Small nit: the analog DAC circuit has MSB and LSB reversed. That is, the LSB goes into the top 128k resistor. If you think of it one bit at a time, the gain of the circuit = -Rf/R --- where R is one of the resistors in the ladder. (Rf is the feedback resistor, which is unlabeled and un-valued in the article.) Clearly, you get bigger output when R is smaller.