Effective decoupling radius of decoupling capacitor

The decoupling radius is a measure of the effectiveness of a decoupling capacitor. It is a function of many factors, including the plane geometry, the frequency as well as the capacitor parameters.An important issue in capacitive decoupling is the decoupling radius of the capacitor. Most of the materials mention that capacitors should be placed as close as possible to the chip, and most of the materials talk about the placement distance from the perspective of reducing the loop inductance. Today qihe smt pick and place machine sharing Effective decoupling radius of decoupling capacitor. Indeed, reducing the inductance is an important reason, but there is another important reason that most materials do not mention, that is, the problem of capacitive decoupling radius. If the capacitor is placed too far away from the chip, exceeding its decoupling radius, the capacitor will lose its decoupling effect.

The best way to understand the decoupling radius is to examine the phase relationship between the noise source and the capacitive compensation current. When the chip’s demand for current changes, a voltage disturbance will be generated in a small local area of the power plane. To compensate this current (or voltage), the capacitor must first sense this voltage disturbance. It takes time for the signal to propagate through the medium, so there is a time delay between when a local voltage disturbance occurs and when the disturbance is sensed by the capacitor. Similarly, there is a delay for the compensation current of the capacitor to reach the disturbance region. Therefore, an inconsistency in phase between the noise source and the capacitance compensation current must be caused.

A particular capacitor works best at compensating noise at the same frequency as its self-resonance, the frequency at which we measure this phase relationship. In practical applications, this distance is best controlled at (1/40-1/50) wavelength, which is an empirical value.

For example: 0.001uF ceramic capacitor, if the total parasitic inductance after installation on the circuit board is 1.6nH, then the resonant frequency after installation is 125.8MHz, and the resonant period is 7.95ps. Assuming that the propagation speed of the signal on the circuit board is 166ps/inch, the wavelength is 47.9 inches. The capacitive decoupling radius is 47.9/50=0.958 inches, which is approximately equal to 2.4 cm.

The capacitor in this example can only compensate for power supply noise within a range of 2.4 cm around it, that is, its decoupling radius is 2.4 cm. Different capacitors have different resonant frequencies and decoupling radii. For large capacitors, because their resonant frequency is very low, the corresponding wavelength is very long, so the decoupling radius is large, which is why we don’t pay much attention to the placement of large capacitors on the circuit board. For small capacitors, because the decoupling radius is very small, they should be placed as close as possible to the chip that needs to be decoupled. This is what is repeatedly emphasized in most materials, and small capacitors should be placed as close as possible to the chip.

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