The impedance of an ideal capacitor is given as Z = Xc = 1/2 (pi)fC and, if graphed, would result in a straight line that gets ever closer to zero as frequency (f) increases. However, no circuit component is ideal, i.e., purely resistive or purely reactive. All circuit components exhibit a combination of complex impedance elements: Inductors show unwanted capacitance and hysteresis effects. Resistors display unwanted inductance characteristics. Capacitors have unwanted inductance, resistance, and dielectric absorption. Different materials and manufacturing techniques produce varying amounts of these unwanted parasitics that affect a component's performance. All components, even those constructed of the finest materials and procedures, exhibit some of these artifacts and therefore should be modeled as complex impedances. (See Figures 1 A & 1 B.)

In addition to capacitance (C), two other parameters, dissipation factor (DF) and equivalent series resistance (ESR) are generally measured to reflect the presence of parasitics. Thus a much more realistic representation of a capacitor is shown in Figures 2A & 2B.

This paper will define several capacitor-based parasitics and explore their relationships and effects upon a capacitor's performance. Dielectric absorption will not be discussed in any depth here: those wanting technical details should refer to Richard Marsh's previous groundbreaking work. (See the bibliography.) Briefly, dielectric absorption has been revealed as a major source of distortion in audio circuits. Film dielectric type capacitors, particularly polystyrene, have very low dielectric absorption and for this reason are preferred in audio circuits.

Figures 1A& B: A capacitor (C) with parasitic elements & equivalent electrical circuit. A true value excludes the effects of the parasitics. In the real world, true values are of academic interest only. The effective value includes the influences of the component's parasitics.

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