I'd like to ask some suggestion about how to connect the clock with this kind of ADC (LTC2172-12 65MSPS). Is not so clear for me how to choose the clock frequency (and so the kind of VCO) and how to connect it to ENC+ and ENC- (if some kind of conditioning circuit are required to mitigate jitter and have best performances).
Thank you so much
The main thing to worry about with the clock frequency selection is how your analog input frequency aliases (nyquist theory). Walt Kester (one of my favorite authors on ADCs) discusses toward the end of this article (see figure 18). Let me know if you need to understand this better, and we can discuss.
If your main concern is clock jitter, that is independent of clock frequency.
The below link has a section on how to calculate the maximum clock jitter your design can tolerate
Staring on page 51 of this datasheet, there is a more detailed description of why clock jitter matters
A device like the LTC6951 has several recommended connections for different types of inputs. Page 54 and 55 of datasheet. This device will meet your jitter requirements. But you maybe able to select a lower power clock after you have an idea of your jitter requirements.
If you provide your ADCs analog input frequency range and amplitude, I can walk you through this also.