Why DC spurs are present in FFT plot of ADC
Compare discussion of "Fundamental leakage(bins)" and "DC leakage(bins)" on p.7, and realise you can model DC as a fundamental at 0Hz. Therefore the ignored bins would typically be approx. half the "Fundamental" bins. 

It does depend on the windowing function (see p.21) selected; in FFT plots you should be able to see spectral lines broaden with the Blackman window, both from the fundamental and from any DC component.

But the acid test is: if you decrease the number of ignored bins, does the S/N measurement degrade? If so, you are measuring spectral leakage from the DC component, not actual noise.