LatticeECP3: When is it appropriate to use dc-coupling with the LatticeECP3 SERDES reference clock?
The dedicated SERDES reference clock inputs for the LatticeECP3 include default ac-coupling capacitors and optional dc-coupled connections. This dc-coupled connection is available only in the case where an external ac-coupling capacitor is being placed on the board. It is not recommended to dc-couple the reference clock input directly to clock sources. This technique may cause undesirable common-mode mismatches and increase device jitter.