100-ohm diff on 2-layer PTH board

Feb 27, 2009 4 Replies

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Andrew Holme 17 years ago

I'm designing a board with a 100-ohm diff. pair carrying LVDS. For economy, I want to make a 2-layer PTH board with a solid copper ground plane on the bottom. The PCB fab said this was impossible because of board thickness. After playing with a trace impedance calculator, I see what they mean, at least as far as normal track widths are concerned. To get 100-ohms differential on a 62 mil thick 2-layer board, I need 60 mil wide tracks spaced 40 mil apart. So my question is: what's wrong with that?

TIA

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Helmut Sennewald 17 years ago

"Andrew Holme" schrieb im Newsbeitrag news:GBYpl.53874$ snipped-for-privacy@newsfe16.ams...

Hello,

In principle it will work. Your other traces should have a distance of 3 times the board thickness if they run in parallel over some length. That's a clearance of 0.2inch.

If you have a small board, you could use a thinner board, e.g. 20mil?

Best regards, Helmut

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John Larkin 17 years ago

TXline claims 55 ohms odd mode Z for your dims on FR4, which is 110 ohms diff. You could also go 30 mil traces with a 10 mil gap for 100 ohm differential, which is 50 ohms odd mode per trace.

Nothing wrong with either one.

John

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Joerg 17 years ago

Agree. Controlled impedance lines anywhere between 50 and 300 ohms have even been done on double-sided phenolic. Still got some of those in the garage.

Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.

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David L. Jones 17 years ago

Nothing is inherently wrong with that.

BTW, don't get fixated on differential impedance, you don't actually need it in your case:

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Dave.

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