Parasitic impedance

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Parasitic impedance is the unwanted extra resistance, inductance, or capacitance that sneaks into electrical components. Even a simple resistor has stray capacitance, and all conductors have some resistance and inductance. Designers try to minimize these parasitics, but they can’t be eliminated. Datasheets for some parts list these parasitics to help designers compensate.

Common sources include the resistance and inductance of leads, and the capacitance in the component package. In wound parts like inductors and transformers, there is parasitic capacitance between turns, which can make the part resonate at a self-resonant frequency and stop acting as an inductor above that point.

Parasitics are often treated as lumped components in circuit models, but that isn’t always accurate—for example, inter-turn capacitance is distributed along the winding. Some designs deliberately use parasitics to achieve a function, such as in helical resonators or analog delay lines.

Nonlinear parasitics can appear, especially in integrated circuits, where unwanted p-n junctions can form stray devices. Dielectric parasitics in capacitors and magnetic parasitics in inductors can also vary with voltage or frequency and may not be well captured by simple linear models.