I prepared this for my Introduction to Microelectronic Circuits class at Olin. It was one of the first post-lab exercises we were challenged with.
Using basic electronics laws (e.g. Ohm's, Kirchoff's), I show how an R-2R ladder network of resistors (shown below) can function as a digital to analog converter. The values of the various digital input switches (labeled as b0 ... bN-1) determine the analog value of the output current, Iout.
PDF available for viewing and download below.