There are many types of capacitors, but no matter how they are classified, the basic principle is to use capacitors to show a low resistance to the alternating signal. The higher the frequency f of the alternating current, the lower the impedance of the capacitor. The main function of the bypass capacitor is to provide a low impedance path for the AC signal; the main function of the decoupling capacitor is to provide a local DC power supply to the active device to reduce the propagation of switching noise on the board and guide the noise to ground After adding the decoupling capacitor, the voltage ripple interference will be significantly reduced; the filter capacitor is often used in the filter circuit.
For an ideal capacitor, without considering the influence of parasitic inductance and resistance, then there is no concern in the design of the capacitor, the larger the value of the capacitor, the better. But the actual situation is very different. It is not that the larger the capacitance is, the more beneficial it is to the high-speed circuit. Instead, the smaller capacitance can be applied to high frequencies.
Filter capacitors are used in power supply rectifier circuits to filter out AC components and make the output DC smoother. Decoupling capacitors are used in amplifier circuits where AC is not needed, to eliminate self-excitation and make the amplifier work stably. The bypass capacitor is used when a resistor is connected, and it is connected to both ends of the resistor to make the AC signal pass smoothly.
1. Understanding of decoupling capacitor energy storage
(1) The decoupling capacitor is mainly to remove the interference of high-frequency such as RF signals. The way of the interference is through electromagnetic radiation. In fact, the capacitor near the chip also has the function of energy storage, which is the second place. You can think of the total power supply as a reservoir. Every household in our building needs water supply. At this time, the water does not come directly from the reservoir. That is too far away. When the water comes over, we are already thirsty. The actual water comes from the water tower on top of the building. The water tower is actually a buffer. From a microscopic perspective, when a high-frequency device is in operation, its current is discontinuous and the frequency is very high, and the device VCC has a certain distance from the total power supply, even if the distance is not long, at high frequencies, the impedance Z =i*wL+R, the inductance of the line will also be very large, which will cause the device to be unable to be supplied in time when it needs current. The decoupling capacitor can make up for this deficiency. This is one of the reasons why many circuit boards place small capacitors on the VCC pins of high-frequency devices (a decoupling capacitor is usually connected in parallel with the Vcc pin, so that the AC component is grounded from this capacitor.
(2) The high-frequency switching noise generated by the active device during switching will propagate along the power line. The main function of the decoupling capacitor is to provide a local DC power supply to the active device to reduce the propagation of switching noise on the board and guide the noise to ground.
2. The difference between bypass capacitor and decoupling capacitor
Decoupling: Remove RF energy that enters the distribution network from high-frequency devices when devices are switched. Decoupling capacitors can also provide a localized DC voltage source for the device, which is particularly useful in reducing inrush current across the board.
Bypass: Transfer unwanted unwanted common-mode RF energy from components or cables. This is mainly by generating AC bypass to eliminate unintentional energy into sensitive parts, and also can provide baseband filtering function (bandwidth limited).
We can often see that there is a decoupling capacitor connected between the power supply and ground. It has three functions: one is the energy storage capacitor of this integrated circuit; the other is to filter out the high-frequency noise generated by the device and cut it off. The propagation path through the power supply circuit; 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 play a role in anti-interference, the capacitors are located in different positions, the term is different. For the same circuit, the bypass capacitor takes the high-frequency noise in the input signal as the filtering object, filters the high-frequency clutter carried by the previous stage, and the decoupling capacitor is also called decoupling The capacitor is to filter the interference of the output signal.
From the circuit point of view, there is always a source of driving and a load being driven. If the load capacitance is relatively large, the drive circuit must charge and discharge the capacitor to complete the signal transition. When the rising edge is steeper, the current is relatively large, so that the drive current will absorb a large power supply current. Inductance, resistance (especially the inductance on the chip pin, will produce a rebound), this current is actually a kind of noise relative to the normal situation, will affect the normal operation of the previous stage, this is coupling.
The decoupling capacitor acts as a battery to meet the change of the drive circuit current and avoid coupling interference.
The bypass capacitor is actually decoupled, but the bypass capacitor generally refers to a high-frequency bypass, which is to increase a low-impedance leakage prevention method for high-frequency switching noise. The high-frequency bypass capacitor is generally small, according to the resonant frequency is generally 0.1u, 0.01u, etc., while the decoupling capacitor is generally larger, 10u or greater, according to the distribution parameters in the circuit, and the size of the drive current to determine.
Both decoupling and bypass can be seen as filtering. The decoupling capacitor is equivalent to the battery, to avoid the voltage drop due to the sudden change of current, which is equivalent to filtering ripple. The specific capacitance can be calculated according to the size of the current, the expected ripple size, and the size of the action time. Decoupling capacitors are generally very large and are basically ineffective against higher frequency noise. The bypass capacitor is for high frequency, that is, the frequency impedance characteristic of the capacitor is used. The capacitor can generally be regarded as a RLC series model. At a certain frequency, resonance occurs, and the impedance of the capacitor is equal to its ESR. If you look at the frequency impedance curve of the capacitor, you will find that it is generally a V-shaped curve. The specific curve is related to the medium of the capacitor, so the bypass medium also needs to consider the medium of the capacitor. A safer method is to add more capacitors.
The decoupling capacitor has two functions between the power supply and ground of the integrated circuit: on the one hand, it is the storage capacitor of the integrated circuit, on the other hand, it bypasses the high-frequency noise of the device. A typical decoupling capacitor value in digital circuits is 0.1μF. The typical value of the distributed inductance of this capacitor is 5μH. The 0.1μF decoupling capacitor has a distributed inductance of 5μH, and its parallel resonance frequency is about 7MHz, that is to say, it has a good decoupling effect for noise below 10MHz, and has little effect on noise above 40MHz. 1μF, 10μF capacitors, parallel resonance frequency above 20MHz, the effect of removing high-frequency noise is better. Every 10 or so integrated circuits need to add a charge and discharge capacitor, or an energy storage capacitor, optional about 10μF. It is better not to use electrolytic capacitors. The electrolytic capacitors are rolled up with two layers of film. This rolled-up structure behaves as an inductance at high frequencies. To use tantalum capacitors or polycarbonate capacitors. The selection of decoupling capacitors is not strict, according to C = 1/F, that is, 10MHz to take 0.1μF, 100MHz to take 0.01μF.
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