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Original question:

analog VGA 1(in) : 2ch(out) conversion questions

THS7372: THS7372 Schematic review

Ymkim
Prodigy 130 points
Part Number: THS7372


Hello.

It is my first time working on a video signal and related project, so I am lacking a lot.

I'm going to make a VGA Splitter using THS7372.

I wrote the circuit by referring to the EVM circuit diagram, but I leave an inquiry to check the board before production.

The R,G,B signal processing was configured by selecting the device, but I am not sure how to process the SYNC signal and DDC signal.

Could you tell me what I should consider and proceed?

Thank you.

7268.SCHEMATIC1 _ 03. VGA_Splitter.pdf

  • 0
    TI__Mastermind 23919 points

    Hello,

    I will help you out; it is always an adventure to start a new project.  You have made some good progress; I will take some time today to look over your design and schematic.  I do have some questions for you:

    1) Is the EVM you found the THS7372PWEVM?  That EVM is not available from the TI website and has been obsoleted.  I will look for the old board files in case more information will help.  We definitely support the THS7372, just no longer sell or support the EVM on ti.com.  This is confusing due to the EVM being present on the datasheet.

    2) Are your inputs DC or AC-coupled?  If AC, are you using the SYNC tip clamp circuit (present in datasheet and EVM)?  The EVM defaults are DC or AC (sync tip clamp) inputs with AC-coupled outputs.  From your use of capacitors on the inputs (and no pull-up resistors for Pb, Pr), I assume you are using AC-couple inputs and the sync tip clamp circuitry.

    3) Are you using the Y'PbPr or G'B'R' video signaling?  Use of the first option may require pull-up resistors on Pb & Pr.

    I believe the vertical and horizontal sync signals are contained within the luma, Y', or G' signal for FHD.  Some implementations of G'B'R' embed sync information into all three channels. 

    For your design case of splitting the VGA source to two displays, if you use DC-coupled output you can refer to Figure 88 of the datasheet.  This example shows the THS7372 driving two lines per output.

    If your output is required to be AC-coupled your choice of two separate THS7372's would be correct.  Figure 89 is a decent example of a schematic, and I will investigate your component values and check for any issues with your schematic.  I will also look into the SYNC and DDC signals for your design.

    If you could answer my above questions and share any other details that could help, I would appreciate it.

    When you have some time, please read (again) through the Application Information from pages 28-38.  There are some functions and behaviors in these pages which might help you determine which design approach is best for your project.

    I look forward to seeing an update from you.

    Best,

    Alec Saebeler

  • 0 in reply to Alec Saebeler
    Prodigy 130 points

    1) I refereed figure 89 and the diagram designed by the other user when I modified the circuit.

    2) is AC-coupled generally chosen for Video sync processing application?

    3) We are using G'B'R'(video). Please refer the edited schematic and System block diagram and let me know if it looks okay.

    2781.SCHEMATIC1 _ 03. VGA_Splitter.pdf

  • 0 in reply to Ymkim
    Guru 140200 points

    Hi Ymkim,

    I would remove C108, C111, C114, C117, C120 and C123. They are only for testing purpose. No normal circuit would need them, just the opposite, they would only destabilize the THS72372 :-)

    Kai

  • 0 in reply to kai klaas69
    TI__Mastermind 23919 points

    Hello Ymkim,

    Kai makes a good point about those capacitors.  You may have them as DNP, but I agree they are not recommended for your design. 

    I would recommend using AC-coupling on your inputs (creates best compatibility), but I believe you can do so without the input pullup resistors.  You can instead make use of the internal sync-tip-clamp circuit and not need to add those pullup resistors.  The reasoning here is that for DACs/Encoders that embed sync within the data signals, the sync-tip-clamp circuit works without any pull-up resistors. 

    If you know you will need a separate sync signal, you would in fact have to use pullup resistors, as referenced on page 33 under the INPUT MODE OF OPERATION: AC Bias section.  From your schematic, I can see separate sync signals so you may decide to keep the pullup resistors.  Determining how the motherboard or GPU is sending the sync signal will help determine the proper input method.  

    If your input is referenced to positive supply, you will need to add AC coupling caps.  This is discussed within the sync tip clamp input section.

    Your design looks good.  However, you should investigate the nature of the encoder and/or DAC which is the source for these VGA signals.  Depending on its type, you may need to make small tweaks.  From a high speed amplifiers perspective, your design is in accordance with the datasheet.

    In order to comment more on your design, I would need to know information about the encoder/DAC you are using as your source.  If you know the sync signals will be contained within the data signals, then you can proceed with sync-tip-clamp method for AC coupling.  If your sync signals are separate, then your data signals will likely need some sort of DC-bias, which is how the pullup resistors to positive supply are used.

    Best,

    Alec  

  • 0 in reply to Alec Saebeler
    Prodigy 130 points

    Hello

    Thank you for your answer.

    The Sync signal exists separately, and in this case, how do I branch it and how do I enter it as a connector?

    Should I use THS7372?

    Or should I do it in a different way?

  • 0 in reply to Ymkim
    TI__Mastermind 23919 points

    Hello again,

    Thank you for clarifying your sync signal.  If the signal is separate, your sync signal is likely a combination of vertical and horizontal sync.  You must properly split/direct your sync signal(s) such that the VGA connector will provide the appropriate signals at the display.  This might include some logic/filtering circuitry to separate out horizontal and vertical sync.  To begin, this article from Planet Analog gives a good understanding of considerations for VGA analog video: 

    I found the information to be incredibly helpful and provide additional context to what is provided in the datasheet.  There are various kinds of 'sync' signals; finding the proper implementation depends on your VGA source and destination (PC to monitor, for example).  PC VGA signals may or may not be plug&play compatible with monitor VGA ports (especially as you are introducing your own circuitry here).  Your monitor may expect separate horizontal & vertical sync (HSYNC and VSYNC), a combined sync signal (CSYNC), or the sync signal to be contained in the green (G') signal.  You will need to make sure you can convey the correct signal(s) through the cabling to the display.  VGA cables/connectors have separate pins for horizontal and vertical sync; it is likely you will need to either split your combined signal into VSYNC and HSYNC or send your combined signal through HSYNC.  These two articles from ePanorama give advice and context which appears helpful.  Additionally, another good resource is this Tektronix document on measuring video signals.  The document includes digital signals in addition to analog video, but the context here may help with handling your design.

    Guide to Standard HD Digital Video Measurements.pdf

    Also, I found the following article an interesting read.  The VGA video simulator here may be helpful for your project, though I am not certain it helps with splitting & buffering your VGA as much as it may provide an additional consideration or curiosity.

    Finally, I will leave you with this: 

    As an electronics hobbyist outside of work, I found this individual to be insightful and their projects are interesting.

    I know you probably were hoping for a clearer answer here.  I do not have extensive experience working with VGA analog video systems.  It is however something I am interested in.  As you continue to work on your project, I will explore VGA sync signals so I can improve my knowledgebase.  

    From an amplifier perspective, your choice of THS7372 is good.  The information I have provided until know represents my current knowledge of this subject; the links and references in this reply are selected to guide you into the rest of your design.  

    Best,

    Alec