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According to the rule of thumb, a four-layer PCB is typically employed in high-density and high-frequency applications, offering over 20 dB improvement in EMC compared to a two-layer board. In a four-layer configuration, a dedicated ground plane and a power plane can often be implemented. This allows for straightforward connections of the circuit ground wires, grouped appropriately with the ground plane, while addressing operational noise through specialized methods.
There are several approaches to connect the ground wires of each circuit to the ground plane, including:
1. **Single Point and Multipoint Grounding Modes**
– **Single Point Grounding**: All circuit ground wires connect to a single point on the ground plane, which can be categorized into series single point grounding and parallel single point grounding.
– **Multipoint Grounding**: All circuit ground wires are grounded in close proximity, with short connections that are suitable for high-frequency applications.
– **Mixed Grounding**: This approach combines both single point and multipoint grounding.
For low-frequency, low-power applications with the same power supply layer, single point grounding is preferred, commonly used in analog circuits. A star connection is typically employed to minimize the effects of series impedance, as illustrated in the right half of Figure 8.1. High-frequency digital circuits, on the other hand, benefit from parallel grounding. This can be achieved easily using ground vias, as shown in the left half of the figure. Generally, all modules will integrate both grounding methods, with the mixed grounding approach facilitating the connection between the circuit ground wires and the ground plane.
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**Mixed Grounding Mode**
If the entire plane is not designated as the common ground, such as when a module has two ground wires, the ground plane must be segmented, often interacting with the power plane. It is essential to adhere to the following principles:
(1) Align each plane to prevent overlap between unrelated power and ground planes; otherwise, segmentation will fail, causing interference.
(2) At high frequencies, parasitic capacitance between layers can lead to coupling effects through the circuit board.
(3) Signal lines between ground planes (like digital and analog grounds) should be linked via ground bridges, with the return path configured through the nearest via.
(4) Avoid placing high-frequency lines, such as clock signals, close to isolated ground planes to prevent unnecessary radiation.
(5) Minimize the loop area formed by the signal line and its return path, referred to as the loop Z small rule; a smaller loop area reduces external radiation and interference. Careful consideration of ground plane segmentation and signal routing is crucial to avoid issues arising from ground plane slots.
Here, we outline methods for grounding connections:
1. **Common Wiring Connection:** This method ensures reliable low-impedance conduction between two ground wires but is limited to medium and low-frequency signals.
2. **High Resistance Connection:** Characterized by generating a weak conduction current when there is a voltage difference across the resistance, eventually resulting in zero voltage difference once the charge discharges.
3. **Capacitance Connection:** This allows DC cut-off and AC conduction, making it suitable for floating ground systems.
4. **Ground-to-Ground Magnetic Bead Connection:** Magnetic beads act as frequency-dependent resistors, useful for connecting weak signals with rapid current fluctuations.
5. **Inductance Connection:** Inductance helps stabilize circuit state changes, typically employed between grounds experiencing large current fluctuations.
6. **Low Resistance Connection:** A small resistance introduces damping, mitigating overshoot during rapid changes in ground current, thus slowing the rising edge of impulse currents.
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