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THS7314 resistors

RetroRGB
Prodigy 40 points
Other Parts Discussed in Thread: THS7314

I was hoping someone here could help me. I currently run a website that's dedicated to retro video games. A few of the pages on my site contain how-to guides for adding a THS7314 to older game consoles to amplify their RGB-output. My guides basically describe:

RGB out from the game console's video encoder chip -> THS7314 -> resistors -> display

Your average person following these guides will run the video output through a video switch (along with their other game consoles) and into either a CRT RGB monitor (such as a Sony PVM), video processor (that converts the signal to VGA or HDMI) or the SCART RGB input of a PAL display.

When I first wrote these guides, it seemed 75k Ohm were the recommended resistors to use. After testing, the output of the modified consoles was brighter than unmodified consoles (using their stock RGB-out). I switched the guides to recommend 100k ohm resistors and everything has been perfect ever since: The brightness is exactly the same as unmodified consoles and the picture quality is perfect. Also, I haven't found (or been notified of) any issues with the THS7314 + 100k ohm resistor combination in any scenario.

I was recently contacted by someone who claimed using 100k ohm resistors instead of 75k ohm is bad. The person seemed extremely knowledgeable, however I'd rather come here to and ask TI and it's experts directly: Will using resistors different than 75k ohm have any potential negative side-effects for displays?

Sorry for the lengthy post, but I want to make sure I'm providing the proper information to my readers. Thanks in advance for any help you can provide.

  • 0
    TI__Genius 15975 points

    Hello RetroRGB,

    I wanted to make sure that you are saying 75k/100k ohms or 75/100 ohms? and on the output of the THS7314 as series resistor? Could you also draw up a schematic sketch-up of the output interface of THS7314 with the resistor. One of the applications of THS7314 is buffering the RGB video signals by having a 75ohm series resistor on the output and 75ohm resistor as the buffered load. The gain of 2V/V for THS7314 is essentially to compensate for the resistive divider loss at the output (75/(75+75)) when RGB outputs are buffered to other video signal chain paths. According to your application as you are amplifying RGB output by using the THS7314, I am thinking that you are having the 75k ohm load instead of 75ohms?

    Best Regards,

    Rohit

     

  • 0 in reply to Rohit Bhat
    Prodigy 40 points

    Thanks for your response.  I apologize, I was typing quickly and wrote the resistor incorrectly.  I am using 100 OHM resistors.  Here is a link to the exact ones I use:

    Here's a drawing to show what I'm doing.  Please excuse the simplicity, it's just a basic edit of the previous drawing in MS Paint:
    I'm sorry for the basic diagram and layman terms.  I am not en electrical engineer, but have experience with basic circuits.  As I said, so far, this has worked flawlessly with both 100 and 75 ohm resistors...but with the 75 ohm, the video brightness seems too high.
    Once again, thanks so much for your help.
  • 0 in reply to RetroRGB
    TI__Genius 15975 points

    Hello RetroRGB,

    Thanks. I understand your circuit better now. What might be happening in your circuit is that you are applying the RGB signals to an unterminated input of the THS7314, which is causing lower loss at the input and that in turn is gained up by the THS7314. Now, because you had a 75 ohm series resistor at the output earlier, the net result would be a gain in the RGB video signals and the picture would be brighter than usual. I believe that was what you were seeing earlier and that made you change the series resistor from 75ohms to 100ohms; because changing from 75ohms to 100ohms would result in loss of RGB output at the display assuming that the display load is RL = 75ohms and the picture would be normal again.

    Have you tried terminating the input of THS7314 with 75ohms and changing the 100ohm resistor back to 75ohms to see whether the picture is normal? I think that would help explain as to why you are noticing the picture to be bright with a 75ohm series resistor instead of the 100ohm series resistor.

    Best Regards,

    Rohit

  • 0 in reply to Rohit Bhat
    Prodigy 40 points

    Rohit Bhat said:
    Have you tried terminating the input of THS7314 with 75ohms and changing the 100ohm resistor back to 75ohms to see whether the picture is normal?

    I have not tried that, but I will as soon as I get a chance.  Could another solution be lowering the input voltage from 5v to 3v?

    Also, just to verify, it seems that using 100 ohm resistors is not the proper solution, but will it actually cause harm to use them?

    Thanks again for your reply and help!

  • 0 in reply to RetroRGB
    TI__Genius 15975 points

    Hi RetroRGB,

    Sorry for the delay in reply. Yes, another solution could be lowering the input voltage from 5V to 3V but that's not the best solution I suppose because lowering the input signal swing would be more susceptible to noise and interference which could cause other issues which you are not facing at this moment.

    I don't think using 100ohm resistors at the output without input termination would cause harm in what you are doing currently, I suppose because you are getting the results right for what you are trying to accomplish. What might become a problem is that for higher video standards, due to lack of bandwidth, someone else reading your document might face tough challenges hard to debug because in a system fallout, everything seems suspicious.

    Best Regards,

    Rohit

  • 0 in reply to Rohit Bhat
    Prodigy 40 points

    Hi Rohit,

    Thanks for the info.  I'll test the amp with the suggestions you provided and report my findings.

    Thanks again for all your help!