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PCB News - The difference between bypass capacitor and decoupling capacitor in PCB design

PCB News

PCB News - The difference between bypass capacitor and decoupling capacitor in PCB design

The difference between bypass capacitor and decoupling capacitor in PCB design

2021-11-01
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Author:Kavie

The difference between bypass capacitor and decoupling capacitor


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Bypass: Transfer unwanted common-mode RF energy from components or cables. This is mainly to eliminate unintentional energy from entering the sensitive part by generating an AC bypass. In addition, it can also provide a baseband filtering function (limited bandwidth).

Decoupling: Remove the RF energy that enters the power distribution network from high-frequency devices during device switching. The decoupling capacitor can also provide a localized DC voltage source for the device, which is particularly useful in reducing the surge current across the board.

We can often see that a decoupling capacitor is connected between the power supply and the ground. It has three functions: one is the energy storage capacitor of the integrated circuit; the other is to filter out the high-frequency noise generated by the device and cut it off. The path for propagation through the power supply loop; the third is to prevent the noise carried by the power supply from interfering with the circuit.

In electronic circuits, decoupling capacitors and bypass capacitors both play an anti-interference role. The location of the capacitor is different, and the name is different. For the same circuit, the bypass capacitor takes the high-frequency noise in the input signal as the filtering object to filter out the high-frequency clutter carried by the previous stage, and the decoupling capacitor is also called decoupling. Capacitors take the interference of the output signal as the filtering object.

The reason why a small capacitor is connected in parallel with a large capacitor

Due to the large capacity of large capacitors, the volume is generally relatively large, and they are usually made by multi-layer winding, which leads to relatively large distributed inductance of large capacitors (also called equivalent series inductance, or ESL in English). As we all know, the impedance of inductors to high-frequency signals is very large, so the high-frequency performance of large capacitors is not good. Some small-capacity capacitors are just the opposite. Because of their small capacity, the volume can be made small (shortening the lead, reducing the ESL, because a piece of wire can also be regarded as an inductance), and often use plate capacitors Structure, such a small capacity capacitor has a small ESL so that it has good high frequency performance, but due to the small capacity, the impedance to low frequency signals is large. Therefore, if we want to pass low-frequency and high-frequency signals well, we use a large capacitor and then a small capacitor. The commonly used small capacitors are 0.1uF ceramic capacitors. When the frequency is higher, smaller capacitors can be connected in parallel, such as a few pF or hundreds of pF. In digital circuits, a 0.1uF capacitor is generally connected in parallel to the power pin of each chip to the ground (this capacitor is called a decoupling capacitor, of course, it can also be understood as a power filter capacitor, the closer the chip is, the better), because The signals in these places are mainly high-frequency signals, and it is enough to use a smaller capacitor to filter.


The above is an introduction to the difference between bypass capacitors and decoupling capacitors in PCB design. Ipcb is also provided to PCB manufacturers and PCB manufacturing technology.