Well Grounded, Digital Is Analog

文章摘要：Well Grounded, Digital Is Analog Abstract: An inexperienced designer may think that power decoupling capacitors and grounds are magic mystical things that absorb every bad noise. They are not. This application note discusses proper capacitor selection. It explains the failure of capacitors to decoup

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Well Grounded, Digital Is Analog

Abstract: An inexperienced designer may think that power decoupling capacitors and grounds are magic mystical things that absorb every bad noise. They are not. This application note discusses proper capacitor selection. It explains the failure of capacitors to decouple power buses above their self-resonance point. To illustrate the forces and characteristics of capacitors on a ground plane, an analogy to electrons is used. Electron behavior is comically illustrated. Guiding currents toward the ground star point in ground planes is demonstrated by slotting the plane.

Introduction

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 printed circuit board 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 place so much emphasis 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.

Why Digital Is Analog

Figure 1. An input signal, noise, and the two added together.

Designers expect analog signals to be easily affected by noise. The lower waveform in Figure 1 is the direct result of adding noise to the signal, obviously a bad thing. However, digital is thought to be protected by thresholds. This thinking is erroneous, as we will see.


Figure 2. The top waveform is the input and noise. The bottom waveform has the noise added.

The thresholds in the digital portion of Figure 2 mean that the top waveform is really analog. How can this be? The region above the top threshold is the area where we call the signal a "one," and below the bottom threshold is the area that we call a "zero." What do we call the undermined area between the thresholds? That is analog. So the top waveform has noise present. Because the noise is below the threshold, digital thinking says that everything is OK. The bottom waveform does not cross a threshold, so it has to be OK!

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