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I'm trying to analyze a circuit from the ~40 year old TI TMS9928/29 Interface to Color Monitors application note. It shows how to convert the (almost) Component video straight from the graphics chip to RGB or Composite out.

All three of the YUV video lines have what is presumably a filter (with identical component values) that I wasn't able to find in any of the filter recipe lists in AoE or elsewhere:

220pF parallel with 390R to ground

(I'm guessing C14 is an unrelated coupling capacitor.)

It looks vaguely like a low-pass filter, but tinkering around in LTSpice, I wasn't able to make the output look any different than the input (above, at, or below the ~3MHz signal frequency).

Walking through the application note schematic is pushing the limits of my analysis ability (in a productive, trial-by-fire sort of way), but this one has me stumped. Thanks!

EDIT: Of course it's not filtering the voltage; that whole top rail is the same node! (This is how you can tell I need more practice.) Wikipedia says "the parallel RC circuit is generally of less interest...", so maybe a better question is "Why did the TI engineers put those components there?"

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Without a source impedance, that 220pF will have no effect.

390 ohms and 10uf is a 40 Hz corner.

This looks like a video signal or a chroma signal, possibly with sync pulses stripped off (that 40 Hz corner will upset the black level unless clamped for DC restoration.)

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  • \$\begingroup\$ Both the sync stripper and DC restoration "blocks" of the schematic come just after this initial input stage. I'll have to check the TMS9928 datasheet to see what sort of impedance is coming from the video lines. (My guess would have been "zero", but this little wrinkle seems to say otherwise.) Thanks! \$\endgroup\$
    – Nicholas
    Feb 24 '20 at 5:51
  • \$\begingroup\$ Ah ha, mystery mostly solved: in section 3.4.2 of the datasheet they show the video chip heading into the same chunk described in this application note, but mention "The internal output buffer devices on these pins are source-follower MOS transistors that require an external pull-down resistor to VSS. A 330ohm resistor is recommended." In my LTSpice model, that makes a beautiful low-pass filter that gets rid of all sorts of junk. Thanks! This pointed me in the right direction. \$\endgroup\$
    – Nicholas
    Feb 24 '20 at 6:10

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