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Original question:

TS5V330C: RGB/Composite Switch Guidance

TS5V330C: 75R clarification for 2 sources to single monitor application

Jens Schönfeld
Prodigy 30 points
Part Number: TS5V330C
Other Parts Discussed in Thread: TS5V330

Hi,

 

I'm in the process of designing a card that generates it's own VGA, but shall also be able to pass through a different VGA source. The "application clip" brochure for the older TS5V330 chip shows this image:

TS5V330_wiring.png

This is confusing to me, as both video sources are shown with a 75R load, and the display also loads the signal with 75R. I'd expect this kind of wiring if the muxer chip would also be a re-driver, but the datasheet is clearly speaking about a switch. So if I really add 75R resistors to GND for each of the three analogue signals, I'd expect the resulting picture to be too dark.

 

Please clarify: Do I really need to add 75R resistors to GND for the RGB input signals of my VGA input, or is that a blooper in the "Application clip" prochure?

 

thanks

Jens

 

  • 0
    TI__Intellectual 2450 points

    Hi Jens,

    It is dependent on if the video source being used has a internal termination or requires the 75ohm resistor. If the devices you have has internal termination then you would not need to add the 75ohms, but if it specifically says to add some sort of termination then it is recommended to add the 75ohms.

    Thank you,
    Arya

  • 0 in reply to Arya Hussein
    Prodigy 30 points

    Let's assume the source is a legacy VGA card from a 1990s PC - since all monitors load the RGB signals with 75R, that output stage is designed to drive exactly that. However, it doesn't *need* that, as the card won't be damaged if no monitor is connected. The output level is just geared towards having that load.

    If the switch is configured to route the signal(s) to the monitor, then the 75R resistors of that monitor should be exactly what that VGA card was designed for (plus the 3R of the switches, but I'm ignoring that here). So given that in case of "user wants to see the pass-through picture", the additional resistors at the input of the switch would sum up to about 38R, resulting in a too-dark picture on the monitor. I don't really see a scenario where resistors on the inputs are required - at least not in any KVM-switch-like application. Correct me if I'm wrong here!

    Jens

  • +1 in reply to Jens Schönfeld
    TI__Intellectual 2450 points

    Hi Jens,

    You are correct, 

    For any KVM or pass-through application, you should omit those input resistors.

    The monitor at the end of the chain provides the required 75ohm termination. If you add them to your board, you will "double terminate" the signal to $37.5ohms, cutting the video signal voltage in half and resulting in a very dark picture