What two values for VGA R-2R resistor ladder?

tonyp12

This shows that for a 4bit dac I would need five of the 2R and three of the R


Finding a complete r2r array with right values for the prop vga is probably impossible.

This R=200ohm one could be close, it's SIP and I'm looking for SMD
http://www.mouser.com/ProductDetail/Murata/RGSD8L201G/?qs=sGAEpiMZZMvrmc6UYKmaNXFn7JV0JJEYnMFa1MNj5Yc%3d


So for my 12bit-rgb vga I was thinking going with isolated array(s) with a total 15 individual 2R
and a second array of nine individual R


One option would be to get 33 of the 2R and put nine sets of two in parallel to make the R

Toby Seckshund

The characteristic output of these networks is "R", so for 75 Ohms the R is 75 and 2R is 150 Ohms

Phil Pilgrim (PhiPi)

With just a simple R-2R network, even if the output impedance is correct, the P-P voltage levels will be wrong if driven from 3.3V. I think you will need some attenuation in there somewhere.

-Phil

kwinn

There used to be R2R resistor networks available for this, and the resistors were more accurately matched than using individual resistors. With DAC's being so common and inexpensive that may not be the case any longer but it is worth checking out.

davejames

Tonyp12,

Attached is some info I gleaned off the interweb while surfing for R2R information.

Bourns and IRC make R2R ladder networks (not sure if they meet your requirements).

tonyp12

This formula should work
3.3 * 75 / (75 + R) = peek volt

As the props output will be closer to 3.0v
So with R=180ohm and R2=360: peek Vout=0.88

But using all 470 ohm (+470s in parallel) probably best
http://mouser.com/search/ProductDetail.aspx?qs=aMbtsk9EOIZKUcUZAgQOww%3D%3D&cm_mmc=findchips

Isn't peek suppose to be 0.7v ?
But Linus made his 0-1v peek, maybe due to blanking level is 350mV that offsets it?
Or that some monitors can adjust for it?
http://www.linusakesson.net/hardware/beagleboard/vga.php