Explain the difference between "decoupling" and "bypass" (I)

In general, 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 actually decoupled, except that the bypass capacitor generally refers to the high-frequency bypass, which is to improve the low-frequency leakage prevention 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.

However, is this the case? of course not!

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 value 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 should also consider the medium of the capacitor. A safer method is more than a few capacitors.

Generally speaking, people who are decoupled will also ask about the bypass. What is the difference between the two, it can be said clearly, but there is no strict distinction.