1 Why do you want to divide digital ground and analog ground?
Although it is interlinked, it is different when the distance is long. The voltage at different points of the same wire may be different, especially when the current is large. Because there is resistance in the wire, the voltage drop will occur when the current flows. In addition, the wire has distributed inductance, and the influence of distributed inductance will be shown under AC signal. Therefore, we have to divide it into digital ground and analog ground, because the high-frequency noise of digital signals is very loud. If the analog ground and digital ground are mixed, the noise will be transmitted to the analog part, causing interference. If grounded separately, high-frequency noise can be isolated by filtering at the power supply. But if the two places are mixed, it is not easy to filter.
How to design digital ground and analog ground
Two basic principles of electromagnetic compatibility (EMC) must be understood before design: the first principle is to minimize the area of current loop; The second principle is that the system only uses one reference plane. On the contrary, if there are two reference planes in the system, it is possible to form a dipole antenna (Note: the radiation of a small dipole antenna is proportional to the length of the wire, the current flowing through it and the frequency); However, if the signal can’t return through the smallest possible loop, a large loop antenna may be formed (Note: the radiation of a small loop antenna is proportional to the loop area, the current flowing through the loop and 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 that the digital ground and analog ground can be isolated. Although this method is feasible, there are many potential problems, especially in complex large-scale systems. The key problem is that the wiring can’t cross the split gap. Once the wiring crosses the split gap, the electromagnetic radiation and signal crosstalk will increase sharply. The most common problem in PCB design is that the signal line crosses the ground or power supply, resulting in EMI.
We adopt the above division method, and the signal line spans the gap between the two grounds. What is the return path of the signal current? Assuming that the two divided grounds 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 big loop will produce radiation and high ground inductance. If the low-level analog current flows through the big loop, the current is easily interfered by external signals. Worst of all, when the divided grounds are connected together at the power supply, a very large current loop will be formed. In addition, the analog ground and the digital ground are connected together by a long wire to form a dipole antenna.
Knowing 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, but ignore the specific path of the current. If the ground plane must be divided and wired through the gaps between the divisions, a single point connection can be made between the divided grounds to form a connecting bridge between the two grounds, and then the wiring can be made through the connecting bridge. In this way, a direct current return path can be provided under each signal line, so that the formed loop area is small.
Optical isolation devices or transformers can also be used to realize the signal crossing the split gap. For the former, the optical signal crosses the split gap; In the case of transformer, it is the magnetic field that spans the split gap. Another feasible method is to use differential signals: signals flow in from one line and return from another signal line, in which case they are unnecessarily used as return paths.
To deeply discuss the interference of digital signals to analog signals, we must first understand the characteristics of high-frequency current. The high-frequency current always chooses the path with the lowest impedance (lowest inductance) and directly below the signal, so the return current will flow through the adjacent circuit layer, regardless of whether this adjacent layer is the power layer or the ground layer. In practice, it is generally preferred to use a unified ground, and divide the PCB into analog and digital parts. Analog signals are routed in analog areas of all layers of the circuit board, while digital signals are routed in digital circuit areas. In this case, the return current of the digital signal will not flow into 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 occur. This kind of problem is not because there is no division, but the real reason is that the wiring of digital signals is not appropriate. The design of PCB is uniform, through digital circuit and analog circuit partition and appropriate signal wiring, which can usually solve some difficult layout and wiring problems, and at the same time, it will not cause some potential troubles 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, improper routing of a signal line 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 suggest that AGND and DGND pins be connected to the same low impedance ground through the shortest lead (note: because most A/D converter chips are not connected with analog ground and digital ground together, the connection between analog and digital ground must be realized through external pins), and any external impedance connected with 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 kind of square leads to problems such as whether the grounding 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 problem 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 adopting this method, it is necessary to ensure that the width of the bridge between the two grounds is as wide as that of the IC, and that no signal line can cross the division gap.
If there are many A/D converters in the system, for example, how can 10 A/D converters be connected? If the analog ground and digital ground are connected together under each A/D converter, multi-point connection will occur, and the isolation between analog ground and digital ground will be meaningless. If you don’t connect in this way, it violates the requirements of the manufacturer.
The best way is to start with a uniform land. As shown in Figure 4, the unified ground is divided into analog part and digital part. This layout not only meets the requirements of IC device manufacturers for low impedance connection between analog ground and digital ground pins, but also does not form loop antenna or dipole antenna to cause EMC problems.
If you have doubts about the uniform design of mixed-signal PCB, you can use the method of dividing the ground plane to lay out the whole circuit board. During the design, you should try your best to make the circuit board easy to be connected together by jumpers with a spacing of less than 1/2 inch or 0 ohm resistors in the later experiments. Pay attention to zoning and wiring, and make sure that there is no digital signal line on the analog part and no analog signal line on the digital part 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, and then connect two grounds together through a 0 ohm resistor 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 scheme is superior to the split scheme in terms of function and EMC performance.
Is the method of dividing land still useful?
This method can be used in the following three situations: some medical devices require that the leakage current between the circuits and systems connected with the patient is very low; The output of some industrial process control equipment may be connected to mechanical and electrical equipment with high noise and power; The other situation is when the layout of PCB is restricted.
There are usually independent digital and analog power supplies on the mixed-signal PCB board, and the split power supply plane can and should be adopted. However, the signal lines 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 a large area of ground. In some cases, the analog power supply is designed with PCB connecting lines instead of a plane, which can avoid the problem of dividing the power supply plane.
