Differential Impedance error

My ethernet 10/100 physical (KS8721) chip tracks to the RJ45 integrated magnetics connector should have had a differential impedance of 100 ohms according to the manufactures spec, ( 5 thou tracks and 5 thou gap). However the board was laid with 10 thou tracks 5 thou gap giving

70 ohms differential impedance (ground plane 5 thou below). The tracks are about 25mm long and the board seems to work ok but I am about to do a few changes and wondered if its worth changing, 5 thou tracks are very thin and I have noted that the manufacturers development board also uses 10 thou tracks.
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No problem - it's so short compared to the wavelength that it can be regarded as a short stub and the mismatch is not too serious.

The maximum base frequency is 62.5 MHz, giving nearly 5 meters wavelength in free space and about 3 meters on your board trace.


Tauno Voipio
tauno voipio (at) iki fi
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Tauno Voipio

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