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Issue with impedance matching of differential pair

Sugreev

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

  • in reply to redwire

    Thanks Red.

    The diff pair is kind of confusing in that the actual CM has no clue as to what it is because the CM lives in the world of single ended impedance only. It threw me for a loop too Slight smile

  • in reply to Sugreev

    Sugreev.

    Since you are trying to design a diff pair on a STD 2Lyr board my suggestion to you is to not use the CM Impedance at all. "Disable it" it is not helping you !

    Because you are trying to achieve a diff-pair of 90Z your best option is to go with what I indicated above and use physical design based on the board material instead.

    The best option in your case is the coplainer waveguide.

    Trace width = 10 Mil
    Trace Space =  6Mil
    Trace to top ground wall space = 6mil.
    Remove the mask over the traces.

    Just a FYI. The docs will tell you that a diff line impedance of 90 Ohms is required and this is correct. But in the real world your going to be adding maybe ESD protection to that transmission line
    which will effect the impedance of the line. The app note goes into some good detail for that chip you wish to use. Perhaps dig in and read it very closely as it has some good info.

    All the best.

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