Power Spectrum in dBFs
Hello again guys after a lot of thought, I finally broke this down and I would like to share it with you hoping it'll help.

So what I did was: 

1- I used matlab's pwelch to get the power spectrum like this

[AutoPowerSpectrum ,F] = pwelch(signal, Nsamps, 0, Nsamps, fs, 'power');


you can read about the different parameters for pwelch in matlab's documentation right HERE.

2- I calculated Vpeak squared, because as far as I know pwelch produces Vrms values.

AutoPowerSpectrum = 2.*AutoPowerSpectrum;


3- got the Vpeak by squaring the above equation

AutoPowerSpectrum = sqrt(AutoPowerSpectrum);


4- converted it to dBFs

p_dBFs = 20*(log10(AutoPowerSpectrum./(vPerLevel*2^(N-1))));


where: vPerLevel is the voltage corresponding to each level in my ADC. It is calculated like this Vref/2^N.

and N is the resolution of the ADC.

Finally used plot() to display it. This solution worked with me and gave me the signal in dBFS, i'm opened to edits and suggestions.

Hope this helped. 

Cheers.