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DS16EV5110A: Vocm Explanation

Russell Cheatham
Prodigy 40 points
Part Number: DS16EV5110A


Tool/software:

The data sheet gives the CML output Vocm as Vdd-0.3 to Vdd-0.2 and Vo differential swing as 800mV - 1200mV p-p.  At 3.3V Vdd, that means measuring single ended Vo could be 3.1V ocm + 0.6V = 3.7V max which is above Vdd.  Wouldn't it clamp at Vdd and cause an assymetric signal?  What am I misunderstanding?

  • 0
    TI__Mastermind 37083 points

    Hi,

    We are looking into this.

    Thanks,

    Drew

  • 0 in reply to Drew Miller1
    TI__Mastermind 37083 points

    Hi Russell,

    800mV - 1200mVppd is a differential measurement.  This means 400mV-600mVpp single ended, so 3.1Vocm + 0.2V = 3.3.  Note this is CML logic (see Figure 11 in data sheet).  Typically, there are 50 Ohm pullups to Vdd in this case.  For CML, Vocm will be determined by swing, so Vocm will work out for 600mVpp case too.

    Thanks,

    Drew

  • 0 in reply to Drew Miller1
    Prodigy 40 points

    Thanks Drew, but I'm not following you in a couple of ways.  I agree with the 400mV-600mVpp single ended, but Vocm is also measured single ended.  The spec defines Vocm as Vdd-0.3 to Vdd-0.2 which is independent of the signal level, so wouldn't worst case single ended be Vocm + max signal (3.1 + 0.6)?  I can't say I've ever understood or been told that Vocm is determined by swing. 

  • +1 in reply to Russell Cheatham
    TI__Mastermind 37083 points

    Hi Russell,

    1. I'm a bit confused about why you're adding the full swing of the signal (600mVpp) to 3.1V.  Can you help clarify?
      VCM = (V1 + V2)/2.  Let's say D_OUT+ = 3.3V and D_OUT- = 2.7V (600mVpp swing), then VCM = 3.0V.  D_OUT+ will swing from 2.7V to 3.3V.
    2. Please take a look at slide 20 of these slides for a good image of CML driver topology.  Note that VOD will increase as the driver current increases.  Note that the common mode voltage will also change as the VOD changes since the common mode voltage will remain centered between min/max voltage of D_OUT.
      https://people.engr.tamu.edu/spalermo/ecen689/lecture5_ee720_termination_txdriver.pdf

    Please let me know if you have any follow up questions.

    Thanks,

    Drew