I have a circuit that drives a VGA monitor using a simple 4-bit (per color) resistor DAC. This works well, colors look great, image is stable, etc. However, I also see circuits in datasheets (like the typical use schematic from AD725) or on other boards that use buffer ICs or discrete transistor amp circuits. How do I know if I should be using something similar? Since my circuit is working fine, what would I gain by using something like an AD8073? Here are three examples of circuits that I have seen. Can anyone explain why/when you would use one over the other?
Thanks,
Matthew
Resistor DAC, similar to what I'm using. Works fine.
Very often the video chip just drives current through the 75 ohm resistors in the VGA monitor, its just a relatively high impedance current source.
This works fine if the cable is very short with a low shunt capacitance, as is usually the case.
But if you wanted to send that video down a very long cable, a proper video buffer amplifier with a 75 ohm output impedance would give much better results.