We recommend the ease of using AC couple to the input of HMC799LP3E to maintain it's usable bandwidth and dynamic range. In order to keep the 650MHz bandwidth, the total Parasitic capacitance including the Cpd to be kept below < 1pF.) To DC couple, first of all there is not DC offset function and proper bias must be taken to keep the input bias voltage at 2.1V.
Is your application has strict DC information that needs to be maintained?