PCIe REFCLK capacitor value

[wmsoto]

Hi,

The value for C188 and C189 in the AC701 schematic is 0.01uF for the REFCLK coupling capacitor but ug482 on page 226 show 0.1uf. I will be using this for PCIe Gen 1 or Gen2  refclk signaling.

Thanks,

[venkata]

You should be using the capacitor value (Cext) as mentioned in Table 51 of DS181. It is 100nF. Please use this value as reference for your design.

[allanherriman]

The PCIe reference clock frequency is what, 100MHz?  Both 10nF and 100nF capacitors have a capacitive reactance of well under 1 ohm at that frequency.

Also, both capacitors are likely to have an SRF of below 100MHz, meaning the impedance will determined by the ESL, not by the capacitance.  Assuming the same case size, both capacitors will have about the same impedance at frequencies that matter.

The performance will differ for frequencies in the range of tens of kHz through to some hundreds of kHz, as this is in the region of the cutoff caused by the capacitance and the load resistance.  (But there shouldn't be any frequency components in that range for a PCIe reference clock.)

Summary: One would expect that either capacitor would work equally well for this application.