System Classification - Signal Processing of Random Physiological Signals - page 10

has no memory elements, that is, stored charge in a capacitor, and is considered an
instantaneous system. There is no need to express the system as a differential equation,
only as constants.
If a system response depends on past value of excitation, then the system is classified
as a Dynamic System, which has memory (meaning it stores energy). Circuits with
capacitors and magnetic inductors components are Dynamic systems, since the capacitors
initial state is based on past excitation. Similarly, magnetic components with residual
magnetism store energy based on past excitation, thus making such systems “Dynamic.”
Dynamic system mathematical models are expressed as differential equations in either
time or frequency domain.
In general, most problems are considered initially to be

1. causal,

2. linear,

3. fixed,

4. lumped-parameter,

5. continuous time, and

6. dynamic systems.