PCB Technical - How to realize the partition design of mixed signal PCB?

The design of mixed signal circuit PCB is very complex. The layout and wiring of components and the processing of power supply and ground wire will directly affect the circuit performance and EMC performance. The partition design of ground and power supply introduced in this paper can optimize the performance of mixed signal circuit.

How to reduce the mutual interference between digital signal and analog signal? Two basic principles of electromagnetic compatibility (EMC) must be understood before design: one is to reduce the area of current loop as much as possible; the other is that only one reference surface is used 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 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 smallest loop as possible, a large loop antenna may be formed The current of the loop is proportional to the square of the frequency. These two situations should be avoided as much as possible in the design.

It is suggested that the digital ground and 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 key problem is that it is impossible to route across the split gap. Once it is crossed, electromagnetic radiation and signal crosstalk will increase rapidly. The common problem in PCB design is EMI problem caused by signal line crossing over divided ground or power supply.

We use the above segmentation method, and the signal line crosses the gap between the two ground. What is the return path of the signal current? It is assumed that the two separated grounds are connected at some place (usually a single point 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 produce radiation and high ground inductance. If the current flowing through the large loop is low-level analog current, the current is easy to be interfered by external signals. What's worse is that when the split ground is connected together at the power supply, it will form a very large current loop. In addition, analog and digital connection through a long wire will 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, ignoring the specific path of the current. If the ground wire layer must be divided and the wiring must be conducted through the gap between the partitions, a single point connection can be made between the divided ground layers to form a connection bridge between the two ground layers, and then the wiring is conducted 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 very small.

Optical isolation devices or transformers can also be used to cross the gap. For the former, the optical signal crosses the gap, while in the case of transformer, the magnetic field crosses the gap. Another possible approach is to use differential signals: signals flow in from one line and return from another, in which case they do not need to be used as a return path.

In order to explore the interference of digital signal to analog signal, we must first understand the characteristics of high frequency current. High frequency currents always choose the impedance (inductance) path directly below the signal, so the return current will flow through the adjacent circuit layer, 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 routed in the analog area of all layers of the board, while digital signals are routed in the digital circuit area. In this case, the return current of the digital signal will 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 a unified design, through the digital circuit and analog circuit partition and appropriate signal wiring, usually can solve some difficult layout and wiring problems, but also will not produce some potential trouble caused by ground division. In this case, the layout and partition of components become the key to 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 analog signal. Such wiring must be carefully checked and checked to ensure 100% compliance with wiring rules. Otherwise, a signal line wiring improper will completely destroy a very good circuit board.

When connecting analog ground and digital ground pins of a / D converter together, most a / D converter manufacturers will recommend that agnd and DGND pins be connected to the same low impedance ground through short leads (Note: since most a / D converter chips do not connect analog ground and digital ground together, they must be connected through external pins) Any external impedance connected to DGND will couple more digital noise to the analog circuit inside the IC through parasitic capacitance. According to this proposal, it is necessary to connect the agnd and DGND pins of the A / D converter to the analog ground. However, this method may cause problems such as whether the ground terminal of the digital signal decoupling capacitor should be connected to the analog ground or the digital ground.

If the system has only one a / D converter, the above problems can be easily solved. As shown in Fig. 3, the ground is divided and the analog and digital parts are connected together under the A / D converter. When adopting this method, it is necessary to ensure that the width of the connecting bridge between the two grounds is equal to that of the IC, and no signal line can cross the dividing gap.

If there are many a / D converters in the system, for example, how to connect 10 A / D converters? If analog ground and digital ground are connected together at the bottom of each a / D converter, there will be multi-point connection, and the isolation between analog ground and digital ground is meaningless. If you do not connect in this way, it is in violation of the manufacturer's requirements.

If you have doubts about the unified design of mixed signal PCB, we can use the method of dividing the ground layer to lay out and route the whole circuit board. In the design, we should try our best to make the circuit board easy to connect the separated ground with Jumpers with spacing less than 1 / 2 inch or 0 ohm resistance in the rear experiment. Pay attention to zoning and wiring to ensure that there are no digital signal lines above the analog section or any analog signal line above the digital section on all layers. Moreover, no signal line can cross the ground gap or divide the gap between power supplies. To test the function and EMC performance of the circuit board, then connect the two ground through 0 ohm resistance or jumper, and retest the function and EMC performance of the circuit board. Comparing the test results, it can be found that in almost all cases, the unified solution is superior to the segmented one in terms of function and EMC performance.

This method can be used in the following three situations: some medical devices require low leakage current between circuits and systems connected to patients; 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 restricted.

In mixed signal PCB, there are usually independent digital and analog power supply, which can and should adopt split power supply. However, the signal line adjacent to the power supply layer cannot cross the gap between the power supplies, and all the signal lines crossing the gap must be located on the circuit layer adjacent to the large area. In some cases, the design of analog power supply with PCB connection line instead of one surface 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. A / D converters are placed across partitions.

4. Do not divide the ground. The circuit board is laid uniformly under the analog part and the digital part.

5. In all layers of the circuit board, the digital signal can only be wired in the digital part of the circuit board.

6. In all layers of the circuit board, the analog signal can only be wired in the analog part of the circuit board.

7. Realize analog and digital power separation.

8. The wiring shall not cross the gap between the split power faces.

9. The signal wire that must cross the gap between the split power supplies shall be located on the wiring layer adjacent to the large area.

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

11. Use correct wiring rules.