You're right. I suppose you'd need a reference voltage and a comparator per output to make it an ADC.
-- theneweuropa
Much more than that. Analog to digital conversion is complex and is usually handled by a dedicated chip. For discrete implementation you would need either 2^n-1 comparators where n is the number of bits (eg 255 comparators for 8 bits) and a string of 2^n resistors plus a complex network of data encoding gates, OR a complicated state machine that performs a quick series of measurements with a set of thresholds per each input sample.