Pros & Cons of Adding a Cap Filter to High-Speed Link

Question:  I added a 2pF cap to the driver side of my high-speed link to reduce HF effects.  However, I am seeing effects that I do not understand in my analysis.  What effects will this cap cause?

Answer: A cap will filter out higher-frequency content.  However, it can also slow down the rising edge of the signal reducing the eye-height.  This filtering can also look like ISI/DDDj/PDj and reduce your eye width.  An alternative technique for removing high frequency content from your analysis is to use the bandwidth limit filters in the scope to determine what the affect of rise-time filtering/adding caps is.

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