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Using 50 ohm trace impedance for single ended signal and 100 for differential signal

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Jebakumar Samuel

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Hi,
Please explain why we are using a 50 ohm trace impedance for single ended signals in board designs and 100 ohm impedance for differential signals.
 

100 ohm differential trace

Hi,

Check this site out,

**broken link removed**

It explains in detail to some extent for your query.

Hope its helped you.



Regards,


Ramesh
 

50 ohm trace

The simplest answer is that historically, coaxial transmission lines have been 50 ohm, so you design your single ended boards at 50 ohms so that you can go board to board with a standard coaxial cable without big reflections.

However, twisted pair cable comes in more like 100 ohm. It is a function of how many twists per inch the cable has. So you should design your PCB to have 100 ohms differential, so that it will launch into twisted pair with a minimum of reflection.

And, BTW, I do mean 100 ohms differential. ie 100 ohms impedance between the two center conductors of a differeential pair. Not 100 ohms for each trace of the pair to ground!
 

50 ohm pcb trace

3 factors are heavily influenced for impedance calculation..

1.The height of the trace above the nearest ground plane should be minimum.
Less height means less radiation..

2.crosstalk dramatically various with nearest ref plane.Cutting height by half reduces the xtalk by 4 times..

3.Less height means less radiation..it 'll reduce the impedance...
 

differential trace impedance

TRACE IMPEADANCE DEPANDANCE ON TRACE WIDTH....

1)TRACE IMPEADANCE LESS FOR LARGE TARCE WIDTH........IT WILL NOT SUITABLE FOR HIGH DENSITY BOARDS.
2)TRACE IPEDANCE IS HIGH FOR THINNER TRACE WIDTH.MANFARACTURING IS NOT POSSIBLE FOR LESS THAN 3 WIDTH TRACE.......

I AM TELLING MECHANICAL POINT OF VIEW.......HE ABOVE PERSON WRITTEN ELECTRICAL POINT OF VIEW..........
 

pcb impedance

consider load i/p impedance 50ohm
and 100 ohm respectively
 

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