Electronics > Beginners

Advice routing a differential pair

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I have this conundrum: USB to serial converter max 2 M baud, D+ and D- on the connector and on the chip have swapped position and so I have following options

1) turn around, as I did here, is it better to match lengths as I did here (even at the cost of having some tracts going in opposite direction), or I couldn't care less?

2) use a couple vias

3) put the IC on the bottom side (I'll do it only if I must).

If you can put your IC a bit higher to have straight lines going to it.

At these speeds and and distances the difference between the length of the traces is negligible.

The speed of the Serial port is not important, what is important is the speed of the USB connection, which
I would assume that it is full speed and not high speed.

At the end, if you are so concerned with trace length matching, then put the IC pependicular to the connector and
not parallel, like it is now.

I would be much more concerned to have ESD protection, if it is not provided by the converter IC,
termination resistors if not provided by the IC and a common mode choke for the lines if you have to pass
emissions test ( not always needed but it helps with conducted ).

You could do a simpler routing like in the sketch below in green. The yellow shows how you could route length matching if you really want to, but it won’t matter at this scale. TrickyNekro’s recommendation to add ESD protection is extremely good advice.

Rotate IC by 90o and don't bother with routing traces between the pads. Or keep rotation as is and run D- with minimum length around D+ pin and wiggle D+ trace for length match. Not that it really matters for LS/FS USB modes to begin with, you could not bother with length match and it still would work fine.

Don't bother with length matching and differential stuff for USB 2.0. On a short distance like this it does not make any difference. For LS/FS is so does not matter that you can route one of the traces around the perimeter of the board and you will not see a difference.


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