Frequency-Dependent Distortion in Class II Ceramic Capacitors: Physics and Simulation Traps

Abstract

Class II ceramic capacitors (e.g., X7R, X5R) exhibit a voltage-dependent capacitance that introduces harmonic distortion into analog signal chains. Counterintuitively, the maximum Total Harmonic Distortion (THD) does not occur at the highest frequencies, but rather near the cutoff frequency of the RC filter. This article explains the physical mechanism behind this frequency-dependent distortion peak. Furthermore, it demonstrates that non-physical harmonic content can appear when a voltage-dependent capacitor is implemented as an instantaneous C(V) behavioral element rather than as a charge-defined device Q(V), especially in simplified fixed-step transient solvers or poorly formulated macromodels.