The switching power supply in the picture supplies power for A and B. The current passes through C1 and then passes through a section of PCB trace (for the time being, it is equivalent to an inductance. Actually, it is wrong to analyze this equivalent with electromagnetic wave theory, but for the convenience of understanding, this equivalent method is still used.) Separate the two paths. Supply A and B. The ripple from the switching power supply is relatively large, so we use C1 to filter the power supply to provide a stable voltage for A and B. C1 needs to be placed as close to the power supply as possible. C2 and C3 are bypass capacitors, which play a decoupling role. When A needs a large current at a certain moment, if there are no C2 and C3, then the voltage of terminal A will be lower due to the line inductance, and the voltage of terminal B will also be affected by the voltage of terminal A and reduce, so the local circuit The current change of A causes the power supply voltage of the local circuit B, which affects the signal of the B circuit. Similarly, the current change of B will also interfere with A. This is "common path coupling interference".
After adding C2, when the local circuit needs a large instantaneous current, the capacitor C2 can temporarily provide current for A. Even if the common circuit part inductance exists, the voltage at terminal A will not drop too much. The impact on B will also be much reduced. So the decoupling function is played by the current bypass.
Generally, large-capacity capacitors are mainly used for filtering, and the speed requirements are not very fast, but the requirements for the capacitance value are relatively large. Generally, aluminum electrolytic capacitors are used. When the surge current is small, it is better to use tantalum capacitors instead of aluminum electrolytic capacitors. From the above example, we can know that as a decoupling capacitor, it must have a fast response speed to achieve the effect. If the local circuit A in the figure refers to a chip, ceramic capacitors should be used for the decoupling capacitors, and the capacitors should be as close as possible to the power pins of the chip. If “local circuit A” refers to a functional module, ceramic capacitors can be used. If the capacity is not enough, tantalum capacitors or aluminum electrolytic capacitors can also be used (provided that each chip in the functional module has decoupling capacitors—ceramic capacitors ). The capacity of the filter capacitor can often be found in the data sheet of the switching power supply chip. If the filter circuit uses electrolytic capacitors, tantalum capacitors and ceramic capacitors at the same time, place the electrolytic capacitors closest to the switching power supply to protect the tantalum capacitors. The ceramic capacitor is placed behind the tantalum capacitor. In this way, the best filtering effect can be obtained.
Decoupling capacitors need to meet two requirements, one is capacity requirements, and the other is ESR requirements. That is to say, the decoupling effect of a 0.1uF capacitor may not be as good as two 0.01uF capacitors. Moreover, 0.01uF capacitors have lower impedance in higher frequency bands. If a 0.01uF capacitor can meet the capacity requirements in these frequency bands, then it will have a better decoupling effect than 0.1uF capacitors.
When we are in the circuit design diagram, we will see different capacitor values, such as 0.1uF and so on. The value of the capacitor is also related to the frequency range of the chip we selected.
Detailed explanation: https://www.21ic.com/jichuzhishi/analog/questions/2013-12-05/197635.html
| Frequency range (HZ) Capacitance value | |
| DC-100K | Tantalum capacitor or aluminum electrolytic above 0uF |
| 100K-10M | 100nF (0.1uF) ceramic capacitor |
| 10M-100M | 10nF (0.01uF) ceramic capacitor |
| >100M | 1nF (0.001uF) ceramic capacitor and the capacitance of PCB ground plane and power plane |
So, don't see 0.1uF capacitors in everything in the future. In some high-speed systems, these 0.1uF capacitors won't work at all.
Ceramic capacitors are generally used for signal source filtering, and electrolytic capacitors are generally used for power supply parts.
On the hardware development board, there are usually many 0.1uF non-electrolytic capacitors and 10uF electrolytic capacitors connected in parallel between the DC power supply and the ground.
The purpose of these capacitors is to make a low impedance between the power line and the ground line, and the power supply is close to an ideal voltage source. You can say that it is filtering, but you need to figure out what wave is being filtered. It is not to filter the ripple of the power supply, but the ripple caused by the current change of a certain chip on the power line, so that it does not affect other chips.
Use 0.1uF non-polar capacitor and 10uF electrolytic capacitor in parallel because the parasitic inductance of electrolytic capacitor is relatively large, and the ability to eliminate high-frequency ripple is poor. The non-polar capacitor has a small parasitic inductance and a better ability to filter high-frequency ripples. However, if the capacity is selected according to the requirements of low frequency, the non-polar capacitor is too large and the cost is high, the electrolytic capacitor is small, and the same capacity is cheaper. Therefore, two types of capacitors are used in parallel.
If you design your own circuit, you should use it in this way, and the location and wiring of each capacitor are very particular.
I can only say two principles:
1) The connecting wires between the two ends of the small-capacity non-polar capacitors and the power pins and ground pins of the chip should be as short as possible, and the shorter the better.
2) The power supply is usually introduced by other circuit boards, and there are usually only one or two electrolytic capacitors on each circuit board. For an electrolytic capacitor, put it where the power supply enters the circuit board. At this time, the electrolytic capacitor is of course far away from the chips, but since the electrolytic capacitor mainly works at a lower frequency, it does not matter if it is a little farther away. If two electrolytic capacitors are used on the circuit board, the other one is placed near the chip that consumes the most power.
These are related to the layout of the circuit board components and the wiring arrangement of the ground wire (multilayer boards usually have a ground layer).
0.1μF capacitor for noise below 10MHz works well.
Press C=1/F, that is, take 0.1μF for 10MHz
Simply put, connect the interference to ground through a capacitor