PCB Blog - PCB board design of Digital-Analog Mixed Circuit

In the design of high speed PCB board, the interference problem in PCB design of digital-analog hybrid circuit has always been a difficult problem. In particular, the analog circuit is generally the source of the signal. Whether the signal can be received and converted correctly is an important factor to be considered in the PCB design. By analyzing the mechanism of the interference of the hybrid circuit, combined with the design practice, the paper discusses the general processing method of the hybrid circuit, and it is verified by a design example. A printed circuit board (PCB) is a support for circuit elements and devices in electronic products, and it provides electrical connections between circuit elements and devices. There are many PCBs now that are no longer a single function circuit, but a mixture of digital and analog circuits. Data is generally collected and received in analog circuits, while bandwidth and gain must be digitized for software control, so digital circuits and analog circuits often coexist on one board, even sharing the same components. Considering the mutual interference between them and the influence on the circuit performance, the layout and wiring of the circuit must have certain principles. The special requirements for power transmission lines and the requirement to isolate noise coupling between analog and digital circuits in mixed-signal PCB design increase the complexity of layout and routing during design. Here, the required PCB design goals are achieved by analyzing the layout and routing design of a high-density mixed-signal PCB.

1. The generation mechanism of digital-analog hybrid circuit interference
Compared with digital signals, analog signals are much more sensitive to noise, because the operation of analog circuits depends on continuously changing current and voltage, and any slight interference can affect its normal operation, while the operation of digital circuits relies on The receiving end detects high level or low level according to the pre-defined voltage level or threshold, and has a certain anti-interference ability. But in a mixed-signal environment, digital signals are a source of noise relative to analog signals. When the digital circuit is working, there are only two kinds of voltages, high and low level, the stable effective voltage. When the digital logic output changes from a high voltage to a low voltage, the ground pin of the device will discharge, resulting in a switching current, which is the switching action of the circuit. The faster the speed of the digital circuit, the shorter the switching time is generally required. When a large number of switching circuits change from a logic high level to a logic low level at the same time, due to the insufficient ability of the ground wire to pass current, a large amount of switching current will cause. The logic ground voltage fluctuates, we call it a ground bounce. As shown in Figure 1. Ground bounce noise and power supply disturbances caused by digital circuits, if coupled into analog circuits, will affect the performance of analog circuits. Since quite a lot of interference sources are generated by the power supply and the ground bus, among which the noise interference caused by the ground wire, the design of the ground and the power supply is particularly important in the PCB design.

2. General processing principles for PCB design of digital-analog hybrid circuits
The above mentioned the generation mechanism of hybrid circuit interference, so how to reduce the mutual interference between digital signals and analog signals? Two basic principles of electromagnetic compatibility (EMC) must be understood before design: The first principle is to minimize the area of the current loop, if the signal cannot return through the smallest possible loop, a large loop may be formed shaped antenna. 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. Both of these situations should be avoided as much as possible in the design.
(1) Layout and routing principles. One of the first factors to consider in component layout is to separate the analog circuit part from the digital circuit part. The analog signal is routed in the analog area of all layers of the board, and the digital signal is routed in the digital circuit area. In this case, the digital signal return current does not flow into the analog signal ground. For some lines with high frequency and special requirements, use differential lines or shielded lines if necessary. Sometimes due to the location of the input/output connectors, it is necessary to mix the wiring of the digital and analog circuits, so there is a high possibility that the analog and digital circuits will interfere with each other. This is to avoid running digital clock lines and high-frequency analog signal lines adjacent to the analog power plane, otherwise, the noise of the power signal will be coupled into the sensitive analog signal. To try to achieve a low impedance power and ground network, the inductive reactance of the digital circuit wires should be minimized, and the capacitive coupling of the analog circuit should be minimized. The frequency of digital circuits is high, and the sensitivity of analog circuits is strong. For signal lines, high-frequency digital signal lines should be kept away from sensitive analog circuit devices as much as possible.
(2) Handling of power and ground. In the design of complex hybrid circuit boards, the layout and handling of ground traces are important factors in improving circuit performance. It has been suggested to separate the digital and analog grounds on mixed-signal boards to achieve isolation between the digital and analog grounds. But this approach tends to route across the split gap, which can cause a dramatic increase in electromagnetic radiation and signal crosstalk. Knowing where and how current returns to ground is key to optimizing mixed-signal board designs. If the ground layer must be divided, and the wiring must be routed through the gap between the divisions, a single-point connection can be made between the divided grounds to form a connection bridge between the two grounds, and then routed through the connection bridge. In this way, a direct current return path can be provided under each signal line, or an optical isolation device, a transformer, etc. can be used to realize the signal crossing the division gap. However, in actual work, PCB design tends to use unified ground. Through the partition of digital circuits and analog circuits and appropriate signal wiring, some difficult layout and wiring problems can usually be solved, and some potential troubles caused by ground separation will not occur. . By comparing the circuit board test results, it is also found that the unified solution is superior to the segmented solution in terms of functionality and EMC performance. There are usually separate digital and analog power supplies on mixed-signal PCBs, and a split power plane should be used, next to and below the ground plane. Power planes may couple RF currents to circuits that can be attached to the space. In order to reduce this coupling effect, the power planes are required to be physically 20H smaller than their adjacent ground planes (H refers to the distance between the power supply and the ground plane).
(3) Handling of hybrid devices. Common hybrid devices include crystal oscillators, high-speed AD devices, etc., and there are two parts of digital circuits and analog circuits inside the device. Generally, the AGND and DGND pins should be externally connected to the same low-impedance analog ground plane, and the lead should be as short as possible. Any additional impedance of DGND will couple more digital noise to the analog circuit inside the device through parasitic capacitance. . Of course, doing this will cause the digital current inside the converter to flow into the analog ground plane, but this is much less intrusive than connecting the converter's DGND pin to a noisy digital ground plane. Like ground, the analog and digital power pins should also be connected to the analog power plane with appropriate bypass capacitors as close to each power pin as possible. If necessary, isolate the analog power pins from the digital power pins by connecting them across inductors.
(4) Add decoupling capacitors. Decoupling capacitors can eliminate high-frequency interference. Since the capacitive reactance of the capacitor is inversely proportional to the frequency, connecting the capacitor in parallel between the signal and the ground can bypass the high-frequency noise. In principle, a 0.01mF~0.1mF ceramic chip capacitor is added to each integrated chip, which not only enables the chip to store energy, provides and absorbs the charging and discharging energy at the moment when the circuit of the chip opens and closes the door, but also bypasses the filter. high frequency noise components of the device. Adding a 10mF~100mF electrolytic capacitor (tantalum capacitor) to the power input can suppress the noise interference of the power supply. Of course, the added capacitor lead should not be too long, because the lead length of the capacitor is a very important parameter, the longer the lead, the larger the inductive inductance is, the lower the resonant frequency of the capacitor will be, and the frequency filtering effect on high-frequency noise will be weakened or even disappear. Keep the capacitors as close to the chip as possible.
(5) A large area of copper clad foil is connected to the analog ground. Cover a large area of copper foil in the analog circuit part and drill dense holes in the blank area to connect to the analog ground, which can play the role of shielding and isolation, thereby reducing mutual interference between analog signals, and can also play a role in heat dissipation.
(6) The power line and the ground line should be as short and thick as possible, especially the lines on the magnetic beads that bridge the digital power supply and the analog power supply must be thick, because in addition to reducing the voltage drop, it is more important to reduce the coupling noise.

3. PCB design example of hybrid circuit
The layout of the printed board separates the analog circuit from the digital circuit, and each channel is completely independent with a certain distance to ensure that the analog signals of each channel will not interfere with each other. Place the analog circuits as close to the edge of the board as possible, and place the digital circuits as close to the power connections as possible to reduce the di/dt effect caused by digital switching. In the division of power supply and ground, the analog signals of this printed board are all on the surface layer, and they are as short and drilled as possible. The second layer and the nineteenth layer next to the analog signal are complete and unified analog ground planes, so as to ensure the return path and impedance of the analog signal, and there will be no EMI problem across the divided ground. The high-speed signal layer is next to the ground plane layer, the important signal lines are routed to the stripline, and the clock and reset sensitive signal lines are routed to the third layer between the two ground planes. Both digital and analog power have separate planes, both are split, but each power plane is also immediately adjacent to the ground plane layer. The high-speed A/D hybrid device is connected to the analog ground on the board, that is, the external ground pins of the device are connected to the analog ground, the power pins are all connected to the analog power supply, and decoupling capacitors are added next to the power pins to eliminate high-frequency interference. The line on the magnetic bead inductor that is connected to the power supply or ground should be thickened, and a few more signal lines should be drilled to connect to the power supply or ground plane, which can reduce the voltage drop and reduce the noise. Sometimes large via holes are used. Connecting to a plane can also meet the requirements. The high-frequency signal lines are strictly controlled by line width and line spacing to make them meet the impedance requirements. They are all wired manually, and dense holes are drilled in the blank area of a large area of copper foil in the analog circuit part to connect to the analog ground. The 100M clock signal line on this printed board has been simulated and analyzed by the design software, and the signal transmission is basically not disturbed, which meets the telecommunication requirements. The printed board produced after debugging shows that the interference of digital signal to analog signal is very small, and the parameter indicators are good.

4. Conclusion
Hybrid circuit PCB design is a complex process. The layout and wiring of components and the handling of power and ground wires will directly affect circuit performance and electromagnetic compatibility. Certain wiring rules must be followed in the design to make the designed PCB board meet the Design requirements.