The primary difference between analog design and digital design is the type of underlying analysis that is used.
In analog design, circuit stimulus is treated as a continuously varying signal over time. The behavior of the circuit is modeled in the time and frequency domains with attention focused on the fidelity/precision, consistency, and performance of the resultant waveforms. Circuit variability, both manufacturing and design induced, must be modeled and compensated for as well.
Digital design treats circuit stimulus as a series of discrete logic “ones” and logic “zeros” over time. A logic “one” is typically represented by the presence of the supply voltage for the IC and a logic “zero” is represented by the absence of this voltage (i.e., zero volts). The devices in digital circuits must spend most of their time at either logic “one” or logic “zero”. As long as the circuits processing these signals are consistent in their response to these logic levels, digital design works well. Analog design is responsible to deliver these qualities.
This allows the behavior of the circuit to be analyzed using combinatorial and sequential models, only considering two voltages (“one” and “zero”), which substantially simplifies the design and verification process.