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Overlooked Characteristics of Capacitors
All capacitors contain a finite amount of lead and device body inductance. Loop inductance (including lead inductance) must be minimized at all times. When the inductance of a routed trace between capacitor and component, plus the internal lead inductance from the solder pad to the plates of the capacitor is added together, a high impedance may be present between the capacitor’s pins and a voltage or return (ground) network. When the impedance of a capacitive connection becomes excessive, a voltage gradient is developed creating undesired common-mode RF currents. RF currents can cause RF radiated emissions. The use of decoupling capacitors mandates the need for minimizing the inductive loop area when installed on a printed circuit board. The inductive loop area includes the inductance of the via and pin escapes (or pad connections from the component pin to the point where the device pin connects to a via), through the power distribution network, and back through the alternate lead of the capacitor, plus its connection length through the bond wires internal to a digital device’s package.
Within any capacitor, the dielectric material determines the impedance magnitude, which is related to the self-resonant frequency of operation. All dielectric material is temperature sensitive. Capacitance will change in relation to the ambient temperature. At certain temperatures, the capacitance value may also change substantially resulting in improper operation when used as a decoupling element. The more stable the temperature rating of the dielectric material, performance of the capacitor is enhanced.
In addition to the sensitivity of the dielectric material to temperature, the equivalent series inductance (ESL) and the equivalent series resistance (ESR) must be low as possible at the desired frequency of operation. ESL acts like a parasitic inductor whereas ESR acts like a parasitic resistor, both in series with the capacitor. Together, ESL and ESR degrade a capacitor’s effectiveness as a bypass or decoupling element. When selecting a capacitor, one should choose a capacitor family that publishes actual ESL and ESR values in their data sheets. Random selection of a standard capacitor may result in improper performance if ESL and ESR are too high. Most vendors of capacitors do not publish ESL and ESR values. It is best to be aware of this selection parameter when choosing capacitors used in high-speed, high-technology PCBs.
Because surface-mount capacitors have significantly less ESL and ESR than radial or axial devices, their use is preferred in higher-speed circuits. Typically for surface mount capacitors, ESL is approximately 1 nH. The ESR should be 0.5 ohms or less. For decoupling, capacitance tolerance is not as important as the inductance loop area, temperature stability, dielectric constant, ESL, ESR, and its self-resonant frequency.
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