According to T1040 Family Design Checklist, DIFF_SYSCLK input is "LVDS type clock buffer with AC/DC characteristics identical to the SerDes reference clock inputs which are HCSL-compatible".
Section 188.8.131.52 of T1040 DS "Differential System clock DC timing specifications" says "For DC timing specification, see (184.108.40.206) DC-level requirement for SerDes reference clocks".
Section 220.127.116.11 constrains input CM voltage between 100 mV and 400 mV for an external DC-coupled connection based on the assumption of maximum average current allowed for each input pin (8 mA).
Is this constraint applicable to the DIFF_SYSCLK input (it seems very doubtful)?
Is there an internal biasing circuit at the DIFF_SYSCLK input or it should be added externally in the case of AC coupling?