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Need TI solution to drive SFP transceiver from LVDS data

Mitchell Kary
Intellectual 630 points
Other Parts Discussed in Thread: SN65CML100, SN65LVCP1412, DS15BA101

Hello,

I require a level translation for a 100 ohm 1GBPS LVDS serialized output (VOD=350mV) to a SFP module which requires a minimum VOD of 500mV. The SFP input is internally AC coupled and has a 100 ohm internal termination.

Is there a TI device which can take a 350mV differential signal and drive a minimum of 500mV into a 100 ohm terminated load? ie The SN65CML100 only generates 400mV into 100 ohms.

Thanks

Mitch

  • 0
    TI__Mastermind 20820 points

    Hi Mitch,

    The output differential swing of the SN65CML100 can be adjusted to meet the needs of your design. I designed a circuit (below) that will give you >500mV of differential voltage into a 100 ohm load:

    The output drive in the SN65CML100 is really strong and can support a wide range of output design needs. The Vod in this particular circuit is 675mV but can be more finely tuned through the termination network. Please let me know if you have any questions?

    Regards,

    Michael Peffers

    High Speed Interface Applications

  • 0 in reply to Michael Peffers
    Intellectual 630 points

    Michael,

    In my application I will be connecting the output of your schematic into a device which has an AC coupled 100 differential termination. Doesn't this cut the swing to 400mV?

    -Mitch

  • 0 in reply to Mitchell Kary
    TI__Mastermind 20820 points

    Hi Mitch,

    You are right. I was thinking that the AC caps would isolate the CML100 from the effects of double termination but a colleague of mine confirmed for me that this is not the case as the AC portion of the signal is just as susceptible to this as the DC.


    We may still have a work around here, I need to do a little more research though. What is your data rate for the application? Is 500mV the absolute minimum that you need in order for the laser driver(?) to work?

    Regards,


    Michael Peffers

    High Speed Interface Applications

  • 0 in reply to Michael Peffers
    Intellectual 630 points

    Michael,

    As I mentioned in my initial post, the application is for a 100 ohm differential 1 Gbps LVDS (TI serialized) signal to be level translated for use with an AC coupled 100 ohm terminated input to an optical SFP transceiver.

    The MSA specification for SFP modules calls for a minimum of 500mV VOD signal.

    Thanks,

    Mitch

  • 0 in reply to Mitchell Kary
    TI__Mastermind 20820 points

    Hi Mitch,

    Have you seen the SN65LVCP1412 device? This device is an equalizer but we can use it to level shift your signal. It will accept the LVDS serialized signal of 350mV and output a 1200mV Vod AC coupled signal.

    The other thing that I am thinking is, how much control do you have over the serializer? Can we change it to a part like the TLK2541 which has a VML output stage that supports higher Vod levels than a CML driver.

    Regards,

    Michael Peffers

    High Speed Interface Applications

  • 0 in reply to Michael Peffers
    TI__Expert 3050 points

    Hi Mitch,

    You could also consider the DS15BA101. This device has easily configurable output VOD, that ranges from 800mVpp to 1.6Vpp differential driving into a 100 ohm differential load with external 50 ohm pull up transmit terminations.

    For a differential output voltage setting of 800mVpp, you can just set Rvo = 953ohms. 

    Mike Wolfe

    DPS APPS / SVA

  • 0 in reply to Mike Wolfe
    Intellectual 630 points

    Mike,

    Perfect! Thanks for the suggestion.

    regards,

    Mitch