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AR# 19270

Virtex-II Pro RocketIO - How do I choose the AC coupling capacitors for speeds other than 3.125 Gbps?


The RocketIO User Guide gives recommended AC coupling capacitors only for 3.125 Gbps 8B10B encoded data. What values should be used for other types of data at other rates?


The following application note from Maxim describes a methodology for choosing AC coupling capacitors.


C = 7.8 * Ncid * Tb / R


C is the capacitance in nanofarads

Ncid is the number of consecutive identical bits (the maximum run length of the data)

Tb = the bit time in nanoseconds

R = the resistance of the line

Another consideration is the Pattern Dependent Jitter (PDJ). The value calculated from the above equation is usually too small and results in higher PDJ. The equations provided for PDJ in the Application Note in conjunction with the above equation will yield a good AC coupling capacitor value.

AR# 19270
Date 12/15/2012
Status Active
Type General Article