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

What is bypass?

Bypass is the provision of a low impedance path to some of the harmful parts of the signal. High-frequency interference in the power supply is a typical useless component that needs to be prematurely removed before entering the target chip. Generally, we use capacitors to achieve this purpose. The capacitor used for this purpose is the so-called bypass capacitor (Bypass Capacitor), which utilizes the frequency impedance characteristic of the capacitor (the frequency characteristic of the ideal capacitor increases with the frequency, the impedance decreases, this earth knows), it can be seen The bypass capacitor is mainly for high-frequency interference (high is relative, generally considered to be high-frequency interference above 20MHz, low-frequency ripple below 20MHz).

What is decoupling?

Decoupled, the earliest used in multi-stage circuits to ensure that signals are transmitted between the front and rear stages without affecting each stage of static working points. Decoupling in the power supply means that when the internal switching operation of the chip or the output changes, it is necessary to instantaneously draw a large current from the power supply line. The instantaneous large current may cause the online voltage of the power supply to decrease, thereby causing interference to itself and other devices. . In order to reduce this interference, a "small pool" of electricity storage needs to be placed near the chip to provide this instantaneous high current capability.

In the power circuit, bypass and decoupling are all designed to reduce power supply noise. The bypass is mainly to reduce the interference of the noise on the power supply to the device itself (self-protection); the decoupling is to reduce the noise generated by the device to the power supply (the home is ugly). Some people say that decoupling is for low frequency and bypass is for high frequency. I think this is inaccurate. The internal switching operation of high-speed chips may be as high as GHz, which causes the interference to the power line to be obviously not in the low frequency range. Decoupling capacitors for this purpose also require good high frequency characteristics. Therefore, we do not deliberately distinguish between decoupling and bypass in the general discussion, which is considered to filter noise, regardless of the source of the noise.