Hi Zhouzhx,
The AD7626 Tcycle range from 100ns to 10,000ns. This mean that it can have a sample of 100khz to 10Mhz.You may do undersampling. As mentioned, the input structure of the AD7626 does not allow for full scale inputs with higher frequencies than 20Mhz to be sampled correctly. You may sample signal with frequency higher than 20Mhz but at lower amplitude.
Regards,
Jonathan