I have an FPGA and want it to drive a VGA monitor. I want more than 3 bits of color so I need a DAC of some sort. So I want to make a resistor dac. My plan is to have 3 bits per subcolor. So total of 9 bits per pixel. I know VGA monitors have an impedance of 75 ohm that I need to take into account, and I know they need 0 for black, and 0.7v for white.(per subpixel)
Problem is I don't know how to continue with this info, I believe I need to use algebra for this, but I'm still learning that and therefore have some issues calculating the correct resistor values.
So first I think I need to figure out what resistor value I need to get 0.7 v taking into account the 75 ohm to ground. Which wounds like a resistor divider to me, if I'm correct, when I use 3.3v as input, I'd need a 280 ohm resistor to approximately get 0.7v.(I used the formula on the wikipedia page for resistor dividers) But I don't know how to continue from here. How do I know what 3 resistor values I need that would give me the correct voltages?(Also, bit 2 needs to give more voltage than bit 0)
Can anyone help me with that? And also explain what you are doing to get me the values so I can learn it too?
In case I do need to use algebra, I only know the basics like solving 3x+3=2x+7 so a little help there would be appreciated.