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Issue with impedance matching of differential pair

Hi,

I am designing a PCB and having issue with the impedance matching of the differential pair. I am using the guidelines of the IC manufacture to design my PCB. Here is the link of:

https://www.ftdichip.com/Support/Documents/AppNotes/AN_146_USB_Hardware_Design_Guidelines_for_FTDI_ICs.pdf

According to this guidelines, the differential pair (DP and DM signals of the USB) must have 90 ohm impedance to each other. But I am not able to match this impedance. I tried using different line width (10mils,15mils,20mils,25mils,30mils) of the differential pair but it doesn’t help me. Also, I tried using 0 ohm resistor in between that didn’t help as well.

The specifications of my PCB are- the conductor thickness is 2oz(2.8mils) and the dielectric thickness is 63 mils (using FR-4 and dielectric constant is 4.5).

Can anyone please help me in resolving this issue ??

Thanks

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redwire over 4 years ago

The guidance is a bit misleading. You don't *require* 45 ohm single ended. You do need 90 ohm differential. The 90 ohm diff pair can be achieved with a tightly coupled pair on the top. You could do CPWG as suggested by excelon or you can widen slightly and do just an edge-coupled microstrip pair.

If you had a line which was 45 ohm single ended, and 90 ohm differential then that would suggest an uncoupled pair and those dimensions are not practical on a 2 layer board again as excelon has mentioned.

What tools were you using to check your impedance? The calculator in Allegro/Orcad will get you the right values assuming your stackup has been entered correctly.
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Sugreev over 4 years ago in reply to redwire

Okay!

Yes,I am using the calculator in Allegro/Orcad. I ma using same values but still not able to resolve the issue.

Thanks
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Wild over 4 years ago in reply to Sugreev

The Allegro PCB calculator, I have found does not do coplanar wave guide impedance. I believe you would need the sigrity tool, or go to the Ansys modelers.

Try this: https://chemandy.com/calculators/coplanar-waveguide-with-ground-calculator.htm

We have used this and it has been very accurate, verified measurements on a TDR.
Also the Saturn PCB toolkit comes up with similar results: saturnpcb.com/.../
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Wild over 4 years ago in reply to Sugreev

The Allegro PCB calculator, I have found does not do coplanar wave guide impedance. I believe you would need the sigrity tool, or go to the Ansys modelers.

Try this: https://chemandy.com/calculators/coplanar-waveguide-with-ground-calculator.htm

We have used this and it has been very accurate, verified measurements on a TDR.
Also the Saturn PCB toolkit comes up with similar results: saturnpcb.com/.../
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