Why An operational amplifier is a high-gain DC-coupled amplifier that works with continuously varying voltages, amplifying, summing, integrating or filtering signals whose values can take any level within the supply rails. That continuous, non-quantized behavior is the definition of an analog device, in contrast to circuits that only recognize discrete high and low logic states.
Watch out Calling it digital confuses it with logic gates and counters, which respond only to two voltage states; MMIC refers specifically to a monolithic microwave IC used at RF/microwave frequencies, not a general purpose op-amp.
Op-amp = smooth, continuous voltages = analog. Gates and logic = only 1s and 0s = digital.