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Looking for a LVDS buffer with Vocm= 0mv to 300mv (user Defined) and Vout=200mv pk-pk

Surender Sharma
Prodigy 60 points

Looking for a LVDS buffer with Output common mode voltage of 0mv to 300mv (user Defined). Most LVDS buffers have 1.2V VCM Out. Is there a way to create such a buffer using amplifier circuit and be able to simulate the behavior.  Vout pk-pk is required to be 200 mV. Any feedback will be helpful.

Thanks

  • 0
    TI__Guru 62920 points

    Hi Surender,

    The 1.2-V common mode voltage is part of the LVDS specification, so you aren't likely to find any LVDS devices with such a low common mode.  The simplest solution in my opinion is to just use series capacitors to AC-couple the buffer's differential output and then set the common mode to the desired level by using bias resistors at the receiver.  I have attached a TI application note that goes into more detail on how to AC-couple different types of differential buffers to one another.  In particular, you may want to take a look at Figures 9 though 11, which show three different methods of setting a receive-side common mode.

    Please let me know if you have any questions.

    Best regards,
    Max Robertson
    Analog Applications Engineer
    Texas Instruments
    m-robertson@ti.com

    AC-Coupling Between Differential LVPECL, LVDS, HSTL, and CML.pdf
  • 0 in reply to Miguel Robertson
    Prodigy 60 points

    Dear Max,

    First Thanks for your kind response. Problem is that I must DC Couple in order to carry Link Start DC conditions used by the Receiver. This is a Special receiver which looks for such conditions at 200mV. I was planning on using National/TI's ADC Differential Driver Buffers which allow VOCM to be adjustable to a defined Reference. I am not sure whether they will do the job or not? Any ideas using analog devices will be helpful.

    Thanks

    Surender

  • 0 in reply to Surender Sharma
    TI__Guru 62920 points

    Surender,

    I understand - if your system needs to be able to support DC levels then AC-coupling is not a viable option.  Your idea of using an amplifier circuit sounds good to me.  TI has several fully differential amplifiers that allow for the output common mode to be set using a CM bias voltage input.  I recommend contacting our high speed amplifiers team to see if they can provide some suggestions.  Here is their support forum:

    http://e2e.ti.com/support/amplifiers/high_speed_amplifiers/f/10.aspx

    One thing to consider when picking out a device is the bandwidth that is needed.  If you are trying to preserve the rise/fall times of your input signal, a rule of thumb for the minimum bandwidth is 0.35 divided by the 10%-to-90% transition time.  You should also make sure that the amplifier's slew rate is fast enough to handle the data transitions.

    Best regards,
    Max Robertson

  • 0 in reply to Miguel Robertson
    Prodigy 60 points

    Thanks Max, I have posted it there now. Thanks for your feedback.

    Surender