![]()
Probe current voltage pin 420*4450 head diameter 5.0 over current current and voltage pin
![]()
Tantalum capacitor
Bypass capacitors are not a theoretical concept, but a practical method that is often used. In the 50s and 60s, the word has its own meaning, and it is not used much now. The tube or transistor needs to be biased to determine the DC supply conditions at the operating point. For example, the gate of the electron tube is required to be negatively charged with respect to the cathode. In order to operate under a direct current power source, a resistor is connected in series from the cathode to the ground, and a positive potential to the ground of the cathode is formed by the plate current, and the gate is grounded to DC. This biasing technique is called “self-biasing”, but for (AC) signals, this is also a negative feedback. To eliminate this effect, connect a large enough point in parallel with this resistor. This is called Road capacitance. Later, some materials extended it to similar situations.
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 0.1μF decoupling capacitor has a distributed inductor of 5μH, and its parallel resonant frequency is about 7MHz. That is to say, it has 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, 0.1μF at 10MHz and 0.01μF at 100MHz.
Generally speaking, a capacitor with a capacity of uf level, such as an Electrolytic capacitor or a tantalum capacitor, has a large inductance, a small resonance frequency, a good pass for a low frequency signal, and a strong inductivity for a high frequency signal. The impedance is large. At the same time, the large capacitor can also function as a local charge pool, which can reduce local interference and be coupled out through the power supply. Capacitors with a capacity of 0.001~0.1uf are generally ceramic capacitors or mica capacitors, small inductance, and resonant frequency. High, the impedance to the high-frequency signal is small, can provide a bypass for the high-frequency interference signal, reduce the external coupling interference to the outside. The bypass is to filter the high-frequency clutter or signal carried by the front stage or the power supply;藕 is a “small pond” designed to ensure the stable output of the positive output (mainly for the work of the device). When other high currents are used, the fluctuation range of the power supply will not affect the operation of the circuit. The additional point is the so-called 藕Combination: The high-frequency switching noise generated by the active components of the device that transmits signals between the front and rear stages without affecting each other's static operating points 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.
Excerpted from Lund's "Board-Level Electromagnetic Compatibility Design" article, the paper speaks well about the use of noise coupling paths, decoupling capacitors, and bypass capacitors. Please see.
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 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. 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 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.
