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AC Coupling cap with different values (Read 257 times)
supermoment
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AC Coupling cap with different values
Aug 01st, 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?
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ac_coupling_values.png
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loose-electron
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Re: AC Coupling cap with different values
Reply #1 - Aug 26th, 2012, 1:11pm
 
They are phase shifted in both cases.

Basic network theory - Whats the impedance you are coupling into?
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Jerry Twomey
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supermoment
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Re: AC Coupling cap with different values
Reply #2 - Aug 26th, 2012, 8:06pm
 
i know the reason already.
it is due to the initial simulation condition, where the nodes are not settle down yet.
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