Selection of coupling capacitors and distributed capacitors

From the circuit, there is always the source of the drive and the load being driven. If the load capacitance is relatively large, the driver circuit must charge and discharge the capacitor to complete the signal transition. When the rising edge is relatively steep, the current is relatively large, so that the drive 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, will affect the normal work of the previous stage. This is the coupling.

To remove the tantalum capacitor is to play a role in the battery, to meet the changes in the drive circuit current, to avoid mutual coupling interference. The bypass capacitor is also actually decoupled, but the bypass capacitor generally refers to the high-frequency bypass, which means that the high-frequency switching noise improves a low-impedance discharge path. High-frequency bypass capacitors are generally small, typically 0.1u, 0.01u, etc., depending on the resonant frequency, while decoupling capacitors are typically larger, 10u or greater, depending on the distribution parameters in the circuit and the magnitude of the drive current.

The bypass is to filter out the interference in the input signal, and the decoupling is to filter out 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 roles between the integrated circuit power supply and the ground: on the one hand, it is the 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 a digital circuit 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. Its parallel resonant frequency is about 7MHz, that is, it has good decoupling effect for noise below 10MHz, and it has almost no effect on noise above 40MHz. A 1μF, 10μF capacitor with a parallel resonant frequency above 20MHz removes high-frequency noise better. Each 10 pieces of integrated circuit should add a charge and discharge capacitance, or a storage capacitor, which can be selected as 10μF. It is better not to use electrolytic capacitors. The electrolytic capacitor is rolled up by two layers of film. This rolled structure shows inductance at high frequencies. Use tantalum or polycarbonate capacitors. The choice of decoupling capacitors is not critical, and can be taken as C=1/F, that is, 0.1 μF for 10 MHz and 0.01 μF for 100 MHz.

Distributed capacitance refers to a distribution parameter formed by non-morphic capacitance. Generally refers to the capacitance formed between the printed circuit board or other form of circuit, the line and the line between the upper and lower layers of the printed circuit board. The capacitance of this capacitor is small, but it may have a certain influence on the circuit. This effect must be fully taken into account when designing printed boards, especially when the operating frequency is high. It also becomes a parasitic capacitance, which must be produced when it is manufactured. It is only a matter of size. In the case of high-speed PCBs, vias can reduce the layer capacitance but increase the inductance. Distributed inductance refers to the increase in impedance due to self-inductance of the conductor when the frequency increases.

Selection and use of capacitors Note:

1, generally low-frequency coupling or bypass, the electrical characteristics of the lower requirements, the choice of paper dielectric, polyester capacitors; in high-frequency high-voltage circuit, you should use mica capacitors or ceramic capacitors; in the power supply filter and decoupling circuit, Electrolytic capacitors can be used.

2, in the oscillation circuit, delay circuit, tone circuit, the capacitor capacity should be consistent with the calculated value. In a variety of filtering and network (selective frequency network), the capacitor requires accurate capacity; in the decoupling circuit, low-frequency coupling circuit, the same two-level accuracy requirements are not strict.

3, The rated voltage of the capacitor should be higher than the actual working voltage, and there must be enough room for it. Generally, a capacitor whose withstand voltage is more than twice the actual operating voltage is used.

4, priority is to use capacitors with high insulation resistance and low loss, but also pay attention to the use of the environment.