"FPGA full Step---Practical Walkthrough" the third chapter of PCB design decoupling capacitor

1. Why do the decoupling capacitors need to be placed nearby?

Most of the information has been mentioned, the proximity of the decoupling capacitance, is from the angle of reducing the loop inductance to talk about the placement problem, in fact, there is a principle is to decouple the radius of the problem, if the capacitance is far from the chip position, more than the decoupling radius, will not be the decoupling effect.

The best way to consider the decoupling radius is to investigate the phase relationship between the noise source and the capacitance compensation current. When the demand for the current of the chip changes, the voltage disturbance is generated in a small local area of the power supply plane, and the capacitance compensates for the current (voltage), which must be sensed by this voltage disturbance. It takes a certain amount of time for the signal to propagate in the medium, so the local voltage disturbance to the capacitive sensing needs a certain time delay, so the phase inconsistency between the noise source and the capacitance compensated current must be caused. The specific capacitance is best for the same noise compensation as the self-resonant frequency, and we measure this phase relationship with this frequency. When the disturbance zone to the distance of the capacitance, the phase of the compensation current and the noise source phase is exactly the same as 180°, that is, the full reverse phase, when the compensation current is no longer in effect, the decoupling effect is invalid, the compensated energy can not be delivered in time, in order to effectively transfer the compensation energy, should make the noise source and the compensation Preferably in the same phase. The closer the distance, the smaller the phase difference, the more compensation energy transfer, if the distance is 0, then the compensation energy is transferred to the disturbance zone, which requires the noise source distance capacitance as close as possible.

For large capacitance, because of its resonant frequency is very low, the corresponding wavelength is very long, because the decoupling radius is very large, so do not have to care how big capacitance in the circuit board placement of the reason, for small capacitance, because the decoupling radius is very small, need to close to the decoupling of the chip.

This is the PCB diagram of the STM core Board that I designed at that time.

Fig. 3.3 Schematic diagram of decoupling capacitance

Schematic diagram of the power supply Section, figure 3.4 is the corresponding PCB diagram.

Figure 3.4 Decoupling capacitor PCB placement

"FPGA full Step---Practical Walkthrough" the third chapter of PCB design decoupling capacitor