Analog/digital partitioning
How do you handle grounding and partitioning between analog and digital circuits on one board?
| Level | ●● onsite technical |
| Track | General board |
| Really testing | Modern practice vs folklore. |
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Say this first: Use one continuous ground plane and partition by placement: keep digital return currents out of the analog area by routing, not by cutting the plane. Split planes create more problems than they solve (signals crossing gaps). Separate the supplies instead — e.g. 3.3 V digital and 3.3 V analog through ferrite/LC filters — and place the ADC across the boundary with its analog and digital pins on their own sides.
When a split is justified: isolated sections or very high-precision designs, with signals crossing only through a bridge or isolator.
Going deeper
Why the datasheet says "AGND and DGND". Those pins separate currents inside the chip. The vendor's own guidance for most mixed-signal parts is to tie both to the same low-impedance ground plane, right at the chip. The split was for the chip, not for your board.
How partitioning by placement works. High-frequency return current flows under its own trace. If no digital trace runs through the analog area, no digital return current flows there either — even on one continuous plane.
Worked failure. A board splits the ground under an ADC, then routes SPI across the split. The SPI return current detours around the gap, through the analog area — the exact problem the split was meant to prevent.
Where people go wrong: joining "analog ground" and "digital ground" through a ferrite bead. At high frequency the bead is an inductor between two grounds — a ground-bounce generator.
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This is PCB-03 from Hardware Engineer Interview Questions. The book has 8 questions on pcb layout, stack-up & manufacturing and 141 in all, each answered the same way: what to say first, how to explain it, the numbers worth quoting and the follow-ups that come next.
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