PCB is a support for circuit components and devices in electronic products, which provides electrical connections between circuit components and devices. Nowadays, many PCBs are no longer single-function circuits, but are composed of a mixture of digital circuits and analog circuits. Data is generally collected and received in analog circuits, while bandwidth and gain are controlled by software and must be digitized, so digital circuits and analog circuits often exist on the same board, and even share the same components.
Considering the mutual interference between them and the impact on circuit performance, the layout and wiring of the circuit must have certain principles. The special requirements for power transmission lines in mixed-signal PCB design and the requirement to isolate noise coupling between analog and digital circuits increase the complexity of layout and routing at design time. Here, the required PCB design goals are achieved by analyzing the layout and routing design of high-density mixed-signal PCBs.
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 tiny interference can affect its normal operation, while the operation of digital circuits depends on The receiving end has a certain anti-interference ability for the detection of high level or low level according to the pre-defined voltage level or threshold. But in a mixed-signal environment, digital signals are a source of noise relative to analog signals. When the digital circuit is working, the stable effective voltage has only two voltages, high and low. When the digital logic output changes from a high voltage to a low voltage, the ground pin of the device is discharged, and a switching current is generated, 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 logic high level to 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, which we call ground bounce. If the ground bounce noise and power disturbance caused by the digital circuit are coupled into the analog circuit, it will affect the working performance of the analog circuit. Since quite a few sources of interference are generated through the power and ground buses, among which the noise interference caused by the ground wire is the largest, the design of the ground and power supply is particularly important in PCB design.
2. General processing principles of digital-analog hybrid circuit PCB design
The above mentioned the 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 designing: 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 shape 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. These two situations should be avoided as much as possible in the design.
2.1. Principles of layout and wiring. One of the first factors to consider in component layout is to separate the analog circuit part from the digital circuit part. Analog signals are routed in the analog area of all layers of the circuit board, while digital signals are routed in the digital circuit area. In this case, the digital signal return current does not flow to the analog signal ground. For some lines with high frequency and special requirements, it is best to manually route them, and use differential lines or shielded lines if necessary.
Sometimes due to the location of the I/O connectors, it is necessary to mix the wiring of the digital and analog circuits, which creates a high potential for the interaction of the analog and digital circuits. This is to avoid running digital clock lines and high-frequency analog signal lines near the analog power supply plane, otherwise, the noise of the power supply 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 as far away from sensitive analog circuit devices as possible.
2.2. Handling of power supply and ground. In the design of complex mixed circuit boards, the layout and handling of ground lines is an important factor in improving circuit performance. Some people recommend splitting the digital and analog grounds on mixed-signal boards to provide isolation between 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.
Understanding the path and manner in which 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 the wiring can be routed through the connection bridge.
In this way, a direct current return path can be provided under each signal line, or optical isolation devices, transformers, etc. can be used to realize signals crossing the division gap. However, in actual work, PCB design tends to adopt a unified ground. Through the partitioning of digital circuits and analog circuits and appropriate signal wiring, some difficult layout and wiring problems can usually be solved, and at the same time, some potential troubles caused by ground division will not occur. . By comparing the test results of the circuit boards, it will also be found that the unified solution is superior to the divided solution in terms of function and EMC performance.
On mixed-signal PCB boards, there are usually separate digital and analog power supplies, and a split power plane should be used, preferably next to and below the ground plane. The power plane may couple radio frequency current to the circuit that can be attached to the space. In order to reduce this coupling effect, the power plane is required to be physically smaller than its adjacent ground plane by 20H (H refers to the distance between the power and ground plane layers).
2.3. 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 must be connected to the same low-impedance analog ground plane externally, and the leads should be as short as possible. Any additional impedance of DGND will couple more digital noise into the analog circuit inside the device through parasitic capacitance. .
Of course, doing this will make the digital current inside the converter flow into the analog ground plane, but this is much less disturbing than connecting the DGND pin of the converter device to the noisy digital ground plane. Like ground, the analog and digital supply pins should also be connected to the analog supply plane with appropriate bypass capacitors connected as close as possible to each supply pin. If necessary, the analog power supply pins should be isolated from the digital power supply pins by means of jumper inductors.
2.4. Add decoupling capacitors. Decoupling capacitors can eliminate high-frequency interference. Since the capacitive reactance of capacitors is inversely proportional to frequency, connecting capacitors in parallel between the signal and ground wires can bypass high-frequency noise. In principle, add a 0.01mF~0.1mF ceramic chip capacitor to each integrated chip, which can not only store energy in the chip, provide and absorb the charging and discharging energy of the chip's circuit at the moment of opening and closing, but also bypass and filter out high frequency noise content of the device.
Adding a 10mF~100mF electrolytic capacitor (preferably tantalum capacitor) at the input end of the power supply can suppress the noise interference of the power supply. Of course, the lead wire of the added capacitor should not be too long, because the lead wire length of the capacitor is a very important parameter. The longer the inductance, the larger the inductance, the lower the resonant frequency of the capacitor, and the frequency filtering effect on high-frequency noise will be weakened or even disappear. Therefore, when designing a high-speed PCB board, special attention should be paid to keeping the lead wire of the capacitor as short as possible. That is, make the capacitor as close as possible to the chip.
2.5. A large area of copper clad foil is connected to the analog ground. Cover a large area of copper foil on the analog circuit part and drill dense holes in the blank area to connect to the analog ground, which can play a role of shielding and isolation, thereby reducing mutual interference between analog signals, and can also play a role in heat dissipation.
2.6. The power line and ground line should be as short and thick as possible, especially the lines on the magnetic beads connecting the digital power supply and the analog power supply must be thicker, because in addition to reducing the voltage drop, the more important thing is to reduce the coupling noise.
3. PCB design example of hybrid circuit
A designed 20-layer circuit board with a typical digital-analog hybrid circuit with 32-channel digital reception and conversion. The highest frequency is high-speed optical fiber signal up to 2.5GHz. The layout of this 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. The analog circuit should be placed as close to the edge of the circuit board as possible, and the digital circuit should be placed as close as possible to the power connection terminal, which can reduce the di/dt effect caused by the digital switch.
In terms of the division of power and ground, the analog signals of this printed circuit board are all routed on the surface layer, and the route is as short as possible and less drilled. The second layer and the nineteenth layer next to the analog signal are a complete and unified analog ground plane, which ensures that the analog signal has the best return path and impedance, and there will be no EMI problems across the split ground. The high-speed signal layer is adjacent to the ground plane layer, the important signal lines go through the strip line, and the clock and reset sensitive signal lines go on the third layer, between the two ground planes. There are separate planes for digital and analog power that are split, but each power plane is also immediately adjacent to the ground plane plane.
The high-speed A/D hybrid device is connected to the analog ground on the board, that is, the external ground pin of the device, and the analog power supply is connected to the power pin, and decoupling capacitors are added next to the power pin to eliminate high-frequency interference. The lines on the bead inductors connected to the power supply or ground should be thickened. It is better to connect a few more signal lines and drill holes to connect to the power supply or ground plane, which can reduce the voltage drop and reduce the noise.
Sometimes it is also possible to meet the requirements by using drilled large vias to connect to the plane. The high-frequency signal lines are strictly controlled in line width and line spacing to meet the impedance requirements. Manual wiring is used. Finally, dense holes are drilled in the blank area covered by 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 telecommunications requirements. The printed boards produced have been debugged and shown that the interference of digital signals to analog signals is very small, and the parameters are good. Hybrid circuit PCB design is a complex process. The layout and wiring of components, as well as the treatment of power and ground wires will directly affect the circuit performance and electromagnetic compatibility performance. Certain wiring rules must be followed in the design to make the designed PCB board reach Design requirements.
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