Neat. Basically, a summing amplifier hooked up to an inverting amplifier where each summing input resistor value is double the next. For real use, each digital input could also use some sort of signal level filtering, probably some sort of logic buffer which can drive all these pins and reduce load on the input. (In idealized simulation, it's fine as is.)
https://en.wikipedia.org/wiki/Operational_amplifier_applicat...
https://en.wikipedia.org/wiki/Operational_amplifier_applicat...