I noticed that if I change the value of register 0x18 from 0x04 to 0x00, the input span of the AD9635 become 1Vpp.
Do I have to expect a change in SNR in this configuration?
Thank you for your interest in AD9635.
AD9635 Register 0x18 Bits[2:0] does a digital scaling so that a smaller input swing results in a full-scale output.
The AD9635 datasheet SNR numbers are based on a full-scale input swing of 2Vpp_differential. The definition of SNR is directly tied to the amplitude of the signal; lower signal means lower SNR (assuming noise is unchanged). For this reason, if you set Register 0x18 = 0x00 SNR will be reduced compared to datasheet numbers.