PCB News - Digital-analog mixed circuit PCB design

General principles of PCB design for digital-analog hybrid circuit

I talked about the generation mechanism of hybrid circuit interference, so how to reduce the mutual interference between digital signal and analog signal? Before designing, we must understand the two basic principles of electromagnetic compatibility (EMC): The first principle is to reduce the area of the current loop as much as possible. If the signal cannot return through the smallest possible loop, a large loop may be formed.状 Antenna. The second principle is that the system uses only one reference plane. On the contrary, if the system has two reference planes, it may form a dipole antenna. Avoid these two situations as much as possible in the design.

(1) Principles of layout and routing. One of the first factors to consider for 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 circuit board, and the digital signal is routed in the digital circuit area. In this case, the digital signal return current will not flow into the analog signal ground. For some high-frequency lines with special requirements, it is best to manually route them, and use differential lines or shielded lines when necessary. Sometimes due to the position of the input/output connector, the wiring of the digital and analog circuits must be mixed together, which may cause the mutual influence of the analog part and the digital part of the circuit. This is necessary to avoid running digital clock lines and high-frequency analog signal lines near the analog power layer, otherwise, the noise of the power signal will be coupled to the sensitive analog signal. To try to achieve a low-impedance power and ground network, the inductance of the digital circuit wire should be minimized, and the capacitive coupling of the analog circuit should be minimized. The frequency of the digital circuit is high, and the sensitivity of the analog circuit is strong. For the signal line, the high-frequency digital signal line should be as far away as possible from the sensitive analog circuit device.

(2) Power and ground handling. In the design of complex hybrid circuit boards, the layout and handling of grounding wires are important factors to improve circuit performance. It is suggested to separate the digital ground and analog ground on the mixed-signal circuit board to achieve isolation between the digital ground and the analog ground. However, this method tends to cross the separation gap wiring, which will cause a sharp increase in electromagnetic radiation and signal crosstalk.

Knowing the path and method of current return to ground is the key to optimizing mixed-signal circuit board design. 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 wiring 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 also be used to achieve the signal across the segmentation gap. However, in actual work, PCB design tends to adopt a unified ground. Through the division of digital and analog circuits and appropriate signal wiring, some difficult layout and wiring problems can usually be solved, and it will not cause some potential troubles due to local division. . By comparing the test results of the circuit board, you will also find that the unified ground solution is superior to the split ground in terms of function and EMC performance.

There are usually independent digital and analog power supplies on the mixed-signal PCB board. Split power planes should be used, preferably next to and below the ground plane. The power plane may couple radio frequency currents to circuits 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 supply and the ground plane).

(3) Treatment of hybrid devices. Common hybrid devices include crystal oscillators, high-speed AD devices, etc. There are two parts of digital circuit and analog circuit inside the device. Generally, the AGND and DGND pins should be connected externally 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 internal analog circuit of the device through parasitic capacitance. . Of course, this will cause the digital current inside the converter to flow into the analog ground plane, but this will cause much less interference than connecting the DGND pin of the converter to the noisy digital ground plane. Like grounding, the analog and digital power pins should also be connected to the analog power plane, and appropriate bypass capacitors should be connected as close as possible to each power pin. If necessary, the analog power supply pin and the digital power supply pin should be isolated by a jumper inductor.

(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 will serve as a bypass for high-frequency noise. In principle, each integrated chip is added with a 0.01mF~0.1mF ceramic chip capacitor, which not only enables the chip to store energy, but also provides and absorbs the instantaneous charging and discharging energy of the chip's circuit when the door is opened and closed, and it can also be bypassed and filtered. The high frequency noise component of the device. Add a 10mF~100MF electrolytic capacitor (preferably a tantalum capacitor) to the power input terminal to suppress the noise interference of the power supply. Of course, the capacitor lead should not be too long, because the lead length of the capacitor is a very important parameter. The longer the inductance, the greater the inductance and the lower the resonance frequency of the capacitor. The frequency filtering effect on high-frequency noise will be weakened or even disappear. Therefore, in the design of high-speed PCB boards, special attention should be paid to making the lead of the capacitor as short as possible. That is, make the capacitor as close as possible to the chip.

(5) A large area of copper-clad foil is connected to the analog ground. Cover the analog circuit with a large area of copper foil and drill dense holes in the blank area to connect to the analog ground. This can play a shielding and isolation effect, thereby reducing the mutual interference between analog signals, and it can also play a role in heat dissipation.

(6) The power and ground wires should be as short and thick as possible, especially the wires on the magnetic beads connecting digital power and analog power must be thick, because in addition to reducing the voltage drop, it is more important to reduce the coupling noise.