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BUFFER FOR VIDEO LINE

eli
Guru 13485 points

looking for at least 18bit buffer, for video lines.

I only found this: http://www.ti.com/product/SN74ALVCH16825

 

  • 0
    TI__Mastermind 24485 points
    Hello Eli,

    Can you provide more information on what exactly you need?
    What is the loading on the line? Is there a large capacitive load on the output?
    What voltage would you like it to operate at?
    Do you require a specific data rate?

    Here is a list of buffer devices that have 18 or more channels:
    www.ti.com/.../products.html

    Best,
    Michael
  • 0 in reply to Michael J Schultis
    Guru 13485 points

    Hello Michael

    Thanks.

    Data rate should be up to 50MHz

    Voltage: 3.3V

    lines are 50ohm control impedance – digital video lines.

     They go to OLED display. 

  • 0 in reply to eli
    TI__Guru* 96591 points
    Hi eli,
    It's a bit tricky to use a logic device across a controlled impedance line (output of logic is low impedance, transmission line is 50 ohms, input to next stage is typically high impedance). This setup usually results in large over/undershoots we refer to as 'ringing'

    I would recommend that you look at using the SN74LVCR32245A or similar for your application - this includes an output damping resistor to limit the ringing at the distant end and should meet all of your requirements.
  • 0 in reply to Emrys Maier
    Guru 13485 points

    Thanks!

     If the output is low impedance, I can control the layout according to stackup to make the lines 50ohm.

     So in that case why I should care about low impedance output ?

     

     

  • 0 in reply to eli
    Guru 13485 points

    need 20bit devices like

  • 0 in reply to eli
    TI__Guru* 96591 points

    The primary issue is at the distant end, where the 50Ω line terminates into a high-impedance input. Unfortunately, this can't be fully remedied by doing impedance matching because the logic device at the driving end isn't designed to source enough current for a 50Ω load. The best solution (that I know of) is to add some resistance in series with the device's output to dampen the oscillations and use a device on the distant end with input clamp diodes.

    We have an application report that goes into some detail on this subject: Design Considerations for Logic Products

    You can technically use any buffer with the appropriate supply voltage and current ratings.

  • 0 in reply to Emrys Maier
    Guru 13485 points
    Hi
    I'm sorry but I didn’t quit get it.

    I have video lines, pixel clock 27Mhz, current is about 8mA, and 50ohm is impedance control.

    The video is coming out from a DSP and going into CPLD.

    On its way, I want to amplify it by a buffer.

    Output from the buffer should be like the DSP output no ? and can't I match it to 50ohm ?
  • 0 in reply to eli
    TI__Guru* 96591 points

    It really depends on the length of your impedance matched lines -- as long as you are transmitting over less than 1/4 wavelength of wire, the transmission line effects don't really show up and everything is fine.  Assuming you have fast edges on your 27MHz clock (1ns edges) the analog bandwidth of the signal is around 350MHz -- which means the wavelength is ~85.7cm, and 1/4 wavelength is ~21cm.  So, as long as your traces are less than 21cm (ignoring reflections caused by boundary conditions), then the impedance of the line doesn't really matter for the logic signal.

    The typical application of logic is shown on the left of the drawing here -- a short line from the DSP to the Buffer, with the low-impedance output of the logic device tied to the high-impedance input of the buffer.  This typically results in good signal integrity.

    The right part of the image shows the area where signal integrity issues tend to happen. A buffer drives a transmission line which then terminates into the high-impedance input of the CPLD.

  • 0 in reply to Emrys Maier
    Guru 13485 points

    Thank you very much for that.This is really helpful.

     Also, forgot to mention that you used Er=1 which is the best case scenario,

     For FR4 Er can reach to 4 which will result 1/4 wavelength of 10.70 cm.

     So the basic idea is: as long as my transmission line is less than the 1/4 wavelength of wire I can use the buffer without the termination resistors ?

     Meaning those drivers will be good for my purpose : (as long as I'm ok with the TPD of course)

  • 0 in reply to eli
    Guru 13485 points

    Sorry.

     The photo was for transceiver instead of a buffer. I meant those drivers:

  • 0 in reply to eli
    TI__Guru* 96591 points
    Yes, sir, any of those would work - and you are correct, I was just throwing an estimate out there. Using the FR4 numbers is a much better idea -- it sounds like you don't need my help after all :)