# How Significant is VGA Impedance Matching?

I've seen a multitude of different resistor DAC solutions for a VGA signal, however they almost always have an impedance of >200 ohms per color, e.g.

when I've been lead to believe that we want to match the 75 ohm input impedance of the VGA destination. How significant is this large difference in impedance? Will the signal reflection effects be more visible on a CRT vs. LCD monitor, or even at all?

For an implementation where number of resistors and obscurity of their values isn't an issue, this can be solved with an R2R ladder DAC:

where each color output has an effective impedance of 75 ohms.

However for a project where the expectation is that people should be easily able to acquire and construct the circuit, this isn't ideal whatsoever.

Thank you!

• Impedance mismatch is easy to see on a CRT since the cable length is usually at least 6ft. On an LCD VGA screen you may be slightly less prone to visible artifacts due to the slicer used on the input to digitize the incoming signal. These are normally not just an free running A/D but synchronized with the expected resolution of the LCD. Since they have such a small acquisition window they are better at missing most of the reflected signal. You should be able to correctly build the R2R with two R96 values for each ladder value you need. At least then you have close to the right impedance. – Jack Creasey Jul 10 '18 at 17:25
• Thanks for the insight! Your proposed R96 ladder however doesn't appear to be accurate enough. For example, both "all bits on" and "MSB only off" are above 700 mV, resulting in no visible difference. – Connor Spangler Jul 10 '18 at 17:32
• I didn't propose ladder network values ….. You can use a simple calculator to build very accurate values with two resistors: sengpielaudio.com/calculator-paralresist.htm For example building a 310 Ohm resistor with a 316 // 16200 is more than accurate for this application. – Jack Creasey Jul 10 '18 at 19:04
• You need a video amplifier which will buffer the resistors and usually will have a 75 ohm impedance on its output. – EE_socal Jul 10 '18 at 20:41
• @EE_socal If you use a video amp with 75 Ohm output impedance, then you don't need a 75 Ohm ladder network of course. – Jack Creasey Jul 10 '18 at 22:16