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Classical treatment method for grounding in PCB design
In analog and digital circuits, distinguishing between analog ground/digital ground and analog power/digital power is essential, especially when digital interference becomes unbearable for analog circuits. The conventional approach involves dividing the ground wires into two after rectification and filtering, with one segment serving as the analog ground for all analog circuitry and the other as the digital ground for all digital circuitry. Similarly, the DC power supply voltage regulator chip's output is divided into two after filtering: one part, filtered by LC/RC, serves as the analog power supply for the analog section, while the other serves as the digital power supply for the digital section.
Analog ground (AGND) and digital ground (DGND), along with their respective power supplies, can be connected at the start of the power supply, but beyond that, they should remain independent. Components like AVCC (analog power supply) and DVCC (digital power supply) are designed to minimize interference between the two sections. However, complete isolation isn't feasible, so AGND and DGND should still be connected, ideally via a 0-ohm resistor or magnetic bead to reduce interference while maintaining a single connection point.
When designing electronic systems, it’s critical to anticipate and address potential interference issues early on to avoid costly redesigns later. Three primary elements contribute to interference: the source, the propagation path, and the sensitive device. Suppressing the interference source—by reducing du/dt and di/dt—is paramount. This can be achieved by adding capacitors across the source and using inductors or resistors in series with the source loop. Additionally, protecting sensitive devices like A/D and D/A converters is vital.
Conducted interference, which travels through wires, and radiated interference, which spreads through space, require specific countermeasures. Filtering and isolation optocouplers help mitigate conducted interference, while increasing distance, grounding, and shielding combat radiated interference.
To cut off interference pathways, consider several strategies: ensuring a clean power supply, isolating I/O ports controlling noisy devices, optimizing crystal wiring, and dividing circuit boards into digital and analog zones. Grounding high-power devices separately from the microcontroller helps reduce mutual interference. Using anti-interference components like magnetic beads, magnetic rings, and shields at critical points enhances overall performance.
Improving the resilience of sensitive devices involves minimizing noise pickup and quick recovery from disturbances. Techniques include reducing loop areas, thickening power and ground wires, grounding idle I/O ports, and incorporating power supply monitoring circuits. Choosing slower crystal oscillators and directly soldering ICs onto the board can also enhance stability.
While grounding is essential, splitting digital and analog grounds entirely isn’t always necessary. Instead, separating them and connecting them at one point minimizes noise interference. Identifying whether a component belongs to the digital or analog domain depends on its associated chips. For example, power supplies feeding analog circuits are analog, whereas those powering microcontrollers are digital.
For low-frequency analog circuits, one-point grounding is ideal to prevent mutual interference due to shared ground impedance. High-frequency circuits, however, require a combination of separate and one-point grounding to account for inductive effects. Thickening ground lines and full grounding, leaving no unused copper areas, can help suppress high-frequency radiation noise.
Connecting analog and digital grounds directly leads to mutual interference, so alternatives like magnetic beads, capacitors, inductors, or 0-ohm resistors are preferred. Magnetic beads act as band-stop filters, suppressing specific noise frequencies, while capacitors may cause floating issues. Inductors are bulky and unstable, whereas 0-ohm resistors provide effective current path limitation and superior attenuation across all frequencies.
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