You may think that this responsibility is for DCDC or LDO? Yes, they can be done at low frequencies, but high-speed digital systems are different.

Let's take a look at the capacitor first. The function of the capacitor is simply to store charge. We all know that a capacitor should be added to the power supply to filter, and a 0.1uF capacitor is placed on the power supply pin of each chip to decouple.

Why the capacitors next to the power pins of some board chips are 0.1uF or 0.01uF. Is there anything to pay attention to?

To understand this, it is necessary to understand the actual characteristics of the capacitor. The ideal capacitor is just a storage of energy, that is, C.

The actual manufactured capacitors are not so simple. When analyzing power integrity, the commonly used capacitor models are shown in the figure below.

ESR and ESL are determined by the manufacturing process and materials of the capacitor and cannot be eliminated. What impact will these two things have on the circuit?

ESR affects the ripple of the power supply, and ESL affects the filter frequency characteristics of the capacitor.

We know that the capacitive reactance of the capacitor Zc=1/ωC, the inductive reactance of the inductor Zl=ωL, (ω=2πf), the complex impedance of the actual capacitor is Z=ESR+jωL-1/jωC=ESR+j2πfL-1/j2πfC.

It can be seen that when the frequency is very low, the capacitor works, and when the frequency is high, the effect of the inductance cannot be ignored. When the frequency is higher, the inductance takes the leading role and the capacitor loses its filtering effect.

So remember, the capacitor is not a simple capacitor at high frequency. The filter curve of the actual capacitor is shown in the figure below:

From the filter curve of the capacitor above, we can also see that it is not flat. It is like a 'V', which means that it has frequency selection characteristics. Sometimes, we hope that it is as flat as possible (previous board-level filtering)， sometimes it is hoped that the sharper the better (filtering or notching).

What affects this characteristic is the quality factor Q of the capacitor, Q=1/ωCESR. The larger the ESR, the smaller the Q and the flatter the curve. On the contrary, the smaller the ESR, the larger the Q and the sharper the curve.

Usually tantalum capacitors and aluminum electrolysis capacitors have relatively small ESL, but large ESR, so tantalum capacitors and aluminum electrolysis have a wide effective frequency range, which is very suitable for the board level filtering for power supply.

That is, in the input stage of DCDC or LDO, a larger capacity tantalum capacitor is often used for filtering.

And put some 10uF and 0.1uF capacitors close to the chip for decoupling, ceramic capacitors have very low ESR. Whether we put 0.1uF or 0.01uF close to the pins of the chip, the following are listed for your reference:

Frequency Hz |
Capacitor selection |

DC-100K |
10uF or above tantalum/ aluminum capacitors |

100K-10M |
100nF(0.1uF) ceramic capacitors |

10M-100M |
10nF(0.01uF) ceramic capacitors |

>100M |
1nF(0.001uF) ceramic capacitors |

So, don't put 0.1uF capacitors in every circuit in the future. In some high-speed systems, these 0.1uF capacitors won't work at all.