In this paper, we show why the descriptions of the sampled signal used in calculation of its spectrum, that are used in the literature, are not correct. And this finding applies to both kinds of descriptions: the ones which follow from an idealized way of modelling of the signal sampling operation as well as those which take into account its non-idealities. The correct signal description, that results directly from the way A/D converters work (regardless of their architecture), is presented and dis-cussed here in detail. Many figures included in the text help in its understanding.

This paper crowns efforts, made by its author, aiming in showing and proving that the current formula for calculation of the spectra of output signals at A/D converters requires a correcting factor in it. A number of partial results obtained and published in the last years are referred to here. They paved the way to a fully satisfactory and correct result; it is presented in this work. The corrected formula for spectrum calculation is derived using a description of the output signal of an A/D converter by means of the so-called Dirac comb, however not in a direct form, but with taking into account physical reality. In addition, the paper contains a number of interpretative remarks, comments, and explanations - clarifying those matters that have so far been omitted in analyses of the sampling process, despite the fact that they raised various types of doubts.