Capacitors are very important in EMC design and are also our commonly used filtering components! However, in the process of my training, I found that everyone is not very clear about the use of capacitors! Here I will introduce the two main points of capacitive filtering:
    1, capacitive filter is a frequency band, many people think that the larger the capacitor is the better, in fact, each capacitor has a certain filter frequency band, large capacitor filter low frequency, small capacitor filter high frequency, mainly according to the resonant frequency of the capacitor to decide, the capacitor at the resonant frequency point has the best filtering effect! In the resonant point as the center of a band within the better filtering effect, the other parts of the filtering effect is not good! The resonant point of the capacitor is related to the capacitance value of the capacitor and ESL (equivalent series inductance), and you can check the online information, as well as the theoretical analysis of the series resonant circuit to learn in the conference school! It is usually recommended to add a UF capacitor to the power port to filter the differential mode interference between a few hundred KHZ and 5MHZ, because the resonant point of the UF capacitor is about 1MHZ. In addition, it is recommended to add 1nF chip capacitor on high frequency digital circuits, the reason is that the resonant frequency of 1nf capacitor is between 100MHZ, and the resonant frequency points of different manufacturers are different, so it is better to filter tens of MHZ to 200MHZ interference, which is conducive to solving EMI problems!
    2, the capacitor is selected, does not mean that it can filter out interference! When the river floods and reaches the high water level, we often add a ditch to divert the water, then the place that is led to must be a low water level, if it is led to a high water level reservoir, it will cause the water back and raise the water level. Capacitive filtering is the same as the water control problem, the capacitor only plays a role in a ditch, and whether it can be filtered depends on the size of the ground interference connected with the capacitor. We often find that engineers solve interference problems with capacitors have no effect, and to a large extent, the ground interference itself is very large! Instead, it leads the ground interference to the signal or power supply! It is important to note that in some cases, ground interference is not the least! Therefore, we emphasize that there is an important basis for filtering, that is, the ground should have little interference, which is usually said to be "static".
    Therefore, to achieve the filtering effect when we take capacitive filtering, we must choose the appropriate capacitance and connect the ground with relatively small interference! The capacitor can be based on the device manual and experience, and the instrument method can be taken when debugging the ground with small interference, and experienced engineers should consider the schematic diagram and PCB in the early stage.