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Similarities between analog and digital routing strategies in PCB design

2020-01-03 09:58:19

Bypass or decoupling capacitors

When wiring, both analog and digital devices require these types of capacitors, and a capacitor needs to be connected near its power supply pin. This capacitor value is usually 0.1uF. Another type of capacitor is needed on the power supply side of the system. Usually this capacitor value is about 10uF.

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In analog wiring design, bypass capacitors are usually used to bypass high-frequency signals on the power supply. If no bypass capacitors are added, these high-frequency signals may enter sensitive analog chips through the power pins. In general, these high-frequency signals exceed the ability of analog devices to suppress high-frequency signals. If bypass capacitors are not used in the analog circuit, noise may be introduced in the signal path, and even more serious conditions may even cause vibration.
In digital circuits, a large current is usually required to perform the switching of the gate states. Since switching transient currents are generated on the chip when switching and flow through the circuit board, it is advantageous to have additional "backup" charges. If there is not enough charge during the switching operation, the power supply voltage will change greatly. Too large a change in voltage will cause the digital signal level to enter an indeterminate state and will likely cause the state machine in a digital device to malfunction.

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Differences between analog and digital routing strategies

Ground plane is a problemThe basics of circuit board wiring apply to both analog and digital circuits. A basic rule of thumb is to use an uninterrupted ground plane. This common sense reduces the dI / dt (current change over time) effect in digital circuits, which changes the potential of the ground and causes noise to enter the analog circuit.

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The wiring techniques for digital and analog circuits are basically the same, with one exception. For analog circuits, there is another point to note, that is, keep the digital signal lines and the loops in the ground plane as far away as possible from the analog circuits. This can be achieved by individually connecting the analog ground plane to the system ground connection, or placing the analog circuit at the far end of the circuit board, which is the end of the line. This is done to keep the signal path from external interference to a minimum.
This is not necessary for digital circuits, which can tolerate a large amount of noise on the ground plane without problems.