How to distinguish between the filter capacitor, decoupling capacitor, a bypass capacitor

The types of capacitors are complicated, but no matter how they are classified, the basic principle is to use capacitors to make the alternating signal low impedance. 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 to 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 direct the noise to ground. The ripple interference of the voltage after the addition of the decoupling capacitor is significantly reduced; the filter capacitor is often used in the filter circuit.

For an ideal capacitor, regardless of the effects of parasitic inductance and resistance, there is no concern about the capacitor design. The larger the value of the capacitor, the better. However, the actual situation is quite different. It is not that the larger the capacitor, the better the high-speed circuit, but the small capacitor can be applied to the high frequency.

The filter capacitor is used in the power rectifier circuit to filter out the AC components and make the output DC smoother. The decoupling capacitor is used in the amplifier circuit where no AC is needed to eliminate self-excitation and stabilize the amplifier. The bypass capacitor is used when there is a resistor connection and is connected to both ends of the resistor to make the AC signal pass smoothly.

1. Understanding the decoupling capacitor energy storage

(1) The decoupling capacitor is mainly to remove the interference of high frequency such as RF signal, and the way of entering the interference is through electromagnetic radiation. In fact, the capacitor near the chip also has the function of energy storage, which is the second. You can think of the total power supply as a reservoir. Every household in our building needs water. At this time, the water is not directly from the reservoir. It is too far away. When the water comes, we are already thirsty. The actual water is from the water tower on the top of the building. The water tower is actually a buffer. If microscopically, when the high-frequency device is working, its current is discontinuous and the frequency is high, and the device VCC has a distance to the total power supply, even if the distance is not long, at a high frequency, the impedance Z =i*wL+R, the inductance of the line will also be very large, which will cause the device to be supplied in time when current is needed. Decoupling capacitors can make up for this deficiency. This is one of the reasons why many boards place small capacitors on the VCC pin of the high-frequency device (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 direct the noise to ground.

2. The difference between the bypass capacitor and the decoupling capacitor

Decoupling: Removes RF energy from the high-frequency device into the distribution network during device switching. Decoupling capacitors also provide a localized DC voltage source for the device, which is especially useful in reducing cross-board surge currents.

Bypass: Remove unwanted common-mode RF energy from components or cables. This is mainly to eliminate unintentional energy into the sensitive part by generating AC bypass, and also to provide baseband filtering (bandwidth limited).

We can often see that a decoupling capacitor is connected between the power supply and the ground. It has three functions: one is the 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 through which the power supply loop propagates; the third is to prevent the noise carried by the power supply from interfering with the circuit.

In the electronic circuit, the decoupling capacitor and the bypass capacitor both play an anti-interference function. The position of the capacitor is different, and the calling is different. For the same circuit, the bypass capacitor uses the high-frequency noise in the input signal as the filtering object to filter the high-frequency noise carried by the pre-stage, and the decoupling capacitor is also called decoupling. Capacitance is to filter the interference of the output signal.

From the circuit, there is always a source of drive and a driven load. 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 steep, the current is relatively large, so that the driven current will absorb a large supply current, due to the circuit. The inductance, the resistance (especially the inductance on the chip pin, will produce a rebound), this current is actually a kind of noise compared to the normal situation, which will affect the normal operation of the previous stage, which is the coupling.

The decoupling capacitor acts as a battery to meet the changes in the drive circuit current and avoid mutual coupling interference.

The bypass capacitor is also actually decoupled, except that the bypass capacitor generally refers to the high-frequency bypass, which is to increase the low impedance leakage path for the high-frequency switching noise. The high-frequency bypass capacitor is generally small, and the resonant frequency is generally 0.1u, 0.01u, etc., and the decoupling capacitor is generally large, 10u or more, depending on the distributed parameters in the circuit and the magnitude of the change in the drive current.

Both decoupling and bypassing can be seen as filtering. The decoupling capacitor is equivalent to the battery, avoiding a voltage drop due to a sudden change in current, which is equivalent to a filter wave. The specific capacitance can be calculated according to the magnitude of the current, the desired ripple size, and the magnitude of the action time. Decoupling capacitors are generally large and are essentially ineffective for higher frequency noise. The bypass capacitor is for high frequency, that is, the frequency impedance characteristic of the capacitor is utilized. Capacitors can generally be viewed as an 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 plot 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 choice of bypass capacitor also considers the medium of the capacitor. A safer method is more than a few capacitors.

The decoupling capacitor has two functions between the integrated circuit power supply and ground: on the one hand, the storage capacitor of the integrated circuit, and on the other hand, the high-frequency noise of the device is bypassed. A typical decoupling capacitor value in a digital circuit is 0.1 μF. The typical value of the distributed inductance of this capacitor is 5μH. The 0.xn--1f-99b decoupling capacitor has a distributed inductor of 5μH, and its parallel resonant frequency is about 7MHz. That is to say, it has a better decoupling effect for noise below 10MHz, and has little effect on noise above 40MHz. 1μF, 10μF capacitor, parallel resonance frequency above 20MHz, the effect of removing high-frequency noise is better. For every 10 or so ICs, add a charge and discharge capacitor, or a storage capacitor, about 10μF. It is best not to use electrolytic capacitors, which are rolled up by two layers of film. This rolled structure behaves as an inductor at high frequencies. Use tantalum or polycarbonate capacitors. The selection of decoupling capacitors is not critical. It can be taken as C=1/F, that is, 0.xn--1f-99b at 10MHz and 0.xn--01f-yyc at 100MHz.

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