Hi,
46dB does sound low for RAMP SNR, I would have expected your result to be in the mid fifties. What are you using as a video source and video measurement equipment? Could you try the following:
1. video source directly in to the video measurement system, what is the SNR?
2. Try using Y (YPbPr) instead of CVBS through the system. What is the SNR?
3. Try using our evaluation platform for the ADV7391, the front end board uses and ADV7403 which should have similar performance to the ADV7180.
Regards,
Paul