Capacitors have the function of filtering, which should be the most basic knowledge that every hardware engineer has. In some chip IO ports, we can see filter capacitors with a capacitance of 0.1UF 100NF 4.7UF.
Capacitor, a small material, the selection of its capacitance often plays a very important role in hardware circuit design and simulation.
When filtering with capacitors, engineers often like to divide capacitors into bypass capacitors and decoupling capacitors. After searching a lot of information, everyone distinguishes according to the location, but in Xiaobai's view, the essence is the same. Some also say that it can be distinguished according to the function, that is, the decoupling capacitor has one more energy storage function than the bypass capacitor. I think, since they are all capacitors, why can't bypass capacitors store energy, and the capacitor itself has the function of energy storage, and it is not limited to any type of capacitor.
In digital circuits, everyone knows the role of capacitors: energy storage and filtering (providing a low-impedance path for high-frequency noise currents).
In essence, it is almost completely unified in a way. You can think that the energy storage of the capacitor provides an instantaneous charge for high-frequency switching (transistors in the chip switch back and forth), thus avoiding the high-frequency noise generated by the switch from spreading to the direction of the chip, because the energy required for switching is already close to the chip. obtained from the capacitance of the chip. You can also think that the bypass capacitor provides a low-impedance path for high-frequency noise currents, thereby avoiding the need to request charge energy from a farther power source during high-frequency switching.
Although the capacitor plays a very good role in filtering, in some circuits, the selection of its capacitance value is often not easy to determine. A 0.1UF capacitor is placed at the chip pin position, which should be the most common one we see. However, the chips on some boards are placed at 0.01UF. What is the point of this?
To understand these, we must first understand the characteristics of capacitors. An ideal capacitor is just a memory for a capacitor, C. The actual capacitor is not so simple. Its model is as follows:
ESR is the series equivalent resistance of the capacitor, ESL is the series equivalent inductance of the capacitor, and C is the ideal capacitor. ESR and ESL are determined by the manufacturing process and materials of capacitors and cannot be eliminated. ESR tends to affect the ripple of the power supply, while ESL affects the filtering frequency of the capacitor. By the following formula:
Zc=1/WC
ZL=WL, W=2πf
Z=ESR+j(WL-1/wC)=ESR+j(2πfL-1/2πfC) It
can be seen from the formula that when the frequency is very low, its capacitance effect . However, when the frequency is high to a certain extent, the effect of inductance cannot be ignored. No matter how high the inductance is, it plays a leading role. Because the inductance passes through DC and resists AC, the capacitor is completely unable to provide a low-impedance path for high-frequency noise current.
Theoretically speaking, the impedance of an ideal capacitor decreases with the increase of frequency, but due to the inductance effect of the pins at both ends of the capacitor, it should be regarded as an LC series resonant circuit at this time, and the self-resonant frequency is the SFR parameter of the device, which is It shows that when the frequency is greater than SFR, the capacitor becomes an inductance. If the ground is filtered, when the frequency exceeds SFR, the suppression of interference will be greatly reduced.
At the same time, the smaller the capacitance value, the more the curve shifts to the right, the larger the SFR, and the higher the filtered frequency.
Through this figure, it also explains why low capacitance can filter high frequencies, and high capacitance can filter low frequencies.
Having said so much, how much capacitance is appropriate to place near the pins of the chip? The following is a reference list for everyone on the Internet.
Therefore, 0.1UF is not a panacea, but in most scenarios, 0.1UF will be used. In addition, those who are engaged in the mobile phone industry should also know that the filtering frequency of 68pf is about 800M, the filtering frequency of 33pF is about 900M; the filtering frequency of 15pF is about 1800M;
Capacitors have a self-resonant frequency, which is why the filter capacitors of digital circuits rarely exceed 1UF, and most of them are 4.7UF. Of course, the capacity should not be too small, too small to meet the instantaneous charge required when the switch is switched.