• State Resolved
  • Locked Locked
  • Replies 3 replies
  • Subscribers 34 subscribers
  • Views 361 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LVDS line width

juergen.duerrwang
Intellectual 420 points

Hi,

I have one question about LVDS line width. I calculate (using slna035) the impedance of one and of differentials lines. But when I take the values for my board (4layer 150um line width an 35um line hight) I got 62.4ohm for one line and 115ohm for differential line. In your guides I read that Z_0 should be 100 ohms, is that for one or for differential lines? If it is for one line, that means I have to use 0.9mil line width, is that correct? Because 0.9mil seems very tiny...

Regards[

Juergen

I attached my calculation (the first T/W/h are in [cm] the second in [mil])  View:http://e2e.ti.com/cfs-file.ashx/__key/communityserver-discussions-components-files/138/4135.Differential_5F00_Microstrip.pdf]

  • 0
    TI__Genius 13475 points

    Typically LVDS lines are run as 100 Ohm differential lines.  I personally often lay out boards with two, 50Ohm lines (coupled to GND), that are loosely coupled to one another.  This is most often done as edge coupled lines.

     

  • 0
    TI__Mastermind 43795 points

    Hi Juergen,

    As Mark indicated, the differential Zo of two lines for LVDS is usually set to 100 ohms.  When very little coupling is present (ie. the lines are far apart), the individual lines will have a Zo of 50 ohms and a Zo = 100 ohms when taken as a pair.

    In your case the individual lines have a Zo = 62.4 ohms and Zo = 115 ohms as a pair.  Since 115 ohms is less than 124.8 ohms (62.4 x 2), this is showing that some coupling between the lines is present.  In order to achieve a 100 ohm differential impedance you can do two things with your transmission line design -

    1. Move the traces closer together to increase the coupling - which will lower the differential impedance

    2. Increase  the individual trace width to decrease the impedance of each trace - which will lower the differential impedance

    Of course it is possible to use a combination on 1 and 2 above as well.

    Regards,

    Lee

  • 0 in reply to Lee Sledjeski
    Intellectual 420 points

    Thanks for your posts!

    Now Zo an Zdiff is clear for me. To get 100ohm for Z_0 i will use an combination of increasing line width and increasing coupling.

    Thanks for your help!

    Regards Juergen