Large capacitance filter low frequency, small capacitance filter high frequency? ——Selection of filter capacitance

Article 1: there has always been a puzzle: capacitance inductance is 1 / JWC, large capacitance C is large, high frequency w is also large, impedance should be very small, is not more suitable for filtering high-frequency signals? However, the fact is that large capacitance filters out low-frequency signals.

Today, I found the answer as follows:

General capacitance of about 10PF is used to filter out high-frequency interference signal, and 0.1uF is used to filter out low-frequency ripple interference, which can also play a role of stabilizing voltage

The specific capacitance value of filter capacitance depends on the main working frequency on your PCB and the harmonic frequency that may affect the system. You can check the capacitance data of relevant manufacturers or refer to the database software provided by manufacturers, and select according to the specific needs. As for the number, it's not certain. It depends on your specific needs. It's good to add one or two more. If it's not useful for the time being, you can not paste it first, and then choose the capacity value according to the actual debugging situation. If the main working frequency on your PCB is relatively low, you can add two capacitors, one to remove the ripple and the other to remove the high frequency signal. If a larger instantaneous current will appear, it is recommended to add a larger tantalum capacitor.

In fact, filtering should also include two aspects, that is, what you call large capacity and small capacity, namely decoupling and bypass. I won't say the principle. For practical use, it's OK to decouple 0.1uF for general digital circuit, which can be used for less than 10m; for more than 20m, it's better to use 1-10 UF to remove high-frequency noise, which is about C = 1 / F. The bypass is generally smaller, generally 0.1 or 0.01uF according to the resonance frequency

When it comes to capacitance, all kinds of names will make people dizzy. In fact, the principle of bypass capacitance, decoupling capacitance, filter capacitance and so on is the same. That is to say, using the characteristic of low impedance to AC signal, which can be seen from the formula of equivalent impedance of capacitance: XCAP = 1 / 2 л FC, the higher the working frequency, the greater the capacitance, the resistance of capacitance The less resistance. In a circuit, if the main function of a capacitor is to provide a low impedance path for the AC signal, it is called a bypass capacitor; if the main purpose is to increase the AC coupling between the power supply and the ground and reduce the impact of the AC signal on the power supply, it is called a decoupling capacitor; if it is used in a filter circuit, it is also called a filter capacitor; in addition, for DC voltage, the capacitor It can also be used as circuit energy storage and play the role of battery by impulse discharge. In reality, the role of capacitance is multifaceted, so we don't need to spend too much time on how to define it. In this paper, we call these capacitors used in high speed PCB design bypass capacitors

The essence of capacitance is to pass AC and isolate DC. Theoretically, the larger the capacitance is, the better.

However, due to the wiring of lead wire and PCB, in fact, capacitance is a parallel circuit of inductance and capacitance (and the resistance of capacitance itself, sometimes it can not be ignored)

This introduces the concept of resonance frequency: ω = 1 / (LC) 1 / 2

Capacitance is capacitive below resonance frequency and inductive above resonance frequency.

Therefore, large capacitance filters low-frequency waves and small capacitance filters high-frequency waves. This can also explain why the capacitance filter frequency of STM package with the same capacitance value is higher than that of dip package.

As for how much capacitance to use, this is a reference

Resonance frequency of capacitor

Capacitance dip (MHz) STM (MHz) 1.0 μ f 2.5 0.1 μ F 8 16 0.01 μ f 25 50 1000pf 80 160 100 pf 250 500 10 pf 800 1.6 (GHz)

But it's just for reference, in the words of old Engineers - mainly by experience. A more reliable way is to connect two capacitors, one large and one small, in parallel, with a difference of more than two orders of magnitude, so as to obtain a larger filtering frequency band. Generally speaking, large capacitance filters low-frequency wave and small capacitance filters high-frequency wave. The capacitance is inversely proportional to the square of the frequency you want to filter.

The specific capacitance can be selected by formula C = 4Pi * pi / (R * f * f)

How to select the power filter capacitor and master its essence and method is not difficult.

1) Theoretically, the impedance of an ideal capacitor decreases with the increase of frequency (1 / JWC). However, due to the inductance effect of the pins at both ends of the capacitor, the capacitor should be regarded as a LC series resonant circuit. The self resonant frequency is the FSR parameter of the device, which means that when the frequency is greater than the FSR value, the capacitor becomes an inductance. If the capacitor is filtered to the ground, when the frequency exceeds the FSR, the interference suppression will be greatly reduced, So we need a smaller capacitor in parallel to the ground. Why?

The reason lies in small capacitance and large SFR value, which provides a path to ground for high-frequency signals. Therefore, in power filter circuits, we often understand that the basic reason for large capacitance to consider low frequency and small capacitance to consider high frequency is that the SFR (self resonance frequency) value is different, of course, we can also think about why? If we think from this point of view, It can also understand why the capacitance of power filter should be as close to the ground as possible

2) So in the actual design, we often have questions, how do I know the SFR of the capacitor? Even if I know the SFR value, how do I choose the capacitance value of different SFR values? Is it to choose one capacitor or two capacitors?

The SFR value of the capacitor is related to the capacitance value and the pin inductance of the capacitor, so the SFR value of 04020603 with the same capacitance value or the SFR value of the direct plug-in capacitor will not be the same, of course, there are two ways to obtain the SFR value: 1) device data sheet, such as 22pf0402 capacitor SFR value is about 2G, 2) directly measure its self resonance frequency through the network analyzer, think about how to measure? S21?

After knowing the SFR value of the capacitance, use software simulation, such as rfsim99, to select one or two circuits for your power supply circuit