The real capacitor can be modeled as an ideal capacitor in series with an ideal inductor in series with an ideal resistor. The ideal inductor models the capacitors Equivalent Series Inductance (ESL) and the ideal resistor models the capacitors Equivalent Series Resistance (ESR). The ideal capacitor's impedance is infinite at DC and decreases as the applied voltage increases in frequency. Eventually, the ideal capacitor's impedance reaches zero. The real capacitor's impedance never reaches zero because of ESR and ESL. Remember that an ideal inductor's impedance increases with increasing frequency. An ideal resistor is frequency independent. So put all this together and the impedance vs. frequency curve has a very interesting shape. AVX Corporation has an excellent and detailed explanation of inductive parasitic effects and how to measure them in the Technical Article at : http://www.avx.com/docs/techinfo/parasitc.pdf . This article brings a lot of insight into many aspects of analog circuit design.
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