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PCIe AC coupling capacitors values

Question asked by xabiven on Nov 2, 2017
Latest reply on Nov 6, 2017 by xabiven

Hello all,


I have a question on the value of the required AC coupling capacitor for PCIe links.

Indeed, the QorIQ data-sheets (Txxx and LSxxx) specify the AC coupling capacitors depending on the targeted protocol. So, it recommends:

 * PCIe GEN2 (2.5 GT/s): CTX =   75 to 200 nF

 * PCIe GEN2 (5.0 GT/s): CTX =   75 to 200 nF

 * PCIe GEN3 (8.0 GT/s): CTX = 176 to 265 nF


I observed that the reference boards always have 100 nF capacitor, like on traditional GEN2 designs.


=> Does GEN3 is implemented on demo-boards ?

=> Should I use capacitor in the range 176-200 nF if I design an interface which can run wether at GEN2 or GEN3 rates ?

=> Or, should I use a classic 220nF capacitor for a PCIe GEN3 capable interface, considering it will be able to negotiate the link down to GEN2 speed ? Will it provide a reliable GEN 2 link ?


Best regards,