PCB Blog - Examples of PCB board wiring techniques

The design of the mixed-signal circuit PCB board is very complicated. The layout and wiring of the components and the handling of the power and ground wires will directly affect the circuit performance and electromagnetic compatibility performance. This article will introduce digital and analog circuit partition design to optimize the performance of mixed-signal circuits. In the PCB board, to reduce the mutual interference between digital signals and analog signals, two basic principles of electromagnetic compatibility (EMC) must be understood before design: reduce the area of the current loop as much as possible; the system only uses one reference surface. If there are two reference planes in the system, it is possible to form a dipole antenna. If the signal cannot return through the smallest possible loop, a large loop antenna may be formed, which should be avoided as much as possible in the design. Separating the digital and analog grounds on mixed-signal boards allows for isolation between the digital and analog grounds. Although this method is feasible, this method also has many potential problems, especially in complex large-scale systems. The key problem is that the wiring cannot be routed across the division gap. Once the wiring is crossed, the electromagnetic radiation and Signal crosstalk increases dramatically. A common problem in PCB board design is that the signal line crosses the split ground or power line to generate EMI problems.

1. Partition method 1
The signal line spans the gap between two grounds, so what is the return path for the signal current? Assuming that the two divided 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, and the ground current will flow through the large loop. High frequency currents generate radiation and high inductance. If a low-level analog current flows through the large loop, the current can be easily disturbed by external signals. When the split grounds are connected together at the power supply, a very large current loop is formed. In addition, the analog ground and the digital ground are connected together through a long wire to form a dipole antenna. Knowing where and how current returns to ground is key to optimizing mixed-signal board designs. Many designs only consider where the signal current flows, ignoring the specific path of the current. If the ground layer must be divided, and the wiring must be routed through the gap between the divisions, a single-point connection can be made between the divided grounds to form a connection bridge between the two grounds, and then routed through the connection bridge. In this way, a direct current return path can be provided under each signal line, so that the loop area formed is small. Signals across the splitting gap can also be achieved using optical isolators or transformers. For the former, it is the optical signal that crosses the division gap; for the latter, it is the magnetic field that crosses the division gap. Another possible way is to use differential signaling: the signal flows in from one line and returns from the other signal line. In this case, there is no need to use the ground as a return path.

2. Dividing method 2
In practical work, a unified ground is generally used, and the PCB board is divided into an analog part and a digital part. Analog signals are routed in the analog area on all layers of the board, while digital signals are routed in the digital circuit area. In this case, the digital signal return current does not flow into the analog signal ground. Digital-to-analog interference occurs only when digital signals are routed over the analog portion of the board, or analog signals are routed over the digital portion of the board. This kind of problem is not because there is no split ground, the real reason is the improper wiring of the digital signal. The PCB board design adopts a unified ground. Through the partition of digital circuits and analog circuits and appropriate signal wiring, some more complex layout and wiring problems can usually be solved, and some potential troubles caused by ground separation will not be caused. In this case, the layout and partition of components becomes the key to determine the quality of the design. With proper layout, digital ground currents will be confined to the digital portion of the board and will not interfere with analog signals. Such wiring must be carefully checked and checked to ensure compliance with wiring rules, otherwise, improper routing of a signal line will completely destroy the design of a circuit board.

3. A/D partition
When connecting the analog ground and digital ground pins of the A/D converter together, most A/D converter manufacturers recommend connecting the AGND and DGND pins to the same low impedance ground with short leads. Because most A/D converter chips do not connect the analog ground and digital ground together, the analog ground and digital ground must be connected through external pins. Any external impedance connected to DGND will pass more parasitic capacitances. Digital noise is coupled to the analog circuitry inside the IC. Following this recommendation, both the AGND and DGND pins of the A/D converter need to be connected to analog ground. If the system has only one A/D converter, the above problem can be solved easily. Divide the ground and connect the analog ground and digital ground together under the A/D converter. If there are many A/D converters in the system, if the analog ground and digital ground are connected together under each A/D converter, multiple points will be connected, and the isolation between the analog ground and the digital ground will be minimal. It's pointless, and if you don't connect it this way, you're violating the manufacturer's requirements. It is to use the unified ground at the beginning, and the unified ground is divided into an analog part and a digital part. This kind of layout and wiring not only meets the requirements of IC device manufacturers for low impedance connection of analog ground and digital ground pins, but also does not form loop antennas or dipole antennas on PCB board.