PCB News - How to realize partition design of mixed signal PCB

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

How to reduce the interference between digital and analog signals? Two basic principles of electromagnetic compatibility (EMC) must be understood before design: the first principle is to minimize the current loop area; The second principle is that the system uses only one reference plane. On the contrary, if the system has two reference planes, it is possible to form a dipole antenna (note: the radiation of a small dipole antenna is proportional to the length of the line, the amount of current flowing, and the frequency). If the signal does not return through the smallest possible loop, a large circular antenna may be formed. Avoid both in your design as much as possible.

It has been suggested to separate the digital ground and the analog ground on the mixed-signal circuit board to achieve isolation between the digital ground and the analog ground. Although this approach is feasible, it has many potential problems, especially in large and complex systems. The key problem is not to cross the gap wiring, once crossed the gap wiring, electromagnetic radiation and signal crosstalk will increase dramatically. A common problem in PCB design is EMI problem caused by signal line crossing the ground or power supply.

We use the above segmentation method, and the signal line spans the gap between the two ground, what is the return path of the signal current? Suppose the two partitioned lands are connected at some point (usually a single point at one point), in which case the earth 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 easy to be interfered by external signals. The bad thing is that when the sections are connected together at the power source, a very large current loop is formed. In addition, analog and digital ground connected by a long wire form a dipole antenna.

Understanding the path and mode of current backflow to ground is the key to optimize mixed-signal circuit board design. Many design engineers only consider where the signal current flows, ignoring the specific path of the current. If the ground layer must be partitioned and must be routed through the gap between the partitions, a single point connection can be made between the partitioned ground to form a connection bridge between the two ground layers and then routed through the connection bridge. In this way, a direct current backflow path can be provided below each signal line, resulting in a small loop area.

Optical isolation devices or transformers can also be used to realize the signal crossing the segmentation gap. For the former, it is the optical signal that spans the segmentation gap. In the case of a transformer, it is the magnetic field that spans the partition gap. Differential signals are also possible: signals flow in from one line and return from the other, in which case they are used as backflow paths unnecessarily.

To explore the interference of digital signal to analog signal, we must first understand the characteristics of high frequency current. The high-frequency current always selects the impedance (inductance), the path directly below the signal, so the return current will flow through the adjacent circuit layer, regardless of whether the adjacent layer is the power or ground layer.

In practice, it is generally preferred to use a uniform PCB partition into analog and digital parts. Analog signals are routed in the analog region of all layers of the board, while digital signals are routed in the digital circuit region. In this case, the digital signal return current does not flow into the ground of the analog signal.

Interference from digital signals to analog signals occurs only when the digital signals are routed over or analog signals are routed over the digital parts of the circuit board. This problem is not due to the lack of segmentation, the real reason is the improper wiring of digital signals.

PCB design uses unified, through the digital circuit and analog circuit partition and appropriate signal wiring, usually can solve some of the more difficult layout and wiring problems, but also does not have some potential trouble caused by ground segmentation. In this case, the layout and partitioning of components becomes critical in determining the quality of the design. If properly laid out, the digital ground current will be limited to the digital part of the board and will not interfere with the analog signal. Such wiring must be carefully checked and checked to ensure 100% compliance with wiring rules. O

therwise, an improper signal line will completely destroy a very good circuit board.

When connecting analog and digital ground pins of A/D converters together, most A/D converter manufacturers recommend connecting AGND and DGND pins to the same low-impedance ground using short leads (Note: Because most A/D converter chips do not connect analog and digital ground together internally, the analog and digital ground must be connected via external pins), any external impedance connected to DGND will couple more digital noise to the analog circuit inside the IC via parasitic capacitance. Following this recommendation, both the A/D converter AGND and DGND pins need to be connected to the analog ground, but this approach raises questions such as whether the ground end of the digital signal decoupling capacitor should be connected to the analog or digital ground.

If the system has only one A/D converter, the above problem can be easily solved. As shown in Figure 3, the ground is split and the analog and digital ground sections are connected together under the A/D converter. When this method is adopted, it is necessary to ensure that the bridge width between the two sites is equal to the IC width, and that no signal line can cross the partition gap.

If the system has many A/D converters, for example, 10 A/D converters how to connect? If analog and digital ground are connected under each A/D converter, A multipoint connection will result, and the isolation between analog and digital ground will be meaningless. If you do not, you violate the manufacturer's requirements.

The way to do that is to start with a uniform. Divide the ground uniformly into analog and digital parts. This layout not only meets the requirements of IC device manufacturers for low impedance connection of analog and digital ground pins, but also avoids EMC problems caused by loop antenna or dipole antenna.

If you have doubts about the unified approach of mixed-signal PCB design, you can use the method of ground layer partition to lay out and route the entire circuit board. In the design, attention should be paid to make the circuit board easy to be connected together with jumpers or 0 ohm resistors spaced less than 1/2 inch apart in the later experiment. Pay attention to zoning and wiring to ensure that no digital signal lines are above the analog section on all layers and that no analog signal lines are above the digital section. Moreover, no signal line should cross the ground gap or divide the gap between the power sources. To test the board's function and EMC performance, retest the board's function and EMC performance by connecting the two floors together via a 0 ohm resistor or jumper. Comparing the test results, it was found that in almost all cases, the unified solution was superior in terms of functionality and EMC performance compared to the split solution.

This approach can be used in three situations: some medical devices require very low leakage current between circuits and systems connected to the patient; The output of some industrial process control equipment may be connected to noisy and high-power electromechanical equipment; Another case is when the LAYOUT of the PCB is subject to specific restrictions.

There are usually separate digital and analog power supplies on a mixed-signal PCB board that can and should have a split power supply face. However, the signal lines adjacent to the power supply layer cannot cross the gap between the power supplies, and all the signal lines that cross the gap must be located on the circuit layer adjacent to the large area. In some cases, the analog power supply can be designed with PCB connections rather than one face to avoid power face splitting.

# Mixed-signal PCB design is a complex process, the design process should pay attention to the following points:

1. Divide the PCB into separate analog and digital parts.

2. Proper component layout.

3.A/D converter is placed across partitions.

4. Don't divide the ground. The analog part and the digital part of the circuit board are laid uniformly.

5. In all layers of the board, the digital signal 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. Analog and digital power separation.

8. Wiring should not span the gap between the split power supply surfaces.

9. The signal lines that must span the gap between the split power supplies should be located on the wiring layer adjacent to a large area.

10. Analyze the actual path and mode of earth current flow.

11. Use correct wiring rules.