Mixed signal refers to digital and analog signal. The first principle is to reduce the area of current loop as much as possible; the second principle is that the system only uses one reference plane. On the contrary, if there are two reference planes in the system, a dipole antenna may be formed (Note: the radiation of a small dipole antenna is directly proportional to the length of the line, the current flowing through and the frequency); if the signal cannot return through the loop as small as possible, a large loop antenna may be formed (Note: the radiation of a small loop antenna is directly proportional to the loop area and the current flowing through) The loop current is proportional to the square of the frequency. These two situations should be avoided as far as possible in the design.
It is suggested that the digital ground and the analog ground on the mixed signal circuit board should be separated so as to realize the isolation between the digital ground and the analog ground. Although this method is feasible, there are many potential problems, especially in complex large-scale systems. The most important problem is that we can’t cross the gap. Once we cross the gap, the electromagnetic radiation and signal crosstalk will increase sharply. In PCB design, the most common problem is the EMI problem caused by signal lines crossing the divided ground or power supply.
We use the above segmentation method, and the signal line crosses the gap between the two grounds. What is the return path of the signal current? It is assumed that the two separated grounds are connected together at some place (usually at a single point). In this case, the ground current will form a large loop. The high-frequency current flowing through the large loop will produce radiation and high ground inductance. If the low-level analog current flows through the large loop, the current is vulnerable to external signal interference. The worst thing is that when the split ground is connected together at the power supply, it will form a very large current loop. In addition, analog ground and digital ground are connected by a long wire to form a dipole antenna.
Understanding the path and mode of current return to ground is the key to optimize the design of mixed signal circuit board. Many design engineers only consider where the signal current flows, and ignore the specific path of the current. If it is necessary to divide the ground layer and route through the gap between the divisions, a single point connection can be made between the divided ground layers to form a connection bridge between the two ground layers, 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 is very small.
Optical isolation devices or transformers can also be used to realize the signal across the division gap. For the former, the optical signal crosses the division gap; for the transformer, the magnetic field crosses the division gap. Another way to use the return signal path from the differential line is not feasible.
In order to further explore the interference of digital signal to analog signal, we must first understand the characteristics of high frequency current. High frequency current always chooses the path with the lowest impedance (inductance) and 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, PCB is usually divided into analog part and digital part. Analog signals are wired in the analog area of all layers of the circuit board, while digital signals are wired in the digital circuit area. In this case, the return current of the digital signal does not flow to the ground of the analog signal.
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, the interference of the digital signal to the analog signal will appear. This problem is not because there is no division, the real reason is that the digital signal wiring is not appropriate.
PCB design adopts unified layout, through digital circuit and analog circuit partition and appropriate signal wiring, it can usually solve some difficult layout and wiring problems, and at the same time, it will not produce some potential trouble caused by ground partition. In this case, the layout and partition of components become the key to the design. If the layout and wiring are reasonable, the digital ground current will be limited in the digital part of the circuit board, and will not interfere with the analog signal. Such wiring must be carefully checked and checked to ensure 100% compliance with the wiring rules. Otherwise, a wrong signal line will completely destroy a very good circuit board.
When connecting analog ground and digital ground pins of a / D converter, most a / D converter manufacturers recommend that agnd and DGND pins be connected to the same low impedance ground through the shortest lead (Note: since most a / D converter chips do not connect analog ground and digital ground together internally, analog ground and digital ground must be connected through external pins), Any external impedance connected to DGND will couple more digital noise to the analog circuit in IC through parasitic capacitance. According to this suggestion, it is necessary to connect both agnd and DGND pins of a / D converter to analog ground. However, this method may cause problems such as whether the ground terminal of 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, the ground is divided and the analog and digital parts are connected under the A / D converter. When adopting this method, the width of the bridge between the two ground 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 ground and digital ground are connected under each a / D converter, the multipoint connection will be generated, and the isolation between analog ground and digital ground will be meaningless. If you don’t connect in this way, it will violate the requirements of the manufacturer.
The best way is to start with a unified approach. As shown in Fig. 4, it is uniformly divided into analog part and digital part. This 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 cause EMC problems due to loop antenna or dipole antenna.
If you have doubts about adopting a unified approach to the design of mixed signal PCB, you can use the method of dividing the ground wire layer to lay out and route the whole circuit board. In the design, you should pay attention to make it easy to connect the circuit board with a jumper or 0 ohm resistor with a spacing less than 1 / 2 inch in the back experiment. Pay attention to zoning and wiring, and make sure that no digital signal line is above the analog part on all layers, and no analog signal line is above the digital part. Moreover, no signal line can cross the ground gap or the gap between the split power sources. To test the function and EMC performance of the circuit board, connect the two ground wires together through 0 ohm resistance or jumper, and retest the function and EMC performance of the circuit board. Comparing the test results, we can find that in almost all cases, the unified solution is better than the split solution in terms of function and EMC performance.
Is the method of dividing land still useful?
This method can be used in the following three cases: some medical equipment requires low leakage current between the circuit and system connected with the patient; the output of some industrial process control equipment may be connected to electromechanical equipment with high noise and high power; another case is when the layout of PCB is limited.
Generally, there are independent digital and analog power supply on mixed signal PCB board, and split power supply can and should be used. However, the signal line adjacent to the power layer cannot cross the gap between power sources, and all the signal lines crossing the gap must be located on the circuit layer adjacent to the large area ground. In some cases, the analog power supply is designed as PCB connecting line instead of one side, which can avoid the problem of power supply side segmentation.
The design of mixed signal PCB is a complex process. The following points should be paid attention to in the design process
1. Divide PCB into independent analog part and digital part.
2. Proper layout of components.
3. The A / D converter is placed across partitions.
4. Don’t divide the ground. Under the analog part and the digital part of the circuit board, the ground wire is laid uniformly.
5. In all layers of the circuit board, digital signals can only be wired in the digital part of the circuit board.
6. In all layers of the circuit board, analog signals can only be wired in the analog part of the circuit board.
7. Realize the division of analog and digital power supply.
8. The wiring shall not cross the gap between the divided power supply surfaces.
9. The signal line that must cross the gap between the split power supply should be located on the wiring layer adjacent to the large area.
10. Analyze the actual path and mode of return current.
11. Use correct wiring rules