# How to optimize the performance of mixed-signal circuit PCB design

The PCB design of mixed-signal circuits is very complicated. The layout and wiring of components and the processing of power and ground wires will directly affect circuit performance and electromagnetic compatibility. The partition design of ground and power supply introduced in this article can optimize the performance of mixed-signal circuits.

The PCB design of mixed-signal circuits is very complicated. The layout and wiring of components and the processing of power and ground wires will directly affect circuit performance and electromagnetic compatibility. The partition design of ground and power supply introduced in this article can optimize the performance of mixed-signal circuits.

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 the system has two reference planes, it may 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 returns, it 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.

All current must have a loop back to the source. The generation of this loop will automatically find the path of least impedance. Usually in a PCB structure with a power/ground plane, it will be directly on the plane (power or ground) below the signal line. The return signal (current) has the same amplitude and opposite direction as the original signal (current). I0 ― total signal current, AH ― distance from signal line to reference plane, m D ― vertical distance from the observation point to the center of the signal line, mi (D) ― return current density at the observation point, A/in 1: division The digital ground and analog ground on the mixed signal are separated, 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. We use the above 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 that are divided 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 will generate 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. At the same time, pay attention to the following two points: the slot of the reference plane, and the slot of the inappropriate reference plane will increase the loop area of ​​the signal.