I'm studying the Schematic of the AvNet IO Carrier for the Ultrascale+ based SoM. In particular, I have a doubt about the function of the R130 resistor on page 14.

Can someone help me to understand its function?

partial schematic


It’s series damping to reduce ringing on SCL. It serves to match the source impedance to the trace, and provides a back-termination to absorb signal reflections.

  • \$\begingroup\$ It also has other names, such as source termination resistor. Basically it slows down fast signal edges, as it forms an RC filter with stray capacitance. So as it forms a low pass filter, it limits the bandwidth of signal. \$\endgroup\$
    – Justme
    Apr 27 '20 at 16:52
  • \$\begingroup\$ Thank you for the prompt reply, in your opinion @Justme, how the designer calculated it? \$\endgroup\$ Apr 28 '20 at 6:04
  • 1
    \$\begingroup\$ Most likely a designer did not calculate anything, just threw in a random small value resistor that is high enough to remove the problem, and low enough to not introduce other problems. It is also possible that they experimented by trying few different values on a prototype to find out how large the value must be to remove the problem. \$\endgroup\$
    – Justme
    Apr 28 '20 at 6:18

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