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DS90UB933-Q1: Question on why 50 ohm termination is needed for RIN- when SERDES is used in single ended Coaxial mode.

Shaun Chua
Prodigy 60 points
Part Number: DS90UB933-Q1

Hi there, 

I am trying to understand how the high speed data is flowing through the FPD III link in the SERDES pair. Using DS90UB933/934 pair as example. 

I am assuming the differential driver is similar to the typical LVDS driver (in a high level sense ignoring details). See picture below (obtained from TI LVDS owner manual),

Q: can this be confirmed? 

In differential mode, it is straightforward in that high speed current will flow from DOUT+ of serializer through the STP cable to RIN-, go through the internal 100 ohm termination (following picture below regarding the differential mode, obtained from TI FPD3 training) in the deserializer, then exit out of RIN- of deserializer  and flow back to DOUT- of serializer and back to GND, thus completing the current loop. 

Q: However, what is the path that the high speed current takes in the single ended coxial mode case (FIgure 30 in pg 37 of DS90UB933 datasheet)? 

  • Is it that high speed current flows out from DOUT+ (serializer, goes through the coaxial cable, reaches RIN+ of the deserializer), gets terminated by the 50 ohm termination, goes through to the VBIAS (which would be GND in AC) and then flow back out via GND of the deserializer, back to coaxial cable's GND, and back to the serializer? 
  • Why is the external 50 ohm needed for the RIN- port in this single ended case? What is its significance?  

Thanks. 

Shaun. 

  • 0
    TI__Mastermind 30515 points
    Shaun,
    1. similar as LVDS, some use CML.
    2. dependent on the total power consumption.

    best regards,
    Steven
  • 0 in reply to Steven(Shenzhen) Shi
    Prodigy 60 points
    HI Steven,

    Can you elaborate more on pt2. An example would be good.

    Thanks.
  • 0 in reply to Shaun Chua
    TI__Mastermind 30515 points

    You can visit  www.ti.com/fpdlink, and get PoC training. it has details.

    regards,

    Steven

  • 0 in reply to Steven(Shenzhen) Shi
    Prodigy 60 points
    Hi Steven,

    I have already went through the training for both POC and FPD3 Link.

    It does not answer specifically the question I have in Question 2.

    in Q2, please see my original post. In that I phrase my question to ask specific items.

    Thanks.
  • 0 in reply to Shaun Chua
    TI__Mastermind 30515 points
    Chua,
    Q: However, what is the path that the high speed current takes in the single ended coxial mode case (FIgure 30 in pg 37 of DS90UB933 datasheet)?
    A: The output is CML architecture. please refer to TI's recommended circuit. below replies on your further concerns.

    •Is it that high speed current flows out from DOUT+ (serializer, goes through the coaxial cable, reaches RIN+ of the deserializer), gets terminated by the 50 ohm termination, goes through to the VBIAS (which would be GND in AC) and then flow back out via GND of the deserializer, back to coaxial cable's GND, and back to the serializer?
    A: yes, the return path through GND.

    •Why is the external 50 ohm needed for the RIN- port in this single ended case? What is its significance?
    B: it is for the impedance matching for high speed output and input I/f.

    best regards,
    Steven
  • 0 in reply to Steven(Shenzhen) Shi
    Prodigy 60 points
    Hi Steven,

    Thanks for clearer explanation.

    So, according to your explanation. In the single ended coaxial mode, the high speed current flows from serializer into deserializer via a 50ohm coax cable, and gets terminated with the 50 ohm internal termination in the deserializer, and then returns back to the serializer via the GND. (DOUT+ to RIN+) This I can understand, since 50 ohm internal termination is used to match 50ohm Zo of coax cable.

    Q: Okay...then if that is the case, looks like DOUT- and RIN- is not involved in the outputting of high speed current loop. Then why is a 50 ohm EXTERNAL termination still required for RIN- and DOUT- port?

    Q: Is there an appnote/document you can refer me to that describes the details & reasons of how to use a CML output as a single ended IO?

    Thanks.
  • 0 in reply to Shaun Chua
    TI__Mastermind 30515 points
    Yes, this is why the coax cable should be grounded well. you can search "high speed I/f CML" in my.ti.com to get more high speed I/f knowledge, thanks.

    best regards,
    Steven
  • 0 in reply to Steven(Shenzhen) Shi
    Prodigy 60 points

    Hi Steven, 

    With all due respect, please answer my question directly.

    Q: Okay...then if that is the case, looks like DOUT- and RIN- is not involved in the outputting of high speed current loop. Then why is a 50 ohm EXTERNAL termination still required for RIN- and DOUT- port? 

     

    Thanks. 

  • 0 in reply to Shaun Chua
    TI__Mastermind 30515 points
    Chua,
    For single-end application, the unused 50ohms termination is NOT for high speed current loop, please don't be stuck into this.
    1. the high speed diff. I/O should have same LOAD on both diff. pins, and the unused single-end pin also should be terminated to balance output/input (then minimize the total noise).
    2. For diff. input pins (Din+/Din-), the unused input pin also should have proper bias voltage, which is included inside FPD-Link chip.

    so for FPD-Link coax cable application, the unused diff. pin should be terminated correctly based on d/s recommendation.


    best regards,
    Steven
  • 0 in reply to Steven(Shenzhen) Shi
    Prodigy 60 points

    Hi Steve,

    Many thanks. Appreciate the explanation.