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Design >> High-Speed I/O Design >> AC Coupling cap with different values
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Message started by casual on Aug 1^st, 2012, 11:26pm

Title: AC Coupling cap with different values
Post by casual on Aug 1^st, 2012, 11:26pm

Hi

I tested 5Gbps Tx driver with different ac coupling cap value, 4pF & 100nF with 100ohm differential termination. The waveforms are as below:

When the ac coupling cap is 100nF, the differential signals are shifted, is it normal or something wrong?? for PCIe2 , the coupling cap is around 65-200nF right?

Title: Re: AC Coupling cap with different values
Post by loose-electron on Aug 26^th, 2012, 1:11pm

They are phase shifted in both cases.

Basic network theory - Whats the impedance you are coupling into?

Title: Re: AC Coupling cap with different values
Post by casual on Aug 26^th, 2012, 8:06pm

i know the reason already.
it is due to the initial simulation condition, where the nodes are not settle down yet.