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Contributor
Contributor
498 Views
Registered: ‎09-25-2008

Ultrascale GTH coupling capacitors recommendation

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Hi,

I have a question concerning the coupling capacitors recommendations in UG576.

For the reference clock input the recommendet capacitor value is 10nF (see also AR# 61723).

For the Transmit and Receive channels the recommendet value is 100nF and was changed twice in UG576:

Revision 1.1: from 100nF to 10nF

Revision 1.2: from 10nF back to 100nF

Does anyone know the reason for these changes? What is the reason to use two different capacitor values for the clock and for the data?

Many thanks,

best regards

Stefan

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Xilinx Employee
Xilinx Employee
472 Views
Registered: ‎11-29-2007

Re: Ultrascale GTH coupling capacitors recommendation

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hello, I have to change my previous suggestion about the REFCLK: I was thinking to signal integrity only, but we have to think also to the time needed to the common mode to set-up.

Because we want a quick set-up of the common mode, the 10nF decoupling capacitor is preferred for the REFCLK. 

I hope this clarifies.

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Xilinx Employee
Xilinx Employee
484 Views
Registered: ‎11-29-2007

Re: Ultrascale GTH coupling capacitors recommendation

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hello,

usually the serial line decoupling capacitor size depends on datarate and the pattern you are willing to transmit (how many consecutive equal symbols can be in the serial data). 10 or 100nF will both cover the vast majority of supported protocols. If you will target highest datarate and the number of consecutive symbols is low you might select 10nF directly. With slow-medium datarates 100nF will be OK. To be more precise we should make a simulation.

You might perform an IBIS-AMI simulation of your particular case but it is possible you will find negligible differences. This is the reason why sometimes you find both 100nF and 10nF values used.

For REFCLK, 100nF is used typically. 10nF is preferred due to quicker common mode setup.

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Contributor
Contributor
477 Views
Registered: ‎09-25-2008

Re: Ultrascale GTH coupling capacitors recommendation

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Hi,

Thanks for your fast reply. So maybe we will do an IBIS simulation but I guess for most application both values (10nF and 100nF) would be ok, right?

For reference clock 100nF is typically? But why is the official xilinx recommendation 10nF (AR# 61723) ;-)

Regards,

Stefan

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Xilinx Employee
Xilinx Employee
473 Views
Registered: ‎11-29-2007

Re: Ultrascale GTH coupling capacitors recommendation

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hello, I have to change my previous suggestion about the REFCLK: I was thinking to signal integrity only, but we have to think also to the time needed to the common mode to set-up.

Because we want a quick set-up of the common mode, the 10nF decoupling capacitor is preferred for the REFCLK. 

I hope this clarifies.