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

There has been a doubt: the capacitance inductance is 1/jwC, large capacitance C large, high frequency w is also large, the impedance should be very small, not more suitable for filtering high frequency signals? However, the fact is: large capacitors filter low frequency signals.

The answer is as follows:

The general 10PF capacitance is used to filter the high frequency interference signal, 0.1UF is used to filter the low-frequency ripple interference, can also play a stabilizing effect of the filter capacitor specific selection of what capacity depends on the frequency of the main working frequency on your PCB and the possible impact of the harmonic frequency on the system, can check the related manufacturers' electricity. The data or reference software provided by the manufacturer is selected according to specific needs. As for the number is not necessarily, see your specific needs, add more than one or two are also good, temporarily not to use the first do not stick, according to the actual debugging situation, then choose the capacity. If the main frequency of your PCB is relatively low, add two capacitors. If there is a relatively large instantaneous current, it is recommended to add a larger tantalum capacitor.

In fact, filtering should also contain two aspects, that is, what you call large capacity and small capacitance values, that is, decoupling and bypass. The principle I do not say, practical point, the general digital circuit decoupling 0.1uF can be used for 10M below; 20M above the use of 1 to 10 uF, high frequency noise is better, about C=1/f. Bypass is generally smaller, generally based on the resonance frequency is generally 0.1 or 0.01uF.

When it comes to capacitance, all kinds of calls make people dizzy, bypass capacitors, decoupling capacitors, filter capacitors, and so on. In any case, the principle is the same, that is, using the characteristic of low impedance to the AC signal, this point can be seen through the equivalent impedance formula of the capacitance: Xcap=1/2 fC, The higher the working frequency and the larger the capacitance, the smaller the impedance of the capacitor. In the circuit, if the main function of the capacitor is to provide a low impedance channel for the AC signal, it is called the bypass capacitor. If it is mainly to increase the AC coupling between the power and the ground and reduce the influence of the AC signal on the power supply, it can be called the decoupling capacitor; if used in the filter circuit, it can be called the filter capacitor. In addition, for DC voltage, capacitors can also serve as circuit energy storage, and use impulse discharge to play the role of batteries. In fact, the role of capacitance is multifaceted, so we don't need to think too much about how to define it. In this paper, we have unified these capacitors used in high-speed PCB design as bypass capacitors.

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