Detailed description of decoupling capacitor and bypass Capacitor

In electronic circuits, both the decoupling capacitor and the bypass capacitor play an anti-interference role. The capacitor is in different positions and the title is different. For the same circuit, the bypass (bypass) Capacitor Filters out the high-frequency noise in the input signal as the filter object, filters out the high-frequency clutter carried by the front, and decoupling) the capacitor is also called the decoupling capacitor, which filters out the interference from the output signal.

The decoupling capacitor is used in places that do not need to communicate with each other in the amplifier circuit to Eliminate Self-excitation and ensure stable operation of the amplifier. In the circuit, there is always a drive source and a drive load. If the load capacitor is large, the driving circuit needs to charge and discharge the capacitor to complete the signal jump. When the rising edge is steep, the current is relatively large, in this way, the drive current will absorb a large amount of power supply current, because the inductance and resistance in the circuit (especially the inductance on the chip pin will cause a rebound ), compared with normal conditions, this current is actually a type of noise, which will affect the normal operation of the previous stage. This is coupling.

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

Decoupling and bypass can be considered as filtering. The decoupling capacitor is equivalent to a battery, avoiding voltage reduction due to sudden changes in current, which is equivalent to a ripple filter. The specific capacity value can be calculated based on the current size, the expected ripple size, and the applied time. The decoupling capacitor is usually very large, which is basically ineffective for noise at a higher frequency. The bypass capacitor is designed for high frequencies, that is, the frequency impedance of the capacitor is used. Capacitors can generally be considered as a series model of RLC. At a certain frequency, a 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 capacitor medium. Therefore, you need to consider the capacitor medium when selecting the bypass capacitor. An insurance method is to merge several capacitors.

Related functions

Decoupling capacitors play two roles between the power supply and ground of the integrated circuit: one is the energy storage capacitor of the integrated circuit, and the other is to bypass the high-frequency noise of the device. The typical decoupling capacitance value in digital circuits is 0.1 μF. The typical distribution Inductance Value of this capacitor is 5 μh. The 0.1 μF decoupling capacitor has a distribution inductance of 5 μh, and its parallel resonance frequency is about 7 MHz. That is to say, it has a better decoupling effect for noise below 10 MHz, it hardly works for noise above 40 MHz. The capacitance of 1μF and 10μf has a parallel resonance frequency of more than 20 MHz, and the effect of high-frequency noise removal is better. A charge-discharge capacitor or an energy storage capacitor must be added to every 10 or so integrated circuits. The optional value is about 10 μF. It is best not to use electrolytic capacitors. electrolytic capacitors are flushed by two layers of films. The structure of the fl is expressed as inductance at high frequencies. You need to use the TA capacitor or polycarbonate capacitor. The selection of the decoupling capacitor is not strict. The decoupling capacitor can be set to C = 1/F, that is, 0.1 μF for 10 MHz and 0.01 μF for MHz.