I haven't found any information to connect ADRF6806 to the ADCs, is it necessary a driver? Have I to insert a LPF? What you suggest about ADC?
I assume you mean you need DC-56MHz. If this is the case you will need an ADC that samples at least twice 56MHz, or more than 112Msps. Depending on how much filtering you need you will want to sample faster than that. I would recommend the AD9258, a 14 bit 125Msps part, the LTC2208-14 a 14 bit 130Msps, there is also a 160Msps version the LTC2209 if you want more guard band. If you have multiple channels, let me know and I'll recommend other parts.
For the amplifier to drive these, the LTC6409 is good and runs off of a single supply, the ADA4930 is also a good choice. If you need lower power but less performance the ADA4927-1 is also a good choice.
Look through those ADCs and amps and we can start helping with the signal chain.