The design of mixed-signal circuit PCBs is very complex. The layout and routing of components and the handling of power and ground wires will directly affect circuit performance and EMC performance. This article describes ground and power partitioning designs that 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 the two basic principles of electromagnetic compatibility (EMC): one is to reduce the area of the current loop as much as possible; the other is to use only one reference plane in the system. On the contrary, if there are two reference planes in the system, a dipole antenna may be formed (note: the radiation size of a small dipole antenna is proportional to the length of the line, the current flowing and the frequency); if the signal cannot pass through the smallest possible loop path back, a large loop antenna may be formed. The current in the loop is proportional to the square of the frequency. These two situations should be avoided in design.
It is recommended that digital and analog grounds be separated on mixed-signal circuit boards to achieve isolation between digital and analog grounds. While feasible, this approach has many potential problems, especially in complex, large-scale systems. The key issue is that routing across split gaps is impossible. Once crossed, electromagnetic radiation and signal crosstalk will increase rapidly. A common problem in PCB design is EMI problems caused by signal lines crossing the ground or power supply.
We use the above segmentation method, and the signal line passes through the gap between the two grounds. What is the return path for the signal current? Suppose two separate grounds are connected somewhere (usually a single point at some location). In this case, the ground current will form a large loop. High frequency currents flowing through large loops produce radiation and high ground inductance. If the current flowing through the large loop is a low-level analog current, the current is susceptible to interference from external signals. Even worse, when separate grounds are connected together at the power supply, a very large current loop is created. Additionally, connecting analog and digital via long wires will form a dipole antenna.
Understanding the path and manner in which current returns to ground is key to optimizing mixed-signal circuit board designs. 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, wiring must be carried out through the gap between the partitions. A single point connection can be made between the divided ground layers to form a connecting bridge between the two ground layers, and then the wiring is carried out through the connecting bridge. This provides a DC loop under each signal line, resulting in a small loop area.
Opto-isolating devices or transformers can also be used to span the gap. In the former, the optical signal passes through the gap, while in the case of a transformer, the magnetic field passes through the gap. Another possible approach is to use differential signaling: signals flow in from one line and return from another line, in which case they don't need to be used as return paths.
In order to explore the interference of digital signals to analog signals, we must first understand the characteristics of high-frequency currents. High frequency current always takes the impedance (inductance) path directly beneath the signal, so the return current will flow through an adjacent circuit layer, whether the adjacent layer is a power or ground layer.
In practice, PCBs are usually divided into analog and digital parts. Analog signals are routed in the analog areas of each layer of the circuit board, while digital signals are routed in the digital circuit areas. In this case, the return current of the digital signal does not flow to the ground of the analog signal.
Interference from digital signals to analog signals will occur only when digital signals are wired in the analog part of the circuit board or when analog signals are wired in the digital part of the circuit board. This problem is not due to lack of partitioning, the real reason is improper digital signal routing.
PCB design adopts a unified design. Through the division of digital circuits and analog circuits and appropriate signal routing, some difficult layout and wiring problems can usually be solved without causing potential troubles caused by some ground divisions. In this case, the layout and partitioning of components become key to the design. With proper layout, digital ground current will be limited to the digital portion of the board and will not interfere with analog signals. Such wiring must be carefully checked and inspected to ensure 100% compliance with wiring regulations. Otherwise, a poorly routed signal line can completely ruin a perfectly good circuit board.
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 via short leads (Note: Due to the large Most A/D converter chips do not connect analog and digital ground together, they must be connected via external pins) Any external impedance connected to DGND will couple more digital noise to the analog inside the IC via parasitic capacitance in the circuit. According to this proposal, the A/D converter's agnd and DGND pins need to be connected to analog ground. However, this method may lead to problems such as whether the ground end 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 problems can be easily solved. As shown in Figure 3, under the A/D converter, the ground is separated and the analog and digital sections are connected together. When using this method, you need to ensure that the width of the connecting bridge between the two grounds is equal to the width of the IC, and that no signal lines can span the separation gap.
If there are many A/D converters in the system, for example, how to connect 10 A/D converters? If you connect analog and digital grounds together at the bottom of each A/D converter, you will have multiple connections and isolation between analog and digital grounds will be meaningless. Failure to connect in this manner violates the manufacturer's requirements.
If you have questions about the unified design of mixed-signal PCBs, we can use the method of dividing ground layers to lay out and route the entire circuit board. In the design, we should try to make the board easy to connect to separate grounds later in the experiment with jumpers spaced less than 1/2 inch apart or with 0 ohm resistors. Pay attention to zoning and routing to ensure that there are no digital signal lines above the analog part of all layers or any analog signal lines above the digital part. Additionally, there are no signal lines to span the ground gap or divide the gap between power supplies. Test the functionality and EMC performance of the circuit board, then connect the two grounds via a 0 ohm resistor or jumper wire, and retest the functionality and EMC performance of the circuit board. Comparing the test results,
This method can be used in the following three situations: some medical equipment requires low leakage current between circuits and systems connected to the patient; the output of some industrial process control equipment may be connected to high-noise, high-power electromechanical equipment; another The first situation is that the layout of the PCB is restricted.
In mixed-signal PCBs, there are usually separate digital and analog power supplies, and split power supplies can and should be used. However, signal lines adjacent to the power supply layer cannot span the gap between the power supplies, and all signal lines crossing the gap must be located on large adjacent circuit layers. In some cases, replacing a surface analog power supply design with PCB connections can avoid power-side segmentation issues.
Mixed-signal PCB design is a complex process. The following points should be noted during the design process:
1. Divide the PCB into independent analog parts and digital parts.
2. Correct layout of components.
3. A/D converters are placed across partitions.
4. Don’t divide the land. The circuit board is laid out evenly underneath the analog and digital sections.
5. In all layers of the circuit board, digital signals can only be routed in the digital part of the circuit board.
6. In all layers of the circuit board, analog signals can only be routed in the analog part of the circuit board.
7. Achieve separation of analog and digital power supplies.
8. The wiring must not cross the gap between the split power supply surfaces.
9. Signal lines that must pass through the split power supply gap should be located on large adjacent wiring layers.
10. Analyze the actual flow path and return flow method.
11. Use correct wiring rules.