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High speed PCB circuit return reflux
2020-09-12
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Author:Dag      Share

Basic concept of reflux

In the schematic diagram of digital circuit, the transmission of digital signal is from one logic gate to another. The signal is sent from the output end to the receiving end through the wire, which seems to be unidirectional flow. Many digital engineers therefore think that the loop path is irrelevant. After all, the driver and receiver are designated as voltage mode devices. Why consider the current! In fact, the basic circuit theory tells us that the signal is transmitted by the current. Specifically, it is the movement of the electron. One of the characteristics of the electron flow is that the electron never stays anywhere. No matter where the current flows, it must come back. Therefore, the current always flows in the loop, and any signal in the circuit exists in the form of a closed loop. For high frequency signal transmission, it is actually a process of charging the dielectric capacitor between the transmission line and the DC layer.

High speed PCB

High speed PCB

Effect of backflow

Digital circuits usually use the ground and power plane to complete the return. The return path of high-frequency signal is different from that of low-frequency signal. Impedance path is selected for low-frequency signal return, and inductive path is selected for high-frequency signal return.

When the current starts from the signal driver, flows through the signal line and injects into the receiving end of the signal, there is always a return current in the opposite direction: starting from the ground pin of the load, passing through the copper coating plane, flows to the signal source, and forms a closed loop with the current flowing through the signal line. The noise frequency caused by the current flowing through the copper coated plane is equivalent to the signal frequency. The higher the signal frequency, the higher the noise frequency. The logic gate does not respond to the input signal of, but to the difference between the input signal and the reference pin. A single ended circuit responds to the difference between the input signal and its logical reference plane, so the disturbance on the ground reference plane is as important as the interference on the signal path. The logic gate responds to the input pin and the specified reference pin. We do not know which reference pin is assigned (usually negative power supply for TTL and positive power supply for ECL, but not all of them). In terms of this property, the anti-interference ability of differential signal can have good effect on missile noise and power plane sliding.

When many digital signals on PCB board are switched synchronously (such as CPU data bus, address bus, etc.), the transient load current will flow into the circuit from the power supply or from the circuit to the ground wire. Due to the impedance on the power line and ground wire, synchronous switching noise (SSN) will be generated, and the ground plane rebound noise (hereinafter referred to as "ground bounce") will appear on the ground wire. The larger the surrounding area of the power line and ground wire on the printed circuit board, the greater their radiation energy. Therefore, we analyze the switching state of the digital chip, and take measures to control the reflux mode, so as to reduce the surrounding area and radiation degree.