An apparatus and a method for use in an electric circuit. The apparatus includes a sampling module arranged to obtain an approximation of a voltage-current characteristic of at least one electrical circuit component of the electric circuit subjected to an excitation, wherein each of the at least one electrical circuit component is represented by a constant phase element in an equivalent circuit of the electric circuit; and a processing module arranged to estimate a time-domain voltage response and/or a time-domain current response based on the approximation of the voltage-current characteristic under the excitation applied to the electric circuit.