Filter capacitor selection: large capacitor filter low frequency, small capacitor filter high frequency?

There has always been a doubt: the capacitive inductance is 1/jwC, the large capacitance C is large, the high frequency w is also large, the impedance should be small, is not more suitable for filtering high frequency signals?

However, the fact is: large capacitors filter out low frequency signals.

Find the answer today as follows:

The general 10PF capacitor is used to filter out high-frequency interference signals. The 0.1UF filter is used to filter out low-frequency ripple interference. It can also be used as a voltage regulator. The filter capacitor should be selected according to your PCB. The main working frequency and the harmonic frequency that may affect the system, you can check the capacitance data of the relevant manufacturers or refer to the database software provided by the manufacturer, and choose according to the specific needs. As for the number is not necessarily, look at your specific needs, plus one or two is also very good, temporarily useless can not be posted first, according to the actual debugging situation and then choose the value.

If your main working frequency on the PCB is low, add two capacitors, one to avoid ripple and one to avoid high frequency signals. If a large instantaneous current occurs, it is recommended to add a larger tantalum capacitor.

In fact, the filter should also contain two aspects, that is, the large and small values ​​that you said are decoupling and bypass. I will not say the principle, the practical point, the general digital circuit decoupling 0.1uF can be used for 10M or less; 20M or more with 1 to 10 uF, remove high frequency noise better, probably by C = 1 / f. Bypass is generally relatively small, typically 0.1 or 0.01 uF depending on the resonant frequency.

When it comes to capacitors, all kinds of names can make people feel dizzy, bypass capacitors, decoupling capacitors, filter capacitors, etc. In fact, no matter how they are called, the principle is the same, that is, using the AC signal Low impedance characteristics, which can be seen by the equivalent impedance equation of the capacitor: Xcap = 1/2 лfC, the higher the operating frequency, the larger the capacitance value, the smaller the impedance of the capacitor.

In the circuit, if the main function of the capacitor is to provide a low-impedance path to the AC signal, it is called a bypass capacitor; if it is mainly to increase the AC coupling between the power supply and the ground, and reduce the influence of the AC signal on the power supply, it can be called For decoupling capacitors; if used in a filter circuit, it can also be called a filter capacitor; in addition, for DC voltage, the capacitor can also be used as a circuit to store energy, using the discharge to function as a battery.

In reality, the role of capacitors is often multifaceted. We don't have to think too much about how to define them. In this paper, we uniformly apply these capacitors in high-speed PCB design as bypass capacitors.

The essence of the capacitor is through AC and DC. In theory, the larger the capacitor for power supply filtering, the better. However, due to lead and PCB wiring, the capacitor is actually a parallel circuit of inductor and capacitor (and the resistance of the capacitor itself, sometimes not negligible). This introduces the concept of resonant frequency: ω = 1 / (LC) / 1 2 Capacitance is capacitive below the resonant frequency, and the capacitance above the resonant frequency is inductive.

Therefore, the general capacitor filter low frequency wave, small capacitor filter high frequency wave. This also explains why the capacitor filter frequency of the same value STM package is higher than the DIP package.

As for how much capacitor is used, 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)

But just a reference, in the words of the old engineer - mainly rely on experience.

A more reliable approach is to connect two large and one small capacitors in parallel, generally requiring more than two orders of magnitude difference to obtain a larger filter band.

Generally speaking, a large capacitor filters out low frequency waves, and a small capacitor filters out high frequency waves. The capacitance value is inversely proportional to the square of the frequency you want to filter out.

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

How to choose the power filter capacitor, master the essence and method, in fact, it is not difficult.

1) Theoretically ideal capacitors whose impedance decreases with increasing frequency (1/jwc), but due to the inductive effect of the pins at both ends of the capacitor, the capacitor should be considered as an LC series resonant circuit, the self-resonant frequency The FSR parameter of the device, which means that the capacitor becomes an inductor when the frequency is greater than the FSR value. If the capacitor is filtered to the ground, the suppression of the interference is greatly reduced when the frequency exceeds the FSR, so a smaller capacitor is required to be connected in parallel. Can you think about why?

The reason is that the small capacitor has a large SFR value and provides a ground path for the high frequency signal. Therefore, in the power supply filter circuit, we often understand that the large capacitor considers the low frequency and the small capacitor considers the high frequency. The fundamental reason is the SFR (self-resonance). The frequency value is different, of course, you can think about why? If you think from this point of view, you can understand why the capacitor in the power supply filter should be as close as possible to the ground.

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 a capacitor or two capacitors? The SFR value of the capacitor is related to the capacitance value, and is related to the pin inductance of the capacitor. Therefore, the SFR value of the 0402, 0603, or the in-line capacitor of the same capacitance value is not 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 the self-resonant frequency through the network analyzer, think about how to measure? S21?

After knowing the SFR value of the capacitor, use software simulation, such as RFsim99, select one or two circuits to see if there is enough noise suppression ratio in the working frequency band of the circuit you are supplying. After the simulation, it is the actual circuit test, such as debugging the mobile phone. When receiving sensitivity, LNA power supply filtering is the key, good power supply filtering can often improve several dB.

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Generally speaking, the capacitor is regarded as a leaking water. When the peak of the alternating current comes, it is regarded as adding water to the pregnant child. If the amount of water leakage is equal, then the frequency of adding water is high. In this way, the water level can be kept high. On the contrary, the water level in the low frequency of the water is small, and the water level in the arms has dropped much after the second water coming in, so it is necessary to use a large water to alleviate the water leakage. The water level is falling.