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Well grounded, digital is analog

Posted: 21 May 2009 ?? ?Print Version ?Bookmark and Share

Keywords:capacitor selection? design well-grounded? analog circuits?

However surprising, some engineers really think that "ground" is a magic mystical thing that soaks up every bad thing like a black hole. We wish that were so. Designing a circuit would be far simpler.

Several years ago, a customer had system noise issues. Looking at a PCB with a seasoned analog design engineer, we found a lack of power decoupling capacitors. The ground plane had more holes than Swiss cheese. The analog engineer said: "I thought so. The digital guy struck again." The digital engineer, in this case, was brilliant, but he was just unlearned in analog techniques. The digital designer had failed to understand analog concepts like impedance, transmission lines, standing waves, power decoupling, and why grounds are divided in analog and digital areas. Now we do not want to "bash digital," and without digital we would miss out on many, many cool products. Unfortunately, some engineering educators emphasize too much on digital that analog skills are not well developed.

This application note discusses proper capacitor selection and how that decision affects a well-grounded design. Guiding currents toward the ground star point in ground planes is demonstrated by slotting the plane. The discussion begins by emphasizing why this is necessary, and how digital and analog circuits share a sensitivity to noise.

View the PDF document for more information.

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