Two points of capacitive filtering

Capacitors are very important in EMC design and are also our common filter components! But in the course of my training, we found that everyone's use of capacitors is not very clear! Here I introduce two points of capacitance filtering:

1, the capacitor filter is a frequency band, many people think that the capacitance is bigger the better, in fact, otherwise, each capacitor has a certain filter band, large capacitor filter low frequency, small capacitor filter high frequency, mainly based on the resonant frequency of the capacitor Determined that the capacitor has the best filtering effect at the resonant frequency point! In the section of the frequency centered on the resonance point, there is a good filtering effect, and other parts have poor filtering effect! The resonance point of the capacitor is related to the capacitance value of the capacitor and the ESL (Equivalent Series Inductance). You can check the information on the net and the theoretical analysis of the series resonance circuit learned by the conference school will know! Normally, we recommend adding a UF capacitor at the power port to filter differential mode interference from a few hundred kilohertz to 5 megahertz. The reason is that the UF level capacitor has a resonance point of about 1 MHz. It is also recommended to add high-frequency digital circuit, we recommend adding 1nF chip capacitors, because the 1nf capacitor resonant frequency between 100MHZ, different manufacturers have different resonant frequency, this filter is better MHZ to 200MHZ interference, favorable Solve the problem with EMI!

2. Capacitor selection is good, does not mean that it can filter out interference! When the river floods and reaches a high water level, we often increase the drainage of a ditch. Then the location cited must be a low water level. If it leads to a reservoir with a high water level, it will cause water intrusion and raise the water level. Capacitance filtering and water management problems are the same. Capacitance only plays a role in the development of a trench. Filtering can also depend on the size of ground disturbances connected to the capacitor. We often find that engineers have no effect in solving interference problems and adding capacitors. To a large extent, ground disturbance itself is very large! Instead, ground disturbances lead to signals or power sources! Everyone needs to pay attention to the fact that ground disturbance is not the smallest in some situations! Therefore, we emphasize that there is an important basis for filtering, that is, the place to be interfered with is small, which is usually referred to as "static ground."

So, when we use capacitive filtering to achieve the filtering effect, we must choose the appropriate capacitance and then connect the interference to a smaller place! Capacitance can be based on the device manual and experience, small interference can be taken in the debugging method of the instrument, experienced engineers in the previous schematic and PCB should be considered