How to calculate resistor values for a VGA DAC
The simplest thing to do is to build a 3-bit "R-2R" DAC for each channel, using 75&Omega; resistors. The cool thing about this kind of DAC is that its output impedance is simply "R", which means that you can connect it directly to 75&Omega; coax with no termination issues.



If you build a 4-bit DAC, but tie the MSB to ground, you'll get close to the voltage range you need:







simulate this circuit &ndash; Schematic created using CircuitLab

The Thevenin source voltage is reduced by the voltage divider created by the Thevenin source resistance and the monitor termination resistance, resulting in a range of 0.00V to 0.72V on the line.

The worst-case load on any of the FPGA outputs is approximately 3.3V / 225 &Omega; = 15 mA.