Differencial pair impedance calculator

Software
1

Hi,

I’m making a new pcb with differential pair for USB.
The USB need have 90 ohm of impedance.

Where can try calculate the best configuration in spacing and track width?

Thanks

2

Using the embedded calculator tool from kicad?

http://kicad.txplore.com/?p=146


You just need to keep the correct spacing ?

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3

That tool is not for differential impedance.

I use:
Impedance Calculators

You want to select “Microstrip Zdiff”.

You will need to know the thickness of the dielectric you will be using as well as the relative permittivity (dielectric constant).

Also, be aware, 90 ohms is the differential impedance, the characteristic or single ended impedance will be more than half (ie. 50 - 55 ohms). (Zd < 2 x Zo)

Another thing to keep in mind when you are laying out the board is that symmetry and total lengths are also important.

It’s a good idea to communicate with your board manufacturer when it comes to impedance control. Add a note that tells the manufacturer that the 12 mil tracks are intended to be 90 ohm differential pairs and they often will double check and adjust if necessary.

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4

Just to be clear, when I said above that total lengths are important, I mean that the total length of each line in a differential pair should match as close as possible. However, the length does not affect the impedance.

5

I just thought I would point something out as I am sure the subject will come up.

KiCAD’s PCB Calculator, specifically the “Transmission Line” calculator does have a “Transmission Line Type” called “Coupled Microstrip Line” that can be used for differential pairs, it’s just not as straight forward. Differential pairs are “coupled” lines and they operate in the “odd” excitation mode. But differential pairs use a separate return path which must also be taken into account. Therefore the differential impedance of a differential pair is equal to 2 X Zodd while the characteristic, or single ended, impedance is equal to Zeven / 2.

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