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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

Sugreev over 4 years ago in reply to excellon1

I tried your suggestion. I am using 2layer stack up PCB. The bottom side has ground plane, so I added vias across by diff pair traces but still impedance is quite high.

I have attached picture of the design.

Can you please have a look where I am doing wrong ?

I will really appreciate your help.

Thanks
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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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redwire over 4 years ago in reply to Sugreev

Can you take a screenshot? Make sure you have the BOTTOM layer marked as "PLANE" otherwise the trace calculator won't work -- but this also means that the the bottom is really ground in your design on the bottom (under the USB traces). Also, soldermask over trace will pull the impedance slightly. You can adjust that later -- let's get your calculator working

Cadence does not support the CPWG construction but it does support edge-coupled traces properly. You really don't *need* CPWG for the USB application. Lots of manufacturers out there are building 2 layer boards with USB that work great.

Make sure your settings look like mine. You should get the same values.
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Sugreev over 4 years ago in reply to redwire

I did the same but still having same problem.

I have one question. when I have created diff pair set in Electrical Constraints, in that I have declared the impedance of diff pair 90 ohms with 15% tolerance.

Is that right thing to do ??
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