I'm designing a VGA generator and I have found nice and cheap video filter (ADA4430-1) that goes after DAC.

Its datasheet says that it has cutoff frequency around 10MHz. And that it is specially designed as video filter. I want 640x480, 60Hz, so my pixel clock (frequency at which pixels are changing) is 25MHz.

So now my question: If my pixels changes with 25MHz and filter is for 10MHz, so how and why this filter calls itself suitable for video?

I'm a little confused - what should be the frequency cutoff of such filter to operate with this 640x480 VGA (25MHz)?

  • \$\begingroup\$ "Video" is PAL or NTSC where the pixel clock is less than 5Mhz \$\endgroup\$ – Jasen Nov 16 '15 at 11:04
  • \$\begingroup\$ VGA does not equal video it seems. \$\endgroup\$ – Andy aka Nov 16 '15 at 11:23
  • \$\begingroup\$ Ok, so what should be the cutoff frequency for my VGA. ~50MHz? \$\endgroup\$ – zupazt3 Nov 16 '15 at 11:31
  • \$\begingroup\$ I've found THS7316 - it has 36 MHz bandwidth and filters. Should it work? \$\endgroup\$ – zupazt3 Nov 16 '15 at 11:47

For a good-quality computer video display, where fine vertical lines show the same contrast as fine horizontal lines, your video bandwidth should be able to pass at least the 3rd harmonic of the fastest square wave that appears in the image.

The fastest square wave is alternating dark/light pixels, so its frequency is half the frequency of the dot clock. For VGA at 25.175 MHz, this would be 12.59 MHz. Three times this fundamental frequency is 37.76 MHz, so yes, a 36 MHz filter would be a good choice.


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