DS16EV5110A: Vocm Explanation

Russell Cheatham

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?

over 1 year ago

0 Drew Miller1 over 1 year ago

TI__Mastermind 42028 points

Hi,

We are looking into this.

Thanks,

Drew

0 Drew Miller1 over 1 year ago in reply to Drew Miller1

TI__Mastermind 42028 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 Russell Cheatham over 1 year ago 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 Drew Miller1 over 1 year ago in reply to Russell Cheatham

TI__Mastermind 42028 points

Hi Russell,

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.
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

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DS16EV5110A: Vocm Explanation