App note: Design considerations for mixed-signal, How to design a PCB layout

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Another application note from e2v, this time about PCB design involving mixed-signal (Analog and Digital) components. Link here (PDF)

This application note aims at providing you with some recommendations to achieve improved performance.

The initial assumptions are the following:
• Proper grounding and routing of all signals is essential to ensure accurate signal conversion
• Eliminate the loop area return by using both separate ground plane and power plane
• Circuitry placement on mixed-signal PCBs is a crucial design point

In many cases, engineers have preconceived notions about mixed-signal designs and how analog and digital placement, partitioning and associated design should be performed.

When laying out components for a mixed-signal PCB, certain considerations are critical to achieve optimum performance. Mixed-signal is particularly tricky to design since analog devices possess different characteristics compared to digital components: different power rating, current, voltage and heat dissipation requirements, to name a few.

This study shows how to prevent digital logic ground currents from contaminating analog circuitry on a mixed-signal PCB and particularly ADC component. In our attempt to answer this question, let’s keep in mind two basic principles of electromagnetic compatibility. One is that currents should be returned to their source as locally and compactly as possible, through the smallest possible loop area. The second is that a system should have only one reference plane, if not we would create a dipole antenna.

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