In circuit design, signal integrity is often the primary concern. However, many designers tend to focus solely on signal paths while treating power and ground as ideal, which simplifies analysis but becomes impractical in high-speed designs. Although signal integrity is the visible outcome of a circuit, power integrity should never be overlooked. It directly impacts the performance of the final PCB. Power and signal integrity are closely related, and in many cases, signal distortion stems from issues in the power distribution system. For example, excessive ground bounce noise, improper decoupling capacitor placement, significant loop inductance, poor division of power/ground planes, or an improperly designed ground layer can all lead to serious signal integrity problems.
Decoupling capacitors play a critical role in maintaining power integrity. While it's common knowledge that placing capacitors between power and ground reduces noise, the exact number, value, and placement of these capacitors are often decided based on experience rather than precise calculation. Some may even assume smaller capacitors are always better, without considering their parasitic inductance or frequency response. In high-speed design, this approach is no longer sufficient. We must account for the parasitic parameters of capacitors, calculate the required number, capacitance, and optimal placement to keep the system impedance within acceptable limits. The principle is clear: use exactly the number of decoupling capacitors needed—no more, no less.
Ground bounce is another critical issue in high-speed circuits. When a device has a rise time shorter than 0.5 ns, the rapid switching of large data buses can create strong power supply ripples that disrupt signal integrity. This occurs when the current through a ground loop changes, inducing a voltage due to the loop’s inductance. As the rising edge becomes faster, the rate of current change increases, leading to higher ground bounce voltages. At this point, the ground plane is no longer at a stable reference voltage, and the power supply is no longer a perfect DC source. With simultaneous switching of multiple I/O lines, such as in a 128-bit bus with 50–100 I/Os switching at once, the inductance in the power and ground return path must be minimized to prevent voltage fluctuations. Ground bounce can occur at various levels—on the chip, in the package, through connectors, or on the PCB—leading to power integrity challenges.
Power distribution systems are complex, and managing the impedance between power and ground planes is key to achieving good power integrity. Ideally, the lower the impedance, the less noise and voltage drop will occur. In practice, we define a target impedance based on the maximum allowable voltage variation and power supply range. Then, by adjusting the layout, materials, and component placement, we aim to meet this target impedance across the entire power delivery network. This involves careful planning of power and ground planes, minimizing loop areas, and optimizing decoupling strategies to ensure stable and clean power delivery throughout the board.
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