I often see a small capacitor in parallel on a large capacitor. Why?

Link: http://computer00.21ic.org/user1/2198/archives/2006/11027.html

 

Because the capacity of the large capacitor is large, the volume is also relatively large, and it is usually made by means of multilayer winding. (It should be very experienced when you split the aluminum electrolysis capacitor, you can also take several different capacitors to see if they have not been split, but pay attention to safety and do not hurt your hands), which leads to a large distribution inductance of large capacitors (also called Equivalent series Inductor , Abbreviation ESL ). As we all know, inductance has a great impedance on high-frequency signals, so the high-frequency performance of large capacitors is not good. Some small capacity capacitors are just the opposite. Because the capacity is small, the volume can be very small (shortening the lead and reducing it ). ESL Because a wire can also be regarded as an inductor), and the flat capacitor structure is often used, so that small capacity capacitors have very small ESL In this way, it has a good high-frequency performance, but because of the small capacity, the impedance of the low-frequency signal is large. Therefore, if we want to make the low-frequency and high-frequency signals pass well, we will use a large capacitor and then a small capacitor. The commonly used small capacitance is 0.1 UF When the frequency is higher, a smaller capacitor can be connected in parallel, such PF , Hundreds PF . In a digital circuit, the power pins of each chip must be connected in parallel. 0.1 UF The capacitance to the ground (this capacitor is called the decoupling capacitor, of course, can also be understood as the power filter capacitor. The closer it is to the chip, the better), because the signal in these places is mainly a high-frequency signal, you can use a small Capacitor filter.