Does GTX pins require AC coupling for both transmitters and receivers?

[greatmaverick]

Does GTX pins require AC coupling for both transmitters and receivers? I have seen ac coupling is required for both of transmitters and receivers in the user guide, but AC coupling does not exist in the PCIe receiver line and FMC high speed channel (DP line) connection in KC705. please tell me the reason, thank you.

[kenkovaa]

There is always the possibility for DC-coupling if the protocol is designed for that and common mode voltages match, but AC-coupling is much more common and easier to work with.

As you are talking about PCIe it is always AC-coupled.  From the PCIe 2.0 standard 4.3.5.1: "Each Lane of a PCI Express Link must be AC coupled. The minimum and maximum values for the capacitance is given in Table 4-9. Capacitors must be placed on the Transmitter side of an interface that permits adapters to be plugged and unplugged. In a topology where everything is located on a
single substrate, the capacitors may be located anywhere along the channel."

--Kim

[greatmaverick]

Do I need add AC coupling to the wire between FPGA GTX and FMC connector? Why?
THANK YOU

[kenkovaa · Accepted solution]

I remember that ANSI/VITA 57.1 standard (FMC board standard) was considering a change that all AC-coupling should be placed on mezzanine card and not on the carrier board. In that way the mezzanine cards can support either AC or DC-coupling. I do not have anymore access to the standard though. The important thing is to not have two capacitors in series, that can cause problems. 

--Kim