• State Resolved
  • Locked Locked
  • Replies 6 replies
  • Answers 2 answers
  • Subscribers 34 subscribers
  • Views 274 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.

question for document snla158

Shibuya
Intellectual 2985 points
Other Parts Discussed in Thread: LMH0384

Hello,

I have a question regarding document # snla158.

In the figure 3, equivalent output resistance is 75//75 = 37 ohm.  I think it is no much 75 ohm termination. Is my understanding currect?

Best regards,

Tetsuro 

  • 0
    TI__Guru*** 146315 points
    can find this document , which device?
  • 0 in reply to Brian Zhou
    TI__Guru* 93670 points
    Greetings Tetsuro:
    Here is a link to this document: www.ti.com/.../snla158.pdf

    I believe I understand your question. TXOUT is current mode open collector output driver. We have 75-ohm pull up to 3.3V. This sets the output voltage swing. Also, in order to deliver maximum power to the load it needs to see 75-ohm termination. To meet this termination, BNC connector/cable provides this 75-ohm termination. Using the same reasoning, TXOUTbar needs to have 75-ohm pull up and there should be 75-ohm Resistor R5 to ground(termination Resistor). Please note TXOUT/TXOUTbar are differential balanced driver and it needs to see a balanced load/impedance.

    On the Equalizer side - LMH0384 - input impedance is high impedance - about 1.2K. Since cable/BNC is 75-ohm we need to use a 75-ohm termination(R2) in order to receive maximum input voltage swing. This means the overall impedance on SDI side is going to be two 75-ohm in parallel with one another(i.e 37 ohm). Again, we have a differential receiver on LMH0384 device and thus we need to terminate SDIbar with 37-ohm resistor as well.

    Regards,,nasser
  • 0 in reply to Nasser Mohammadi
    TI__Intellectual 2985 points

    Hello Nasser,

    Thank you for your help to understand the circuit. But I am still confusing about R4 for TXOUT and R1 for SDI. BNC and cable has 75 ohm impedance, why those resisters require in series?

    Best regards,

    Tetsuro

  • 0 in reply to Shibuya
    TI__Guru* 93670 points
    Hi Tetsuro,
    These are needed to make sure there is balanced impedance or the same impedance between the negative and positive terminals.
    Regards,,nasser
  • 0 in reply to Nasser Mohammadi
    TI__Intellectual 2985 points

    Hello,

    Sorry, my question was not appropriate.

    Could you tell me what is role of R4 // L2 in serise with TX line? ( or R1 // L1 for SDI )  

    Why it is required for TXOUT but not required for TXOUT-?

    Best regards,

    Tetsuro

  • 0 in reply to Shibuya
    TI__Guru* 93670 points

    Greetings Tetsuro,

    R4//L2 is needed in order to meet SMPTE required return loss requirements.

    Actually, it is preferred or it is a better design practice to have these on TXOUT- as well. But to save components these are not placed on TXOUT-.

    Regards,,nasser