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DS125DF1610EVM: VOD normalization: Driver VOD setting VS calculated VOD. Who is right?

Romain Ducaffy
Prodigy 160 points
Part Number: DS125DF1610EVM
Other Parts Discussed in Thread: DS125DF1610

Hello,

I am exploring the FIR and VOD settings of the DS125DF1610 and I am stalling a bit on the normalization step.

In SigCon Architect, when I select a "Driver VOD setting" (High level page, Tx FIR/VOD section), I can see pre-set values being updated for Pre/Main/Post cursor taps, DEM and DRV_SEL_VOD as well as a "Calculated VOD". However, the value "Driver VOD setting" and the "Calculated VOD" are sometimes very different: differences range from 0.09% for 350 mVpp (calculated VOD =  350.312 mVpp) to 19% for 650 mVpp (calculated VOD =  527.67 mVpp)! I measured the VOD at the output of the DS125DF1610 for all possible settings and the measurement follows the "Driver VOD setting" quite closely: differences below 5% for all but one setting.

How come the calculated VOD is off if the setting is correct? If the calculation is off, does that mean the normalization process (when I click on "Normalize Taps") may be off too?

Say I want to set up a VOD that is not in the provided list, like 625 mVpp. How do I normalize the taps? Is there a formula linking the Pre/Main/Post cursor taps, DEM and DRV_SEL_VOD that I could follow so that if I want 625 mVpp with Pre tap = -7 and Post tap = -7 I can extract the Main tap value out of it?

Thank you,

--

RD

  • 0
    TI__Mastermind 19135 points

    The VOD value resulting from the different rows in the datasheet table were verified via lab bench measurement. So, they are the most accurate.

    The VOD calculation used by the GUI is an approximation. The reason it may be off from the datasheet VOD table is due to the multiple parameters that affect the VOD. See below explanation for how to calculate the VOD resulting from settings of DEM, DRV_sel_VOD and FIR taps.

    For simple VOD control, the absolute values of each tap should sum to 63

    |Pre|+ |Main|+|Post|= 63

    VOD = drv_sel_vod value - DEM

    If transmit equalization is required when using advanced VOD control, then the absolute values for each tap should sum to the default sum for that particular VOD.

    |Pre|+ |Main|+ |Post|=n (default sum)

    VOD = (drv_sel_vod – DEM) x (n/63)

    Example calculation for VOD = 1000mV setting from datasheet based on input parameters

    • VOD = 1000mVpp -> ||+ ||+ ||==58
      • drv_sel_vod = 31 (1400mVpp)
      • DEM = 2 dB -> This means that the VOD setting is being further reduced by 2dB. In linear scale you may multiply VOD by 0.794
      • VOD = (drv_sel_vod – DEM) x (n/63) = (0.794*1400)*(58/63) = 1023.38mV
    Hope this helps.
    Cordially,
    Rodrigo Natal
    HSSC Applications Engineer

     

  • 0 in reply to Rodrigo Natal
    Prodigy 160 points
    Hello Rodrigo,

    Thank you for the information, it helps indeed.

    Regards,
    --
    RD