PCB Blog - Partition Design of Mixed Signal PCB Board

The design of the mixed-signal PCB board is very complicated, and the layout and wiring of the components, as well as the handling of the power and ground wires, will directly affect the circuit performance and electromagnetic compatibility performance. The partition design of ground and power described in this article can optimize the performance of mixed-signal circuits. How to reduce the mutual interference between digital signals and analog signals? Two basic principles of electromagnetic compatibility (EMC) must be understood before design: 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 wire, the magnitude of the current flowing, and the frequency); and if the signal cannot pass as far as possible When a small loop returns, it is possible to 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). Both of these situations should be avoided as much as possible in the design. It has been suggested to separate the digital and analog grounds on a mixed-signal circuit board so that isolation between the digital and analog grounds can be achieved. Although this method is feasible, there are many potential problems, especially in complex large systems. The key problem is that the wiring cannot be crossed across the split gap. Once the wiring is crossed, the electromagnetic radiation and signal crosstalk will increase dramatically. A common problem in PCB board design is that the signal line crosses the split ground or power supply to generate EMI problems.

We use the above division method, and the signal line spans 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 flowing through the large loop is very easy to be disturbed by external signals. Unfortunately, when the split grounds are connected together at the power supply, a very large current loop is formed. In addition, the analog ground and digital ground are connected together through a long wire to form a dipole antenna. Knowing where and how current returns to the ground is key to optimizing mixed-signal board designs. Many design engineers 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; in the case of a transformer, it is the magnetic field that crosses the division gap. Another possible approach is to use differential signaling: the signal flows from one line and returns from the other, in which case the ground is not needed as a return path. To deeply explore the interference of digital signals to analog signals, we must first understand the characteristics of high-frequency currents. High-frequency currents always choose the impedance (inductance), a path directly under the signal, so the return current flows through an adjacent circuit layer, whether that adjacent layer is a power or ground plane.

In practice, it is generally preferred to use a unified ground, 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 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 difficult 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 become the key to determining the quality of the design. With the proper layout, digital ground currents will be limited to the digital portion of the board and will not interfere with analog signals. Such wiring must be carefully checked and checked to ensure 100% compliance with wiring rules. Otherwise, an improperly routed signal line can completely destroy an otherwise very good circuit board. When connecting the analog ground and digital ground pins of the A/D converter together, most A/D converter manufacturers recommend that the AGND and DGND pins be connected to the same low impedance ground with short leads. (Note: Because most A/D converter chips do not connect the analog ground and digital ground together, the analog and digital ground must be connected through external pins), any external impedance connected to DGND will pass the parasitic capacitance. More 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, but this approach creates issues such as whether the ground of the digital signal decoupling capacitor should be connected to analog ground or digital ground. If the system has only one A/D converter, the above problem is easy to solve. As shown in Figure 3 above, split the ground and connect the analog ground and digital ground sections together under the A/D converter. When taking this method, it is necessary to ensure that the width of the connecting bridge between the two grounds is the same as the width of the IC and that no signal line can cross the division gap. If there are many A/D converters in the system, for example, how to connect 10 A/D converters? If the analog ground and the digital ground are connected together under each A/D converter, there will be a multi-point connection, and the isolation between the analog ground and the digital ground will be meaningless. And if you don't connect like this, you're violating the manufacturer's requirements. The solution is to start with a unified ground. As shown in Figure 4 below, 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 to cause EMC problems.

If you have doubts about the unified grounding method for mixed-signal PCB design, you can use the method of dividing the ground layer to layout and route the entire circuit board. When designing, pay attention to making the circuit board easy to use in subsequent experiments. The spacing is less than 1/2 inch jumpers or 0-ohm resistors will connect the split grounds together. Pay attention to partitioning and routing, making sure that no digital signal lines are on top of the analog section, and no analog signal lines are on top of the digital section on all layers. Also, no signal lines can span ground gaps or divide the gap between power supplies. To test the functionality and EMC performance of the board, then retest the board for functionality and EMC performance by connecting the two grounds together through a 0-ohm resistor or jumper. Comparing the test results, it is found that in almost all cases, the unified solution is superior to the segmented solution in terms of function and EMC performance. Is the method of dividing the land still useful? This approach can be used in three situations: some medical equipment requires low leakage current between circuits and systems connected to the patient; some industrial process control equipment outputs may be connected to noisy and high-power electromechanical equipment; another situation is when the layout of the PCB board is subject to certain constraints. There are often separate digital and analog power supplies on mixed-signal PCB and split power planes can and should be used. However, the signal lines immediately adjacent to the power supply layer cannot span the gap between the power supplies, and all signal lines that cross the gap must be located on the circuit layer immediately adjacent to the large-area ground. In some cases, designing the analog power supply as a PCB connection line instead of a plane can avoid the problem of splitting the power plane.

Mixed-signal PCB design is a complex process. The following points should be paid attention to in the design process:

1) Partition the PCB board into separate analog and digital parts.

2) Appropriate component layout.

3) A/D converters are placed across partitions.

4) Do not divide the ground. Run a uniform ground under the analog and digital parts of the board.

5) In all layers of the board, digital signals can only be routed in the digital part of the board.

6) In all layers of the board, analog signals can only be routed in the analog part of the board.

7) Realize analog and digital power split.

8) The wiring cannot span the gap between the split power planes.

9) The signal lines that must cross the gap between the divided power supplies should be located on the wiring layer immediately adjacent to the large-area ground.

10) Analyze the path and method of the return-to-ground current actually flowing.

11) Use correct PCB board wiring rules.