Digital to Analog Conversion isn't always done with a resistor array, capacitor array or a PWM or PPM signal. Analog and Mixed signal IC designers often use inverters and logic gates to create DACs and ADCs. The basic idea is to use the the matched transconductance of simple "unbuffered" gates. This is illustrated well with this circuit using the readily available CD4572. It's a variation of the more commonly used CD4069 hex inverter. This may be your first introduction to this unusual but increasingly important CMOS analog circuit technique where we start out wiring all the outputs of the gates together.