I want to use the AD9653 working at a 125MHz sampling rate, to undersample a IF signal (E5 Galileo band) of 50MHz bandwidth (from 70 to 120MHz). My question is:
Thank you for your interest in AD9653. The AD9653 has no problem converting signals at the frequencies you mention. Even though the datasheet does not mention much about undersampling, the datasheet shows performance up through fIN = 200MHz, and the AD9653 will convert signals at even higher frequencies.
The question is regarding handling the 50MHz bandwidth. One Nyquist zone for the AD9653 is 125MHz/2 = 62.5MHz. Your 50MHz bandwidth signal will theoretically fit within one Nyquist zone, but you will need to do some aggressive filtering to avoid aliasing. If you are aware of the aliasing concerns and are able to manage this at a system level, the AD9653 is an excellent choice.
If aliasing is a concern and you need greater Nyquist bandwidth to ease the filtering burden, you will need an ADC with higher sample rate. The AD9653 is a 16bit converter. Do you need 16bit performance?