Traditional designs strictly distinguish between analog and digital circuits, however, in high-speed digital-analog hybrid circuits, it is better to use multiple grounds and use large area power and ground planes to provide low impedance for power supply decoupling. And how to eliminate the interference between analog signals and digital signals has become one of the key points of hardware design. When designing printed boards, the design rules of layout, wiring, and internal electrical layer division should be applied as basic design criteria.
1. The layout of the printed board
Crosstalk occurs between signals on transmission lines near printed boards due to the mutual coupling of electromagnetic fields. Reasonable placement of components can greatly reduce EMI problems. For example, sensitive devices should be kept away from the output of high-gain amplifiers; switching power supply modules should be kept away from both sensitive devices and high-gain amplifier circuits; analog circuits and digital circuits should be placed separately to avoid interleaving; analog-to-digital conversion devices should be placed in The junction of analog and digital circuit partitions. Divide the circuits on the PCB according to frequency and type, and carefully choose the location of the clock circuit to avoid excessively long clock signal routing. The usual practice is to arrange each functional module according to the signal flow direction, so that the layout is convenient for signal flow, and the signal direction is kept as consistent as possible.
2. PCB wiring
When the signal frequency is >10MHZ, the wiring, vias, device packaging, etc. on the printed board will cause non-negligible distributed inductance and capacitance. When the wiring length is greater than 1/20 of the corresponding wavelength of the noise frequency, an antenna effect will occur, and the noise will radiate outward through the printed wire. The via hole on the signal line will cause a capacitance of about 0.5pF, and the packaging material of the device may also introduce a capacitance of about 2~6pF. The role of these small distribution parameters in high-speed circuits cannot be ignored.
When designing the PCB, the power and ground vias should be as close as possible to the corresponding pins of the device. Thickening the width of the power and ground wires can reduce the loop resistance. At the same time, the direction of the power and ground wires should be basically the same as the data direction. Helps to enhance anti-interference ability. Using narrower traces (4~8mil) can increase high frequency damping and reduce capacitive coupling. Avoid large current loop areas when routing. Use multi-point grounding to make high frequency ground impedance lower. 90° turns should be avoided when routing because 90° turns increase capacitance and cause a change in the characteristic impedance of the transmission line. Keeping the spacing between adjacent traces greater than the width of the traces can minimize crosstalk.
3. Division of the power plane layer
In order to improve the isolation between different power supplies, so that the interference of the digital part is transmitted to the analog signal part as little as possible, it is necessary to divide the power plane. However, improper segmentation will also result in incomplete signal return paths, affecting the integrity of digital signals. Therefore, the principle of dividing the power plane layer is to see whether the return path of the divided signal is increased, and whether the interference of the return signal to other signals will increase. If conditions permit, the power planes can be set in layers. Because of the layered power supply, the occurrence of signal splitting across the plane layers will be greatly reduced, which can effectively improve the signal quality.
Based on the above three aspects, in the actual design of high-speed digital-analog hybrid PCB, the following points should be followed:
(1) divide the PCB into relatively independent analog and digital parts;
(2) distinguish between analog and digital parts when laying out components ;
(3) Only the unified ground is reserved, and the analog circuit and the digital circuit use a common ground plane;
(4) In all layers, the analog signal is routed in the analog part of the circuit board, and the digital signal is routed in the digital part, and the current loop in the circuit should be Keep it to a minimum;
(5) The power line and the ground line should be close to each other;
(6) The wiring should not cross the gap between the divided power supplies as much as possible. If it is inevitable to cross the divided power supply wiring, try to route the signal lines close to the large-area ground plane. on the trace layer.
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