QWhen should I use ac-coupling vs. dc-coupling on the high-speed serial lanes?
AAc-coupling isolated the dc current between a transmitting Tx device and a
receiving Rx device. This is useful for dc level translation, hot-swap
protection and to prevent dc current flowing between different supply domains
such as in card-to-card or chassis-to-chassis connections.
Ac-coupling has the disadvantage of a setting a low frequency cutoff thereby
requiring that the serial data be scrambled or encoded such that it has a equal
number of zeroes and ones on average and a constrained number of consecutive
identical digits (CIDs).