how to interface cmos to VGA

[Dan Moos]

What is the proper way to interface a 5 volt signal to the red, green and blue  channels of a vga monitor? Lets assume I am going for white, full brightness. I know how to create a simple resistor DAC.

I know from research the upper voltage level of the signal is .7 volts (sounds suspiciously like the monitor input is diode clamped)

I've found that leaving the signals at 5 volts results in a "smeared" looking image. a 1k resistor in series cleans it up considerably. Haven't tried a diode to ground yet to clamp the signal. Resistor divider obviously results in a less bright image. I get pretty passable results with the 1k resistor in series between the logic and the monitor input.

This is a home rolled VGA video circuit for a home-brew computer . It works great, but I've kinda procrastinated on properly doing this part.

[NiHaoMike]

The VGA signals have a 75 ohm termination, so 470 ohms is the closest standard resistor value to give the ideal signal level. Maybe bump that down to 390 ohms to compensate for the impedance of the output drivers.

[Dan Moos]

So just to make sure I'm following, What I am trying to do is create a voltage divider with the 75 ohm termination that results in .7 volts at full input? Thus, the 1k I'm currently using is a slightly attenuated signal compared to what I could send?

Also, is any sort of filtering recommended? My pixels are sent at a 25.175 Mhz  clock rate. Should I be high or low pass filtering this signal? With the 1k series resistor, i get very faint smearing of the signal. You have to really look to see it.

[NiHaoMike]

No need to filter, the whole point is to have the pixel values transition as quickly as possible.

[james_s]

This is commonly done with FPGAs using a simple resistor DAC, for an example see here http://searle.x10host.com/Multicomp/index.html

With additional resistors you can get 4 or more bits per pixel.

Keep in mind the above example is 3.3V rather than 5V.