Differential pair impedance calculations

Jun 19, 2019 1 Replies
F

I'm about to lay out a USB 90 ohm diff pair, and I go to the eeweb impedance online calculator



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I punch in my numbers from the fab house



Trace Thickness = 1.4 mil Substrate Height = 14.4 mil Trace Width = 6 mil Trace Spacing = 8 mil Dielectric = 4.5



and I get 89.9 Ohms



Then I try to verify against another impedance calculator, I pull up Saturn PCB Toolkit and plug in the same info



Conductor Width = 6 mil Conductor Spacing = 8 mil Conductor Height = 14.4 mil Base Copper Weight 1oz Dielectric 4.5



and I get 77 Ohms


Does anyone have a tool they like for these type of impedance calculations? I don't know why the solutions are are off, I suspect user error



These are for inner layers with a ground plane...



Top GND Sig1 Sig2


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J

That's tricky, asymmetric buried diff pair. Looks like the eeweb calc assumes symmetric. Saturn does asymmetric as an option.

For tough cases, we use an EM simulator, like ATLC or ATLC2.

It is fun to run various impedance calculators and compare the numbers. Some use the old Motorola formula, where wide traces have negative impedances.

For short runs and modest data rates, a little error doesn't matter.

John Larkin Highland Technology, Inc lunatic fringe electronics
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