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DS90UB941AS-Q1: how to increase the eye diagram height?

Sherry Liang
Genius 10255 points

Part Number: DS90UB941AS-Q1

Dear team,

Below is two port's eye diagram. The eye height is about 200mV which is a little small. Do you know what factors will result in small height? Can I increase the height by changing registers?

Thanks & Best Regards,

Sherry

  • 0
    TI__Mastermind 19720 points

    Hi Sherry,

    can you describe your set up? is this on an EVM? What is the cable length? Are you differentially probing? What is the impedance of the probe?

    Regards,

    Michael W.

  • 0 in reply to Michael.Walker
    TI__Genius 10255 points

    Hi Michael,

    Thanks for your reply!

    My customer always test the eye diagram, so the set up should be ok. I will double check the set up/probing/impedance and then send to you. This is on customer's board, not our EVM. 

    In addition to the set up/probing/impedance, do we have any other method to change the eye height? Can we change the eye diagram by changing some registers?

    Thanks& Best Regards,

    Sherry

  • 0 in reply to Sherry Liang
    TI__Mastermind 19720 points

    Hi Sherry,

    There is no register to change the eye height. There is no way to change the eye height. 

    Regards,

    Michael W.

  • 0 in reply to Michael.Walker
    TI__Genius 10255 points

    Hi Michael,

    Sorry, I make a mistake. This eye diagram is measured on the 948's CMLOUT pins instead of 941's. When we increase the CMLOUT pins' terminal resistor(100ohm), the height increase a little.

    They use the differential probe, and the probe information is as below,

    In addition, there is a 100ohm resistor on the differential probe, please refer to below red mark. If they remove these two resistors, the eye height increases from ~230mV to ~500mV. Could you please tell me whether we need to remove these two resistors when test eye diagram?

    Thanks & Best Regards,

    Sherry

  • 0 in reply to Sherry Liang
    TI__Mastermind 19720 points

    Hi Sherry,

    Are you following the CML circuit recommendations on Figure 48 and 49 on the 948's datasheet? there should not be to 100 Ohms in series when measuring CML.

    Regards,

    Michael W.