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PCB Mixed Signal Partition

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dknguyen

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Hi. I can't remember whether the analog partition or the digital partition is supposed to be closest to the power supply on the PCB. Nor, can I find any of the old files that had the guidelines when my files got wiped. Does anyone remember? I think it's digital is closest, but am not sure.
 
I always use "star" wiring when a circuit has digital and analog. Both connect to the regulator's output or main filter capacitor at the same spots. Both for the positive supply and for ground.
 
In general, you should keep the digital portion closest to the supply. A signal needs a return path to ground. The analog portion of your circuit is more adversly affected by noise. If your analog portion was closer to the supply the digital return current flow accross your analog portion when it returns.

Star grounding is a good technique but should be used carefully. In reality your grounding will vary from circuit to circuit. The goal is to minimize return path impedance, isolate sections based on noise level and susceptibility, and avoid large area for return path loops.
 
You can place partitions carefully, to prevent ground return currents from digital sections from flowing under analog sections. The easiest way to ensure this is to make the analog sections at the edges of the board. Make sure that the digital traces do not cross over the partition, this generates a lot of EMI.
 
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