Large capacitance filter low frequency, small capacitance filter high frequency filter capacitor selection(II)

The essence of capacitance is through AC and DC. Theoretically, the larger the capacitance, the better. However, due to the cause of the wire and PCB wiring, in fact, the capacitance is a shunt circuit of inductance and capacitance. (and the resistance of the capacitor itself, sometimes it can not be ignored). This introduces the concept of resonant frequency: the capacitance of Omega =1/ (LC) 1/2 is capacitive under the resonant frequency, and the resonant frequency is sensitive to the upper capacitance.

Generally, large capacitance filters low frequency waves and small capacitance filters high-frequency waves. This also explains why the capacitance filter frequency of the same value STM package is higher than that of the DIP package.

As for how large the capacitance is, this is a reference capacitor resonant frequency:

Capacitance value DIP (MHz) STM (MHz)

1.0μF               2.5                5

0.1μF                8                16

0.01μF              25                50

1000pF             80                160

100 pF             250               500

10 pF              800             1.6(GHz)

However, it is only a reference. The words of old engineers are mainly based on experience.

A more reliable method is to connect one large, one small, two capacitors in parallel, and generally require a difference of two orders of magnitude to obtain a larger filtering frequency. Generally speaking, large capacitance filters out low-frequency waves, and small capacitance filters out high-frequency waves. The capacitance value is inversely proportional to the square of the frequency you want to filter out.

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