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Partition design of PCB mixed signal

Posted:05:54 PM December 21, 2018 writer: G

The two basic principles of electromagnetic compatibility (EMC) must be understood before design:

The first principle is to reduce the area of the current loop as much as possible;The second principle is that the system uses only one reference plane.

Conversely, if the system has two reference planes, it is possible to form a dipole antenna (note: the radiation size of the small dipole antenna is proportional to the length of the line, the magnitude of the current flowing, and the frequency); and if the signal cannot pass as much as possible With a small loop return, it is possible to form a large loop antenna (Note: The size of the radiation of the small loop antenna is proportional to the loop area, the current flowing through the loop, and the square of the frequency). Both situations should be avoided as much as possible in the design.

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 works, there are a number of potential problems that are particularly acute in complex large systems. The most critical issue is that it is not possible to route across the split gap. Once the split gap wiring is crossed, electromagnetic radiation and signal crosstalk increase dramatically.

The most common problem in PCB design is that the signal line generates EMI problems across the split ground or power supply.

We use the above segmentation method, and the signal line spans the gap between the two grounds. What is the return path of the signal current? It is assumed that the two divided grounds are connected together somewhere (usually at a single point at a certain point), in which case the ground current will form a large loop. The high-frequency current flowing through the large loop generates radiation and high ground inductance. If a large-scale analog current flows through the large loop, the current is easily disturbed by external signals. The worst part is that when the split ground is connected together at the power supply, a very large current loop is formed. In addition, the analog ground and the digital ground are connected together by a long wire to form a dipole antenna.

Understanding the path and manner in which current flows back to ground is key to optimizing mixed-signal 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 split and must be routed through the gap between the splits, a single point connection can be made between the divided grounds to form a bridge between the two grounds, and then route through the bridge. In this way, a direct current return path can be provided below each signal line, resulting in a small loop area.

Signals can also be used to span the split gap using optically isolated devices or transformers. For the former, the optical signal is crossed across the split gap; in the case of a transformer, the magnetic field is crossed across the split gap. Another possible solution is to use a differential signal: the signal flows from one line back from the other, in which case it is not required as a return path.

To delve into the interference of digital signals with analog signals, you must first understand the characteristics of high-frequency currents. The high-frequency current always chooses the smallest impedance (lowest inductance), directly in the path 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 preferred to use a uniform, and the PCB is partitioned into an analog part and a digital part. The analog signals are routed in the analog region of all layers of the board, while the digital signals are routed within the digital circuit area. In this case, the digital signal return current does not flow into the ground of the analog signal.

The interference of the digital signal to the analog signal occurs only when the digital signal is routed over the analog portion of the board or the analog signal is routed over the digital portion of the board. This problem does not occur because there is no division. The real reason is that the wiring of the digital signal is not appropriate.

The PCB design is unified, through digital circuit and analog circuit partitioning and appropriate signal routing, usually can solve some difficult layout problems, and there is no potential trouble caused by ground segmentation. In this case, the layout and partitioning of components becomes the key to determining the pros and cons of the design. If the layout is reasonable, the digital ground current will be limited to the digital portion of the board and will not interfere with the analog signal. Such wiring must be carefully checked and checked, and 100% compliance with wiring rules must be guaranteed. Otherwise, a signal line improperly routed will completely destroy a very good board.

When connecting the analog ground and digital ground pins of an A/D converter, most A/D converter manufacturers recommend connecting the AGND and DGND pins to the same low-impedance ground through the shortest leads. (Note: Since most A/D converter chips do not have analog ground and digital ground connected internally, analog and digital ground connections must be made via external pins.) Any external impedance connected to DGND will pass parasitic capacitance. More digital noise is coupled to the analog circuitry inside the IC. According to this recommendation, the AGND and DGND pins of the A/D converter need to be connected to the analog ground, but this method will cause problems such as whether the ground 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 problem is easy to solve.

If there are more A/D converters in the system, for example, how do 10 A/D converters connect? If the analog ground and the digital ground are connected together under each A/D converter, a multi-point connection is produced, and the isolation between the analog ground and the digital ground is meaningless. If you do not connect in this way, it violates the requirements of the manufacturer.

The best way is to use uniforms at the beginning. Such placement and routing not only meets the requirements of IC device manufacturers for low-impedance connections of analog ground and digital ground pins, but also does not form loop antennas or dipole antennas and cause EMC problems.

If you have a unified approach to the mixed-signal PCB design, you can use the ground layer segmentation method to layout the entire board. When designing, pay attention to making the board easy to use less than 1/2 inch in the back. The jumpers or 0 ohm resistors will be connected separately. Note the partitioning and routing to ensure that no digital signal lines are on top of the analog section on all layers, and that no analog signal lines are above the digital section. Moreover, any signal line cannot cross the ground gap or divide the gap between the power supplies. To test the board's functionality and EMC performance, then reconnect the board's functionality and EMC performance by connecting the two grounds together via a 0 ohm resistor or jumper. Comparing the test results, it will be found that in almost all cases, the unified solution is superior to the segmentation in terms of function and EMC performance.

Is the method of dividing the land still useful?

This approach can be used in three situations: some medical devices require very low leakage currents between circuits and systems connected to the patient; some industrial process control devices may be connected to a very noisy and high-powered electromechanical On the device; another case is when the layout of the PCB is subject to certain restrictions.

There are usually separate digital and analog power supplies on the mixed-signal PCB, and the split power plane can and should be used. However, the signal lines immediately adjacent to the power plane cannot cross the gap between the power supplies, and all signal lines spanning the gap must be located on the circuit layer next to the large area. In some cases, designing the analog power supply with a PCB connection instead of a face avoids the problem of splitting the power plane.

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

1. Partition the PCB into separate analog and digital sections.

2. Suitable component layout.

3. The A/D converter is placed across the partition.

4. Do not divide the ground. Uniformly laid under the analog and digital sections of the board.

5. In all layers of the board, digital signals can only be routed in the digital portion of the board.

6. In all layers of the board, analog signals can only be routed in the analog portion of the board.

7. Implement analog and digital power splitting.

8. The wiring cannot cross the gap between the divided power planes.

9. The signal line that must cross the gap between the divided power supplies is located on the wiring layer immediately adjacent to the large area.

10. Analyze the path and mode through which the return current flows.

11. Use the correct wiring rules.

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