In mixed-signal PCB design, power plane splitting is a fundamental task, but I've seen many engineers—both beginners and veterans—often stumble over it. Sometimes the board looks well laid out, but power-related issues arise as soon as it's powered on: high power supply noise, unstable circuits, and after hours of troubleshooting, it turns out the power plane splitting wasn't done properly. Today, I'll summarize the common pitfalls I've encountered over the years to help everyone avoid these mistakes.

The first pitfall is placing the split line too close to sensitive analog components. Splitting the power plane is meant to separate digital and analog power, but the edges of the split lines create a "fringe field effect," allowing noise from the digital power to couple into the analog power along the edges. Once, a junior engineer I mentored placed an analog op-amp right next to the split line, resulting in the analog power ripple increasing threefold and the op-amp output drifting. The correct approach is to keep sensitive analog components at least 2mm away from the split line. If space allows, add a grounded copper pour beside the split line to block most of the coupled noise.

The second pitfall is incomplete splitting, leaving a "narrow bridge." Many designers inadvertently leave a narrow connecting strip, as thin as 1mm or less, between split sections—essentially defeating the purpose of splitting. Digital power noise can easily travel across this bridge into the analog power section. I once helped a client troubleshoot a board where the power plane appeared split but had a 0.8mm narrow bridge left in the middle, causing digital noise to leak into the analog power and leading to inaccurate ADC sampling. After removing the bridge and re-splitting the plane, the analog power ripple dropped immediately. So, always zoom in and inspect after splitting to ensure the two power regions are completely isolated.

The third pitfall is neglecting the return current path. Many engineers focus solely on splitting the power plane but forget about the return path—digital return currents flow through the digital power and ground planes. If the power plane is split too finely, the return path becomes longer, impedance increases, and noise rises. When I split power planes, I ensure each power region has a corresponding ground plane nearby, with the return path kept as short as possible: for example, the digital power plane adjacent to the digital ground plane, and the analog power plane adjacent to the analog ground plane. In a previous high-speed digital and analog mixed-signal PCB, excessive splitting of the power plane caused digital signal rise times to slow down. After merging some power plane sections to shorten the return paths, signal quality improved immediately.

The fourth pitfall is failing to implement anti-interference measures after splitting. Even with proper splitting, coupling interference between digital and analog power supplies can still occur. This is where adding filtering components at the power entry points or near the split lines becomes essential. For instance, adding EMI filters at the digital and analog power entries, or placing 0.1μF high-frequency decoupling capacitors near the split lines. In my projects, I always include an LDO combined with 10μF and 0.1μF capacitors at the analog power entry of mixed-signal PCBs for both voltage regulation and filtering. In one industrial control board, the power plane was split but lacked filtering, resulting in high-frequency noise on the analog supply. After adding filter capacitors, the noise dropped by an order of magnitude.

The last pitfall is over-splitting to "save space." Some engineers believe smaller boards are always better, fragmenting the power plane into tiny sections, which compromises power stability. In reality, a larger power plane area means lower impedance and less noise. Unless space is extremely limited, try to keep each power region as large as possible—for example, the analog power plane should cover at least all analog components, not just a small section, to avoid excessive analog power ripple.

Here’s a real-life example: A couple of years ago, a client approached me with a mixed-signal PCB complaining about high power noise and unstable analog circuits. Upon inspection, I found multiple issues with the power plane: an op-amp placed only 0.5mm from the split line, an incomplete split leaving a 1mm narrow bridge, and no filtering after splitting. I redesigned it by moving the op-amp 3mm away from the split line, removing the narrow bridge and re-splitting the plane, and adding an LDO and filter capacitors at the analog power entry. After the modifications, the power ripple dropped from 200mV to 5mV, and the analog circuit stability improved dramatically. The client was amazed at how much nuance was involved in power plane splitting.

Power plane splitting may seem straightforward, but the core principles are "complete isolation, keeping sensitive components away, ensuring proper return paths, and implementing adequate filtering." When designing, take your time to plan the power plane layout, inspect carefully after splitting, and avoid these pitfalls. If unsure, refer to proven design examples or seek advice from experienced engineers—it’s better than dealing with debugging headaches later.