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TMDS442 PCB layout reference.

ngohara1991
Intellectual 555 points
Other Parts Discussed in Thread: TMDS442

Hello, I am using the TMDS442   as an 2 input, dual output  switch and splitter (ie  both output will display whichever input is selected).

I am having some difficulties understanding how to impedance match the TMDS lines of  my  4 HDMI signal, and was hoping someone could share a example or reference PCB layout of the TMDS442 in use?


To clarify, I have one HDMI source on board from another video chip, one from a HDMI connector.

My dual outputs are both to connectors (which connect to displays). 

I am using KiCad PCB software, and  have been able to route my TMDS signals to be length matched.

However, the PCB Calc tool indicates that my impedance match is widely dismatched with the desired 100ohm/50ohm  pair/single TMDS lines.

I am attempting to use Advanced Circuits 7mil trace width and seperation specification, but it would seem this cannot match the desired impedance.

Can anyone confirm what settings they use for their TMDS lines?

Thank you

Neal

  • 0
    Intellectual 555 points
    Actually, second question.
    The TMDS Output section of the datasheet describes using 50 ohm resistor in front of the TMDS receiver. If I am routing my TMDS signals to a connector, which then routes them to some receiver internal to my display device, do I still need these termination resistors?
  • 0 in reply to ngohara1991
    TI__Mastermind 23815 points
    Hi,

    Single ended impedance should be 50 ohms
    Differential impedance should be 100 ohms

    You should select trace width and separation in order to achieve these impedance.
    Please take a look at this document
    e2e.ti.com/.../Texas-Instruments-HDMI-Design-Guide.pdf

    50 ohm termination resistors are placed on receiver side, most of the time it is inside the IC, as TMDS442 has internal termination resistors.

    Regards
  • 0 in reply to Moises Garcia
    Intellectual 555 points
    I've read the guide, and I understand that trace width, separation, height, and vias contribute to the total impedance. What I'm struggling with is properly calculating said total, using the numbers I can find on Advanced Circuit's specs.

    For example, I believe, with a 4 layer 0.093" board, the height would be approximately 14.3 mill (after subtracting 59 mil core and 1.35 mil copper layers).
    Then the trace calculator says that 12 mil wide, 7 mil separation, yields approximately 95.9 ohm Zdiff, and 54.6 ohm Z0.
    This seems like it would qualify for the 100+-5, 50+-5 ohm HDMI specs, true?

    But now I need to figure out how my vias affect this impedance.
    The guide gives formula that yield capacitance and inductance for the via.
    Can I say Z = sqrt( L / C), and add this number to my single impedance for total impedance?
    Or is there other math involved?
    And how does that impact differential impedance overall?

    Thank you.
    ngohara
  • 0 in reply to ngohara1991
    TI__Mastermind 23815 points
    Hi,

    For impedance calculation is taken into account:
    t trace width
    separation between traces
    separation to GND plane
    dielectric of isolator between traces and GND
    Even Cadence Allegro just takes into account this values
    You can use this calculator as a reference(at least for diagrams):
    www.skottanselektronik.com/index.html

    Vias add inductance/capacitance and is preferably to trace avoiding them, but most of the time they are not taken into account for impedance.

    Regards
  • 0 in reply to Moises Garcia
    Intellectual 555 points
    Thank you for all the assistance.