PCB News - PCB wiring technology examples

The design of the mixed-signal circuit PCB is very complicated. The layout and wiring of the components and the processing of the power line and the ground line will directly affect the circuit performance and electromagnetic compatibility performance. This article will introduce the partition design of digital and analog circuits to optimize the performance of mixed-signal circuits.

In the PCB board, to reduce the mutual interference between the digital signal and the analog signal, you must understand the two basic principles of electromagnetic compatibility (EMC) before designing:

· Minimize 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, it may form a large loop antenna, which should be avoided as much as possible in the design.

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 with this method. The problem is particularly prominent 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 line and generates EMI problems.

1. Split method 1

If splitting method 1 is adopted, and the signal line crosses the gap between two grounds, what is the return path of the signal current? Assume that the two grounds that are divided are connected together at a certain point (usually a single point connection at a certain position). In this case, the ground current will form a large loop and flow through the large loop. High frequency current will generate radiation and high inductance.

If low-level analog current flows through the large loop, the current is easily interfered by external signals. 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 through a long wire to form a dipole antenna.

Knowing the path and method of current return to ground is the key to optimizing mixed-signal circuit board design. Many designs only consider where the signal current flows, and ignore the specific path of the current. If the ground layer must be divided, and 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 wiring 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.

The use of optical isolation devices or transformers can also achieve the signal across the segmentation gap. For the former, it is the optical signal that crosses the segmentation gap; for the latter, it is the magnetic field that crosses the segmentation gap. Another feasible method is to use differential signals: the signal flows in from one line and returns from another signal line. In this case, there is no need to use the ground as a return path.

2. Split method 2

In actual work, a unified ground is generally used, and the PCB is partitioned into an analog part and a digital part. The analog signal is routed in the analog area of all layers of the circuit board, and the digital signal is routed in the digital circuit area. In this case, the digital signal return current will not flow into the analog signal ground.

Only when the digital signal is wired on the analog part of the circuit board or the analog signal is wired on the digital part of the circuit board, will the digital signal interfere with the analog signal. This kind of problem does not occur because there is no divided ground, the real reason is the improper wiring of the digital signal.

The PCB board design adopts a unified ground, through the digital circuit and analog circuit partition and appropriate signal wiring, it can usually solve some more complicated layout and wiring problems, and it will not cause some potential troubles caused by the division of the ground. In this case, the layout and partitioning of components becomes the key to determining the pros and cons of the design.

If the layout is reasonable, the digital ground current will be limited to the digital part of the circuit board and will not interfere with the analog signal. Such wiring must be carefully inspected and verified, and the wiring rules must be observed, 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 will recommend connecting the AGND and DGND pins to the same low-impedance ground through the shortest lead. Because most of the 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 cause more parasitic capacitance. Digital noise is coupled to the analog circuit inside the IC. According to this suggestion, you need to connect the AGND and DGND pins of the A/D converter to the analog ground.

If the system has only one A/D converter, the above problems can be easily solved. Separate 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, multi-point connection will be produced, and the isolation between analog ground and digital ground will be minimal. It's meaningless, and if you don't connect like this, it violates the manufacturer's requirements.

The best way is to use the unified ground at the beginning, and divide the unified ground into analog and digital parts.

Such a layout 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 a loop antenna or dipole antenna.

The above is the introduction of PCB wiring technology examples. Ipcb is also provided to PCB manufacturers and PCB manufacturing technology.