Digital-Analogic conversion help

1. Oct 16, 2006

esmeco

I'm trying to work out an equation for the circuit below,but unfortunately I'm not having success.I've managed to get the expression for I1 which is:

I1=(Vin)/2R

How do I find now the expression for I2?Following the equation V=RxI where V is the difference at both terminals of the resistance I need to find the value tension at point A.How do I find that value?I've found the parallel resistance between the resistances that correspond to currents I3 and I4 which is R by I don't know how to work it from that!
Any help is really appreciated!Thanks in advance!

http://i75.photobucket.com/albums/i281/esmeco/digitalanalogicconverter.jpg

2. Oct 17, 2006

Staff: Mentor

Two things. First, the simulation schematic looks like it has a couple connection errors in it. The bottom of the + input network is not connected to the - input network side or to a "ground" symbol as far as I can see. Is something missing?

Second, the trick with R:2R ladders is to see that in a ladder, the R is in series with a 2R in parallel with a net 2R. That gives you a consistent voltage divider characteristic that you can use in a DAC ladder. You might see if wikipedia or your textbook describes the R 2R ladder to get a better explanation. Hope that helps some.