Hi,
Yes, a one-time SYNCIN would be sufficient after the ADC has had time fo power up. Now if you are using the SYNCIN to synchronize multiple ADCs, then the SYNCIN pulse must be provided to all the ADCs at the same time and meet the setup and hold time around the sample clock edge at each ADC.
And if you are not synchromizing multiple ADCs, or don't care about the SYNCOUT at all, then you do not need to provide a SYNCIN at all. When we use the EVM with the TSW1400 Capture Card we do not provide a SYNCIN input.
Regards,
RIchard P.