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Sanity Check on R-2R VGA DAC — Parallax Forums

Sanity Check on R-2R VGA DAC

escherescher Posts: 138
edited 2017-10-27 22:16 in Propeller 1
I have to be losing my mind here or something, because I'm at a loss as to why I'm getting the following results from this LTSpice simulation of a KNOWN-GOOD R-2R VGA DAC that I'm going to hook up to my Propeller 1, referenced from @PhiPi:

vga_full_monty.gif

This is the circuit and the results I'm getting:

5R5f4qV.png

I'm getting double the output I should be, and I cannot for the life of my figure out why.

I'm simulating 01, 10, and 11 voltage levels for Red, Green, and Blue (respectively).

Any one see something stupidly obvious? Thanks!

Update:

The physical version of the circuit results in the same behavior:

8PYFHoj.png

You can see the signal properly stepping through 00, 01, 10, 11 voltage levels. However the peak-to-peak measurement proves the DAC is only attenuating to 1.4V, not 0.7V.

Comments

  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    The reason you're getting twice the output is that you've omitted the 75-ohm termination resistance at the receiving end of the cable (i.e. inside the monitor).

    -Phil
  • escherescher Posts: 138
    Phil Pilgrim (PhiPi) wrote: »
    The reason you're getting twice the output is that you've omitted the 75-ohm termination resistance at the receiving end of the cable (i.e. inside the monitor).

    -Phil

    I added those before and saw barely mVs of difference... to simulate that termination resistance where would I properly insert those?
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    Add them in parallel with the 130-ohm resistors to ground.

    -Phil
  • escherescher Posts: 138
    edited 2017-10-27 22:37
    Just figured it out before coming back to thank you; before I was adding them in the wrong place. It's been a long time since my circuits analysis class, and I absolutely hated the damn thing. Something about circuits just does not click with me. Thanks for the guidance!

    Am I correct in assuming that all VGA systems have this internal 75 ohm termination resistance?

    Are you aware of this being the case for older video standards, such as RGBS?
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    escher wrote:
    Am I correct in assuming that all VGA systems have this internal 75 ohm termination resistance?
    Yes.
    Am I correct in assuming that all VGA systems have this internal 75 ohm termination resistance?
    Don't know for sure, but I'd still put money on it. The reason it's there is for impedance matching. Otherwise you'd get reflections in the cable, which would show up on the monitor as ghosting.

    -Phil
  • escherescher Posts: 138
    Thanks for the answers! So I guess the better question to ask is... with a CGA monitor on hand, is there a way w/ a multimeter or oscilloscope to determine a monitor's termination resistance? Would it be as simple as applying a test voltage to one of the ports, measuring the resulting current, and calculating the resulting resistance via V=IR?
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2017-10-27 23:59
    Better than that, just hook an ohmmeter from the inputs to ground and measure the resistance directly.

    Some monitors have two video connectors and a switch to include the termination resistance or not. This is to allow several monitors to be daisy-chained together. The last monitor in the chain is the only one with the termination resistance switched in.

    -Phil
  • escherescher Posts: 138
    Interesting, never seen that before! Thanks for the help Phil!
  • escherescher Posts: 138
    edited 2017-10-28 01:22
    This success story brought to you by Phil Pilgrim...

  • Cluso99Cluso99 Posts: 18,069
    FWIW there is a series out pin resistance of 27R.
  • Mark_TMark_T Posts: 1,981
    escher wrote: »
    Am I correct in assuming that all VGA systems have this internal 75 ohm termination resistance?

    Are you aware of this being the case for older video standards, such as RGBS?
    Without 75 ohm termination the video would be horribly distorted with reflections. Video always
    uses 75ohm to my knowledge, both at RF and baseband.
  • potatoheadpotatohead Posts: 10,261
    I have an old monitor with a switch. 75 or 300 ohm.

  • Toby SeckshundToby Seckshund Posts: 2,027
    Beware of some professional (high-end) monitors that auto sense whether a term is needed or not - they would not measure 75 Ohms. Nor would something that old that has a capacitor in series with the video
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