In analog features clause UG-1578 mentions "Usable RF range up to 8 GHz". I can't line up that characteristic with maximum ADC sample rate of 6GSps.
Could you please clarify what is hidden behind that?
The usable RF frequency range is independent of the ADC's sample rate and is related to the input bandwidth capability of the ADC's input structure (which includes on-chip buffer with 100 ohm differential termination). Depending on the quality of the matching network (which is often a balun) the input -3 dB bandwidth can be up to 8+GHz (see attached figure). Typically, it is desirable to operate within the passband region of the ADC to reduce the output drive requirements of the driving amplifier since the driver will need to deliver higher output power to compensate for any roll-off in ADC frequency response in the region of interest.
Note the term "usable" is subjective since both the linearity of the ADC (harmonic distortion dominated by HD2 and HD3 as well as intermodulation, IIP2, IIP3) will degrade at higher input frequency as well as noise floor (SNR-dBFS) under large signal conditions due to jitter. However the ADC's performance may still be deemed acceptable for a given application when frequency planning is used to avoid any of the large harmonics/spurious from falling back into the band of interest as well as when compared to the traditional alternative approach of using a RF mixer.