From the circuit, there is always a source of drive and a driven load. If the load capacitance is relatively large, the drive circuit must charge and discharge the capacitor to complete the signal transition. When the rising edge is steep, the current is relatively large, so that the driven current will absorb a large supply current, due to the circuit. The inductance, the resistance (especially the inductance on the chip pin, will produce a rebound), this current is actually a kind of noise compared to the normal situation, which will affect the normal operation of the previous stage. This is the coupling. The decoupling capacitor acts as a battery to meet the changes in the drive circuit current and avoid mutual coupling interference. The bypass capacitor is actually decoupled, except that the bypass capacitor generally refers to the high-frequency bypass, which is to improve the low-frequency leakage prevention path for the high-frequency switching noise. The high-frequency bypass capacitor is generally small, and the resonant frequency is generally 0.1u, 0.01u, etc., and the decoupling capacitor is generally large, 10u or more, depending on the distributed parameters in the circuit and the magnitude of the change in the drive current. Bypass is to filter the interference in the input signal, and decoupling is to filter the interference of the output signal to prevent the interference signal from returning to the power supply. This should be their essential difference. The decoupling capacitor has two functions between the integrated circuit power supply and ground: on the one hand, the storage capacitor of the integrated circuit, and on the other hand, the high frequency noise of the device is bypassed. A typical decoupling capacitor value in a digital circuit is 0.1 μF. The typical value of the distributed inductance of this capacitor is 5μH. The 0.1μF decoupling capacitor has a distributed inductor of 5μH, and its parallel resonant frequency is about 7MHz. That is to say, it has better decoupling effect for noise below 10MHz, and has little effect on noise above 40MHz. 1μF, 10μF capacitor, parallel resonance frequency above 20MHz, the effect of removing high frequency noise is better. For every 10 or so ICs, add a charge and discharge capacitor, or a storage capacitor, about 10μF. It is best not to use electrolytic capacitors, which are rolled up by two layers of film. This rolled structure behaves as an inductor at high frequencies. Use tantalum or polycarbonate capacitors. The selection of decoupling capacitors is not critical. It can be taken as C=1/F, that is, 0.1μF at 10MHz and 0.01μF at 100MHz. Distributed capacitance refers to a distribution parameter formed by non-morphological capacitance. Generally refers to the capacitance formed between the printed circuit board or other forms of circuit, between the line and the line, and between the upper and lower layers of the printed board. The capacity of this type of capacitor is small, but it may have a certain effect on the circuit. This effect must be fully considered when designing the printed board, especially when the operating frequency is high. It also becomes a parasitic capacitance, which is sure to occur during manufacturing, just a matter of size. When fabricating high-speed PCBs, vias can reduce the board capacitance, but increase the inductance. Distributed inductance refers to the increase of impedance due to the self-inductance of the conductor when the frequency is increased. Capacitor selection and use precautions: 1. Generally, when the low frequency coupling or bypass, when the electrical characteristics are lower, the paper medium and polyester capacitor can be selected. In high-frequency high-voltage circuits, mica capacitors or ceramic capacitors should be used; in the power supply filtering and decoupling circuits, electrolytic capacitors can be used. 2. In the oscillation circuit, delay circuit, and tone circuit, the capacitor capacity should be as consistent as possible with the calculated value. In various filtering and network (frequency selective network), the capacitor capacity is required to be accurate; in the decoupling circuit and the low frequency coupling circuit, the requirements for the accuracy of the same two levels are not strict. 3, the rated voltage of the capacitor should be higher than the actual working voltage, and there must be sufficient room, generally use the capacitor with the withstand voltage value of more than twice the actual working voltage. 4. It is preferred to use capacitors with high insulation resistance and low loss, and pay attention to the environment. capacitance
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Basic capacitance knowledge summary