Abstract: The design of mixed-signal circuits is very complex. The layout, wiring, and processing of power and ground will directly affect circuit performance and electromagnetic compatibility. The partition design of ground and power introduced in this article can optimize the performance of mixed-signal circuits.
How to reduce the mutual interference between digital signals and analog signals? Before designing, you must understand 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 plane. Conversely, if the system has two reference planes, a dipole antenna may be formed (Note: the radiation 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 return may form a large loop antenna (Note: The radiation size of a small loop antenna is proportional to the loop area, the amount of current flowing through the loop, and the square of the frequency). Avoid these two situations as much as possible in your design.
Someone suggested to separate the digital ground and the analog ground on the mixed signal circuit board, so as to achieve the isolation between the digital ground and the analog ground. Although this method is feasible, there are many potential problems, especially in complex large systems. The most critical problem is that it cannot be routed across the split gap. Once it is routed across the split gap, electromagnetic radiation and signal crosstalk will increase sharply. The most common problem in PCB design is that the signal line crosses the split ground or power source and causes 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? Assume 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 a low-level simulation current flows through the large loop, this current is easily interfered by external signals. The worst part is that when the split grounds are connected together at the power source, a very large current loop will form. In addition, the analog ground and digital ground are connected by a long wire to form a dipole antenna.
Understanding the path and manner of current return to ground is key to optimizing mixed-signal circuit board design. Many design engineers only consider where the signal current flows, and ignore the specific path of the current. If the ground layer must be divided and routed through the gap between the divisions, you can first make a single point connection between the divided grounds to form a connection bridge between the two grounds, and then route 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.
Using optical isolation devices or transformers can also achieve signals across the split gap. For the former, it is the optical signal that crosses the division gap; in the case of a transformer, the magnetic field is crossed. Another possible solution is to use a differential signal: the signal flows from one line to the other and returns from another signal line. In this case, no ground is required as a return path.
To thoroughly discuss the interference of digital signals on simulation signals, we must first understand the characteristics of high-frequency currents. The high-frequency current always chooses the path with the lowest impedance (lowest inductance) directly below the signal, so the return current will flow through the adjacent circuit layer, regardless of whether the adjacent layer is the power layer or the ground layer.
In practice, it is generally inclined to use a uniform, and the PCB is divided into the simulation part and the digital part. The simulation signals are routed in the simulation area of all layers of the circuit board, and the digital signals are routed in the digital circuit area. In this case, the digital signal return current does not flow to the ground of the analog signal.
Only when the digital signal is routed on the simulation part of the circuit board or the simulation signal is routed on the digital part of the circuit board will the digital signal interfere with the simulation signal. This problem does not occur because there is no division, the real reason is that the wiring of digital signals is inappropriate.
PCB design adopts a unified, through digital circuit and simulation circuit partition and appropriate signal wiring, which can usually solve some of the more difficult layout and routing problems, but also does not cause some potential troubles caused by geographical division. In this case, the layout and zoning of components becomes the key to determining the quality 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 simulation signal. Such wiring must be carefully checked and verified to ensure 100% compliance with wiring rules. Otherwise, improper wiring of a signal line will completely destroy a very good circuit board.
When connecting the analog ground and digital ground pins of the A / D converter, most A / D converter manufacturers will recommend: Connect the AGND and DGND pins to the same low impedance ground through the shortest lead (Note: Because most A / D converter chips do not connect the analog ground and digital ground together, you must connect the analog and digital ground through external pins.) Any external impedance connected to DGND will be connected by parasitic capacitance. More digital noise is coupled to the simulation circuits inside the IC. According to this suggestion, the AGND and DGND pins of the A / D converter need to be connected to the simulation ground, but this method will cause problems such as whether the ground of the digital signal decoupling capacitor should be connected to the simulation ground or the digital ground.
If the system has only one A / D converter, the above problems can be easily solved. As shown in Figure 3, the ground is divided, and the analog ground and the digital ground are connected together under the A / D converter. When this method is adopted, the width of the connecting bridge between the two grounds must be the same as that of the IC, and 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 and digital grounds are connected together under each A / D converter, a multi-point connection is generated, and the isolation between the analog and digital grounds is meaningless. Failure to connect this way violates the manufacturer's requirements.
The best way is to use uniformity from the beginning. As shown in Figure 4, the unified ground is divided into a simulation part and a digital part. This layout meets the requirements of IC device manufacturers for low-impedance connection of the analog ground and digital ground pins, and at the same time does not form loop antennas or dipole antennas and cause EMC problems.
If you have doubts about adopting a unified approach to mixed-signal PCB design, you can use the ground layer division method to place and route the entire circuit board. When designing, pay attention to making the circuit board as easy as possible to use less than 1/2 inch in the back experiment A jumper or 0 ohm resistor will connect the split grounds together. Pay attention to the partitioning and routing, and make sure that no digital signal lines are above the analog part, and no analog signal lines are above the digital part on all layers. Moreover, no signal line can cross the ground gap or the gap between the divided power supplies. To test the functionality and EMC performance of the board, then connect the two grounds together through a 0 ohm resistor or jumper, and retest the functionality and EMC performance of the board. Comparing the test results, you will find that in almost all cases, a unified solution is superior to a partition in terms of functionality and EMC performance.
1 Is the land division method still useful?
This method can be used in the following three cases: some medical equipment requires low leakage current between the circuits and systems connected to the patient; the output of some industrial process control equipment may be connected to the noisy and high power electromechanical On the device; another situation is when the PCB layout is subject to certain restrictions.
There are usually separate digital and analog power supplies on mixed-signal PCBs. Split power planes can and should be used. However, the signal lines immediately adjacent to the power supply layer cannot cross the gap between the power supplies, and all signal lines crossing the gap must be located on the circuit layer immediately adjacent to a large area of ground. In some cases, designing the simulated power supply with PCB connection lines instead of one surface can avoid the problem of power supply surface splitting.
2 Mixed signal PCB design is a complicated process. Pay attention to the following points during the design process:
PCB Partition the PCB into separate simulation and digital sections.
Appropriate component layout.
A / D converters are placed across partitions.
Do not divide the ground. Lay uniformly under the analog and digital parts of the circuit board.
In all layers of the circuit board, digital signals can only be routed on the digital part of the circuit board.
In all layers of the circuit board, the simulation signal can only be routed in the simulation part of the circuit board.
Achieve simulation and digital power division.
The wiring must not cross the gap between the divided power planes.
The signal lines that must cross the gap between the split power supplies must be located on the wiring layer next to the large area ground.
Analyze the path and method of the actual return current to ground.
Use correct wiring rules.