Henry Ott Consultants

Electromagnetic Compatibility Consulting and Training

Supplying Power to a Mixed-Signal PCB

Question: Thank you very much for your article Partitioning and Layout of a Mixed-Signal PCB, it has been very useful to me. I now understand why a solid ground plane is better than a split plane. However I still have a question on how to supply power to a mixed-signal board from a single power supply, since all the analog and digital current is provided by the power supply and will flow back to the power supply I think that the analog ground current will be contaminated if it flows back to the power suppy on the same lead as the digital current.

Answer: You are working under a common misconception, that all analog and digital signal currents flow back to the power supply. This is exactly what you want to avoid. See Fig. 2 in my 2001 Printed Circuit Design magazine article. In a properly designed and laid out board all signal and transient currents are confined to the PCB. The power supply supplies only dc current to the boards, all signal and transient currents are then supplied from decoupling capacitors on the PCB and flow in small loops confined to the board. To do otherwise creates major EMC and Signal Integrity problems. 

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Henry Ott Consultants
48 Baker Road Livingston, NJ 07039
Phone: 973-992-1793, FAX: 973-533-1442

May 21, 2009