PCB News - Partition design of mixed-signal PCB

How to reduce the mutual interference between digital signal and analog signal? Before designing, we must understand the two basic principles of electromagnetic compatibility (EMC): The first principle is to minimize the area of the current loop; the second principle is that the system uses only one reference surface. On the contrary, if there are two reference planes in the system, it is possible to form a dipole antenna (Note: the radiation size of a small dipole antenna is proportional to the length of the line, the amount of current flowing and the frequency); and if the signal cannot pass as much as possible A small loop return may form a large loop antenna (Note: the radiation size of a small loop antenna is proportional to the loop area, the current flowing through the loop, and the square of the frequency). Avoid these two situations as much as possible in the design.

It is suggested to separate the digital ground and analog ground on the mixed-signal circuit board, so that the isolation between the digital ground and the analog ground can be achieved. Although this method is feasible, there are many potential problems, especially in complex large-scale systems. The most critical problem is that it cannot be routed across the division gap. Once the division gap is routed, electromagnetic radiation and signal crosstalk will increase sharply. The most common problem in PCB design is that the signal line crosses the divided ground or power supply and generates EMI problems.

As shown in Figure 1, we use the above-mentioned division method, and the signal line crosses the gap between the two grounds. What is the return path of the signal current? Assuming that the two grounds are connected together somewhere (usually a single point connection at a certain location), in this case, the ground current will form a large loop. The high-frequency current flowing through the large loop generates radiation and high ground inductance. If the low-level analog current flows through the large loop, the current is easily interfered by external signals. The worst thing is that when the divided grounds are connected together at the power supply, a very large current loop will be formed. In addition, the analog ground and digital ground are connected by a long wire to form a dipole antenna.

The above is an introduction to the partition design of mixed-signal PCB board. Ipcb is also provided to PCB manufacturers and PCB manufacturing technology.