The role of the capacitor in the PCB LAYOUT
It is used in the power rectifier circuit to filter out the AC components and make the output DC smoother.
It is used in the amplifying circuit where no communication is needed to eliminate self-excitation and make the amplifier work stably.
When used in a resistor connection, connect the two ends of the resistor to make the AC signal pass smoothly.
1. Understanding the decoupling capacitor energy storage
1.1 Decoupling capacitors are mainly used to remove high frequency interference such as RF signals. The way to enter interference is through electromagnetic radiation.
In fact, the capacitor near the chip also has the function of storing energy. This is the second place. You can think of the total power supply as the Miyun Reservoir. Every household in our building needs water supply. At this time, the water is not directly from the reservoir. The distance is too far, and 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 from 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. The decoupling capacitor can make up for this deficiency. This is why many boards are in the high-frequency device VCC tube. One of the reasons for placing a small capacitor at the foot is that a decoupling capacitor is usually connected in parallel with the vcc pin so that the AC component is grounded from this capacitor.
1.2 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 bypass capacitor and decoupling capacitor
Decoupling: Removes RF energy from high-frequency devices into the distribution network during device switching. Decoupling capacitors can also be used as a localized DC voltage source for devices that are particularly useful in reducing cross-board surge currents.
Bypass: Unwanted common-mode RF energy is removed from a component or cable. This is primarily achieved by creating an AC bypass to eliminate unintentional energy from entering the sensitive portion, and also providing baseband filtering (bandwidth limited).
We can often see decoupling capacitors between the power supply and ground. It has three functions:
1. An energy storage capacitor of an integrated circuit;
2. Filter out the high frequency noise generated by the device and cut off the path through which the power supply loop propagates;
3. Prevent noise from the power supply from interfering with the circuit.
In electronic circuits, decoupling capacitors and bypass capacitors are anti-interference. The position of the capacitor is different, and the calling is different. For the same circuit, the bypass capacitor is the input. The high-frequency noise in the signal is used as a filtering object to filter the high-frequency noise carried by the pre-stage, and the decoupling capacitor is also called the decoupling capacitor, which is to filter the interference of the output signal from the circuit. There is always a source of driving and a load to be driven. If the load capacitance is relatively large, the driver 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 driver is driven. The current will absorb a large supply current. Due to the inductance in the circuit, the resistance (especially the inductance on the chip pin will rebound), this current is actually a kind of noise compared to the normal situation, which will affect The normal operation of the pre-level. This 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 actually decoupled, except that the bypass capacitor generally refers to the high-frequency bypass, which is to improve the low-frequency leakage prevention circuit for the high-frequency switching noise. The high-frequency bypass capacitor is generally small, according to the resonant frequency. Generally, it is 0.1μF, 0.01μF, etc., and the decoupling capacitor is generally large, 10μF or more, which is determined according to the distribution parameters in the circuit and the change of the driving current.
Both decoupling and bypass can be seen as filtering. Decoupling capacitors are equivalent to batteries, avoiding sudden changes in current
The voltage drop is equivalent to the filter wave. The specific capacitance can be calculated according to the current, the expected ripple size, and the action time. The decoupling capacitors are generally large, and are basically ineffective for higher frequency noise. The capacitance of the circuit is for the high frequency, that is, the frequency impedance characteristic of the capacitor is utilized. The capacitance can generally be regarded as an RLC series model. At a certain frequency, resonance occurs, and the impedance of the capacitor is equal to its ESR. 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 choice of bypass capacitor should also consider the medium of the capacitor. A safer method is more than a few capacitance.
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. The typical decoupling capacitance value in the digital circuit It 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 resonant frequency is about 7MHz, that is, it is better for noise below 10MHz. Decoupling effect has almost no effect on noise above 40MHz. 1μF, 10μF capacitor, parallel resonance frequency is above 20MHz, the effect of removing high frequency noise is better. Add a charge and discharge capacitor for every 10 ICs, or 1 storage capacitor, about 10μF. It is better not to use electrolytic capacitor. The electrolytic capacitor is rolled up by two layers of film. The rolled structure is an inductor at high frequency. Use tantalum or polycarbonate capacitor. The decoupling capacitor is not strictly selected, and can be taken as C=1/F, that is, 0.1μF at 10MHz and 0.01μF at 100MHz.