Function of Capacitor in High Speed PCB Design(II)

The second part: the specific model and distribution parameters of capacitor should be applied correctly and reasonably, it is necessary to understand the specific model of capacitor and the significance and function of each distribution parameter in the model. Like other components, actual capacitors are different from "ideal" capacitors. "actual" capacitors have an additional characteristic of inductance and resistance due to the effects of their packaging, materials, etc. It must be characterized by additional "parasitic" or "non-ideal" properties in the form of resistive and inductive elements, nonlinear and dielectric storage. The "actual" capacitor model is shown below. Because of the characteristics of the capacitors determined by these parasitic elements, they are usually described in detail in the manufacturer's product description. Understanding these parasitic effects in each application will help you choose the right type of capacitor.

1. Equivalent series resistance (ESR RESR): the equivalent series resistance of the capacitor consists of the pin resistance of the capacitor and the equivalent resistance phase of the two electrodes of the capacitor. When a large AC current passes through the capacitor's RESR, the capacitor consumes energy (resulting in loss). This has serious consequences for RF circuits and power decoupling capacitors with high ripple currents. But it will not have great influence on the precision high impedance and small signal analog circuit. The lowest RESR capacitors are mica capacitors and thin film capacitors.

2. Equivalent series inductance ESLLSL: the equivalent series inductance of the capacitor is composed of the inductance of the capacitor pin and the equivalent inductance of the two electrode plates of the capacitor. Like RESR, there are also serious problems in RF or HF operating environments, although precision circuits themselves work normally under DC or low frequency conditions. The reason is that transistors used in precision analog circuits still have gain to amplify resonance signals with very low inductance when the transition frequency transition frequency increases to several hundred MHz or several gigahertz. This is the main reason why the power end of this circuit should be decoupled properly at high frequency.