PCB design high-speed analog input signal routing method

The wider the line width, the stronger the anti-interference ability and the better the signal quality (the influence of skin effect). But at the same time, the requirement of 50Ω characteristic impedance must be guaranteed. Normal FR4 board, the surface line width 6MIL impedance is 50Ω. This obviously cannot meet the signal quality requirements of high-speed analog input, so we generally use hollowing out GND02 and let it refer to the ART03 layer. In this way, the differential signal can be counted as 12/10, and the single line can be counted as 18MIL. (Note that the line width exceeds 18MIL and then widening is meaningless)

PCB design high-speed analog input signal routing method and rules

The CLINE highlighted in green in the figure refers to the single-line and differential high-speed analog input of the ART03 layer. While doing so, some details must be dealt with:

(1) The simulation part of the TOP layer needs to be packaged, as shown in the figure above. It should be noted that the distance from the ground copper to the analog input CLINE needs to be 3W, that is, the AIRGAP from the edge of the copper to CLINE is twice the line width. According to some electromagnetic theoretical calculations and simulations, the magnetic field and electric field of the signal lines on the PCB are mainly distributed within the range of 3W. (The noise interference from surrounding signals is less than or equal to 1%).

(2) The GND copper of the positive layer of the analog area also needs to be isolated from the surrounding digital area, that is, all layers are isolated.

(3) For the hollowing out of GND02, we usually hollow out all of this area, so the operation is relatively simple and there is no problem. But considering the details or in order to do better, we can only hollow out the analog input wiring part, of course, the same as the TOP layer, the 3W area. This can guarantee the signal quality and the flatness of the board. The processing result is as follows:

PCB design high-speed analog input signal routing method and rules

In this way, the return path of the high-speed analog input signal can be quickly reflowed on the GND02 layer. That is, the simulated ground return path becomes shorter.

(4) Irregularly punch a large number of GND vias around the high-speed analog signal to make the analog signal flow back quickly. It can also absorb noise.

PCB design high-speed analog input signal routing rules

Rule 1: High-speed PCB signal routing shielding rules In high-speed PCB design, the routing of key high-speed signal lines such as clocks needs to be shielded. If there is no shield or only part of it, it will cause EMI leakage. It is recommended that the shielded wire be grounded with a hole per 1000 mil.

PCB design high-speed analog input signal routing method and rules

Rule 2: High-speed signal routing closed-loop rules

Due to the increasing density of PCB boards, many PCB LAYOUT engineers are prone to a mistake in the process of routing, that is, high-speed signal networks such as clock signals, which produce closed-loop results when routing multi-layer PCBs. As a result of such a closed loop, a loop antenna will be produced, which will increase the radiated intensity of EMI.

PCB design high-speed analog input signal routing method and rules

Rule 3: High-speed signal routing open loop rules

Rule 2 mentions that the closed loop of high-speed signals will cause EMI radiation, but the open loop will also cause EMI radiation.

High-speed signal networks such as clock signals, once an open-loop result occurs when the multilayer PCB is routed, a linear antenna will be produced, which increases the EMI radiation intensity.

PCB design high-speed analog input signal routing method and rules

Rule 4: Characteristic impedance continuity rule of high-speed signal

For high-speed signals, the characteristic impedance must be continuity when switching between layers, otherwise it will increase EMI radiation. In other words, the width of the wiring of the same layer must be continuous, and the impedance of the wiring of different layers must be continuous.

PCB design high-speed analog input signal routing method and rules

Rule 5: Wiring direction rules for high-speed PCB design

The wiring between two adjacent layers must follow the principle of vertical wiring, otherwise it will cause crosstalk between the lines and increase EMI radiation.

In short, the adjacent wiring layers follow the horizontal and vertical wiring directions, and the vertical wiring can suppress the crosstalk between the lines.

PCB design high-speed analog input signal routing method and rules

Rule 6: Topological structure rules in high-speed PCB design

In high-speed PCB design, the control of the characteristic impedance of the circuit board and the design of the topological structure under multi-load conditions directly determine the success or failure of the product.

The figure shows a daisy chain topology, which is generally beneficial when used in a few Mhz. It is recommended to use a star-shaped symmetrical structure on the back end in high-speed PCB design.

PCB design high-speed analog input signal routing method and rules

Rule 7: Resonance rule of trace length

Check whether the length of the signal line and the frequency of the signal constitute resonance, that is, when the length of the wiring is an integer multiple of the signal wavelength 1/4, the wiring will resonate, and the resonance will radiate electromagnetic waves and cause interference.