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