The relationship between the capacity of filter capacitance and the frequency of filtered noise

The choice of decoupling capacitance does not have the exact corresponding relationship with frequency, theoretically the larger the better, but in reality all devices are not ideal devices, no matter what kind of capacitance, ESL and ESR are inevitable, so the frequency response curve of the actual capacitance is obviously nonlinear, only in a certain frequency range, it basically conforms to the theoretical calculation results of pure capacitance, beyond a certain limit, it will be theoretical The worse the value is, the farther the value is, and even the capacitance will no longer be a capacitance after exceeding a certain degree. This frequency is called "self resonance frequency". Under the same material and manufacturing process, the smaller the capacity, the higher the self resonance frequency of the capacitance. Therefore, the selection of decoupling capacitor should consider not only the frequency, but also the load. After a certain frequency, the capacitor material and production process should be considered. On this basis, the comprehensive results determine the capacity and type of decoupling capacitor.

Generally, the noise frequency of digital circuit is in the range of megahertz to 100m, and the appropriate effect can be obtained by using 0.1uF capacitor of ceramic monolith medium. If the load is heavy or the noise is strong, the capacitor with larger capacity can be selected or multiple capacitors can be paralleled. Under the same capacity and capacitor material, the effect of small capacitance paralleling is stronger than that of single large capacitance, and the higher the frequency is, the more obvious the effect is For high-frequency decoupling, parallel connection of large and small capacitors is needed to deal with noise of different frequency spectrums.

Selection principle of capacity of general decoupling capacitor:

Light load below 100m: 0.1uF, heavy load or the capacitor with 1-10uf can be added in case of large low-frequency noise, and ceramic or tantalum is suitable as the dielectric material;

100m-1000m: the above-mentioned + 100-1000pf (+ 10PF), whether it is necessary to select according to the high limit of frequency in brackets, the medium selection of small capacitance must be high-frequency ceramics, and mica is often used in the early stage,

More than 1G: above + 1-10pf, the best medium is high Q microwave ceramic material.

When high frequency and heavy load, several small capacitors must be used in parallel to cut off the large capacitance directly.

By the way, technically, decoupling capacitance is not commonly known as filtering capacitance. Filtering capacitance refers to power supply system. Decoupling capacitance is a special filtering capacitance distributed near the device or sub circuit, which is mainly used to deal with the device itself or external noise. Therefore, decoupling capacitance means "removing (noise) coupling".