446RE have inbuilt ADC of 12-bit resolution and reference voltage is 3.3v. Thus theoretical resolution is 3.3/2^12 = 0.0008056640625 is expected, but I am getting the voltage fluctuations of about 25mV. Please let me know how what the best resolution is possible using inbuilt ADC and also the means 

[AScha.3]

first - read ds of your cpu , adc real performance ->

so from this can expect about 0,8mV *7 -> +/- 6mV best resolution.

+ input has to be low impedance, best 100nF cap at input adc pin to gnd. (mlcc cer cap)

+ long sampling time , min. 7.5 

+ averaging , maybe 2000 x samples , will show, how far you can come down :)

+ otherwise need preamp