Video driver / amplifier necessary?

[TQFP]

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.


Discrete amp


Using a video IC

 

[barry]

I think it really depends on what your load is. I thought VGA was 75 ohms, which is why you need a driver. Not sure why your circuit works at all.

 

[Warpspeed]

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.