• State Resolved
  • Locked Locked
  • Replies 6 replies
  • Answers 1 answer
  • Subscribers 33 subscribers
  • Views 553 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.

TMUXHS4212: Not sure how the absolute max and Vcm + Vdiff add up.

Dylan Shekter
Prodigy 60 points
Part Number: TMUXHS4212
Other Parts Discussed in Thread: DS90LV027A, LMK1D1204P

Tool/software:

If Vcm = 0v-1.8v, and Vdiff = 1.8v so each pin can go 0.9v above/below Vcm) as specified in the description paragraph in the first page of the datasheet, this necessitates either a negative voltage of 0-0.9 = -0.9v or 1.8+0.9 = 2.7v. However, the absolute max at the I/O pins is specified to -0.5-2.4v. So, how does this work? Even if we put Vcm to 0.4-1.5v I think the math works out, but the part says it can do 0-1.8.

Thank you.

  • 0
    TI__Mastermind 24430 points

    Hi Dylan,

    In the datasheet, the convention for differential voltage is actually double what you might think. For TMUXHS4212 the max Vdiff is 1.8V. That means the P and N switches would be at +/-0.45.

    So the real max differential you would see is 0.45 - -0.45 = 0.9V. but the convention is to double this to get 1.8V
    So when finding out the max/min recommended voltage on the switch, the math is: Vcm + Vdiff/4

    So for TMUXHS4212 it would be 1.8V + 1.8/4 = 2.25V and  0V - 1.8V/4 = -0.45V at 3.3V supply. 

    For 1.8V supply( 1.2+ 1.2/4=1.5  and 0V-1.2/4=-0.3V)

    Please let me know if this helps and I can try to explain this differently if you need it.

    Regards,

    Kameron

  • 0 in reply to Kameron Hill
    Prodigy 60 points

    Hi Kameron,

    I think I disagree as this being the convention. For example, in the ds99r421 datasheet it says in figure 4: Vdiff = (DOUT+) - (DOUT-). Not divide by 2 again. Likewise the lmk1d1204p datasheet, and the DS90LV027A, and probably a lot more that give the equation in the datasheet. Can you please provide a source for this math you're doing?

    Thank you,

    Dylan

  • 0 in reply to Dylan Shekter
    TI__Mastermind 24430 points

    Hi Dylan,

    I spoke with the designer for this device and here is a explanation for Vdiff/4

    When a certain Vdiff is mentioned, is means Vp – Vn , so imagine Vp as +350mV to -350mV, so corresponding Vn will look like -350mV to + 350mV, now if you subtract the two signals you will get +/-700mV, so that’s a differential signal of 1.4, that’s why the max/min voltage can be explained by VCM +Vdiff/4.

     

    Please let me know if this explanation helps and if I can help any further.

    Regards,

    Kameron

  • 0 in reply to Kameron Hill
    Prodigy 60 points

    Hi Kameron,

    That makes sense but conflicts with figure 7-3. That figure says 2.6V max, but the maximum voltage your math shows is 2.25V. Is figure 7-3 mislabeled?

    Thank you,

    Dylan

  • 0 in reply to Dylan Shekter
    TI__Mastermind 24430 points

    Hi Dylan,

    Thank you for bringing this up.

    Let me speak with the design engineer to confirm what this Figure 7-3 means.

    Please expect me to respond back tomorrow.

    Regards,

    Kameron

  • +1 in reply to Kameron Hill
    TI__Mastermind 24430 points

    Hi Dylan,

    After talking to the design engineer for this part section 7.3( Vin=2.6) is looks to be a typo and our team will not this down for a future revision.

    For the time being we can confirm that the Vcm+Vdiff/4 equation is correct and that the abs max section of the data sheet saying 2.4V is the max Vin needs to be respected.

    The recommended operation for voltages on the io pins follows the Vcm+Vdiff/4 equation.

    Please let me know if you have any additional questions here.

    Regards,

    Kameron