Digital-Analogic conversion help

[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

 

[berkeman]

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.