Parasitic Capacitance

If two parts of an electronic circuit are in close proximity to one another, there is a likelihood of a capacitance effect – known as parasitic or stray capacitance – between them. This is unavoidable and usually unwanted. It arises because the electric field between parts at different potential causes them to store an electric charge. Stray capacitance is associated with conductors such as wires and PCB tracks as well as components. It becomes more of an issue as signal frequency increases.

Self-capacitance is parasitic capacitance found within a component – for example across the windings of an inductor - without linkage to an external component or conductor.

Parasitic capacitance changes the intended output from the circuit or device. In amplifier circuits with extended frequency response, parasitic capacitance between the output and the input can act as a feedback path, causing the circuit to oscillate at high frequency. These unwanted oscillations are called parasitic oscillations.

In high frequency amplifiers, parasitic capacitance can combine with stray inductance such as component leads to form resonant circuits, also leading to parasitic oscillations. In all inductors, the parasitic capacitance will resonate with the inductance at some high frequency to make the inductor self-resonant; this is called the self-resonant frequency.

Electronic design automation computer programs, which are used to design commercial printed circuit boards, can calculate the parasitic capacitance and other parasitic effects of both components and circuit board traces, and include them in simulations of circuit operation. This is called parasitic extraction.