EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: ankeakala on September 12, 2020, 12:14:09 pm
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If I have understood it correctly, I seem to have differential impedance of 159 ohms and single-ended impedance of roughly 80 ohms. Which is way out of spec for USB. How would I correct this? Would I simply use proper value resistor to ground to bring the impedance to correct level? If I try to calculate proper trace width and separation to do that like that, I get trace width of 70 mils..
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Bring the ground plane closer (use thinner prepreg material) so that you don't need 70mils trace. So a 4-layer PCB is practically necessary.
Or, if you don't have budget for 4-layer PCB, keep the traces very short and just accept the impedance mismatch. A few cm is OK for up to USB2.1.
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Bring the ground plane closer (use thinner prepreg material) so that you don't need 70mils trace. So a 4-layer PCB is practically necessary.
Or, if you don't have budget for 4-layer PCB, keep the traces very short and just accept the impedance mismatch. A few cm is OK for up to USB2.1.
Yes, the design goals are such that it may not be an option. And even if it was, the calculators online don't seem to support the idea that changing those factors would make big enough difference. So, not sure how one would correct for such mismatch.
But, in another thread, mikeselectricstuff worded quite strongly to the direction that no, it won't make much of a difference. And worst case, maybe I need to run it at low speed which would still be perfectly fine.
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Whats the length of the trace, and what USB speed do you want to reach?
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Whats the length of the trace, and what USB speed do you want to reach?
Roughly 60mm with 1mm difference between the pair. Low speed 1.5Mbit would be plenty. ATMEGA32U4 would allow that so it would work fine, if it will.
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Whats the length of the trace, and what USB speed do you want to reach?
Roughly 60mm with 1mm difference between the pair. Low speed 1.5Mbit would be plenty. ATMEGA32U4 would allow that so it would work fine, if it will.
Dont worry about the impedance, connect it with two tracks next to each other. 0.3mm wide or so.
If the reference guide or the datasheet has series resistor recommended, than place 0603 or 0402 size on your board.