Selection of coupling capacitors and distributed capacitors

From the perspective of the circuit , there is always a driving source and a driven load. If the load capacitance is relatively large, the driving circuit must charge
and discharge the capacitance to complete the signal jump. When the rising edge is relatively steep, the current is relatively large, so the driving current will absorb a large amount of power supply current. Due to the inductance and resistance in the circuit (especially the inductance on the chip pins, which will rebound), this current is actually a kind of noise relative to normal conditions, which will affect the normal operation of the previous stage. This is coupling. The decoupling capacitor plays the role of a battery to meet the change of the driving circuit current and avoid mutual coupling interference. The bypass capacitor is actually also decoupling, but the bypass capacitor generally refers to high-frequency bypass, that is, to provide a low-impedance leakage protection path for high-frequency switching noise. High-frequency bypass capacitors are generally small, generally 0.1u, 0.01u , etc. according to the resonant frequency , while decoupling capacitors are generally larger, 10u or larger, and are determined according to the distributed parameters in the circuit and the change of the driving current . Bypassing is to filter out the interference in the input signal, while decoupling is to filter out the interference in 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 power supply and the ground of the integrated circuit : on the one hand, it is the energy storage capacitor of the integrated circuit , and on the other hand, it bypasses the high-frequency noise of the device. The typical decoupling capacitor value in digital circuits is 0.1μF. The typical value of the distributed inductance of this capacitor is 5μH. The 0.1μF decoupling capacitor has a distributed inductance of 5μH, and its parallel resonance frequency is about 7MHz, that is, it has a good decoupling effect for noise below 10MHz, and it has almost no effect on noise above 40MHz. The 1μF and 10μF capacitors have a parallel resonance frequency above 20MHz, and the effect of removing high-frequency noise is better. For every 10 integrated circuits, add a charging and discharging capacitor or an energy storage capacitor, which can be about 10μF. It is best not to use electrolytic capacitors. Electrolytic capacitors are two layers of film rolled up. This rolled-up structure behaves as an inductor at high frequencies. Tantalum capacitors or polycarbonate capacitors should be used. The selection of decoupling capacitors is not strict. It can be based on C=1/F, that is, 0.1μF for 10MHz and 0.01μF for 100MHz. Distributed capacitance refers to a distributed parameter formed by non-morphological capacitance. It generally refers to the capacitance formed between lines and between the upper and lower layers of a printed circuit board or other circuit forms. The capacity of this capacitor is very small, but it may have a certain impact on the circuit. This impact must be fully considered when designing a printed circuit board, especially when the operating frequency is very high. It is also called parasitic capacitance, which will definitely be generated during manufacturing, but it is just a matter of size. When laying high-speed PCBs, vias can reduce the capacitance of the board layer, but will increase the inductance. Distributed inductance refers to the increase in impedance caused by the conductor's self-inductance when the frequency increases. Notes on capacitor selection and use: 1. Generally, paper and polyester capacitors can be used in low-frequency coupling or bypass when the electrical characteristics are low; mica capacitors or ceramic capacitors should be used in high-frequency and high-voltage circuits; electrolytic capacitors can be used in power supply filtering and decoupling circuits . 2. In oscillation circuits, delay circuits, and tone circuits, the capacitor capacity should be as consistent as possible with the calculated value. In various filters and networks (frequency selection networks), the capacitor capacity requires accuracy; in decoupling circuits and low-frequency coupling circuits, the requirements for the same two levels of accuracy are not very strict. 3. The rated voltage of the capacitor should be higher than the actual working voltage, and there should be enough room. Generally, capacitors with a withstand voltage value of more than twice the actual working voltage should be selected. 4. Capacitors with high insulation resistance and low loss should be selected first, and attention should also be paid to the use environment.