Well Grounded, Digital Is Analog

文章摘要：It is time to see why this is digital mis-thinking.Figure 3, Signal plus noise with the resulting time errors.Figure 3 repeats parts of Figures 1 and 2, but adds vertical lines to illustrate how the analog noise changed the output timing. Clearly, more than just missing the amplitude (up and down) t

Well Grounded, Digital Is Analog,标签：布线,emc是什么意思,http://www.88dzw.com

It is time to see why this is digital mis-thinking.


Figure 3, Signal plus noise with the resulting time errors.

Figure 3 repeats parts of Figures 1 and 2, but adds vertical lines to illustrate how the analog noise changed the output timing. Clearly, more than just missing the amplitude (up and down) thresholds is necessary. Looking carefully at the transitions, we see that the negative transition "A" is early compared to its intended position. The positive transition "B" is very close, so we drew a single line. Negative transition "C" is early and positive transition "D" is late. The noise has been transformed from amplitude error to time-jitter positional errors. If the signal is sampled at the arrows, the information will be preserved. However, if signal frequency was higher, the time error would be larger as a percentage, Eventually the sample points will be compromised.

These illustrations show why we say that digital is analog. That is, both analog and digital require care to preserve the signal purity. At this point we can consider the techniques that safeguard our wanted signals.

Selecting the Capacitor for the Application

One misconception is that capacitors take the bad noise of the power-supply lines and make it disappear into ground. Not true. Another notion is that capacitors are created equal. They are not. Poor capacitors do not help, and even high-quality capacitors are sensitive to frequency. All capacitors have unavoidable series resistance and inductance.

AVX® and Kemet® are capacitor companies that specify parasitic components and provide free Spice tools. The application notes on both sites are also very informative. These Spice tools allow us to graph the actual performance of the capacitors. For example, a 0.1µF capacitor is used as decoupling in many circuits. The impedance is frequency dependent, as Table 1 illustrates.

Table 1. Frequency vs. Impedance of a Typical 0.1µF Capacitor Frequency (Hz) Impedance (Ω) 1k 1.5k 33k 50 1.8M 1 10M 0.1 15.8M 0.023, self-resonance point
The Spice program allows us to evaluate and change several parameters to understand the circuit's function. The capacitor type can be ceramic or electrolytic. The size and working voltage are also variables.

In sensitive circuits we might see many decoupling components. Series inductors, resistors, and ferrite beads will form lowpass filters in conjunction with several capacitors. The capacitors are chosen to attenuate specific frequency ranges. It is not unusual to see four decoupling capacitors on a circuit. A large electrolytic serves as bulk current storage or low-frequency suppression; two ceramic capacitors are used to reduce interference in the 10MHz to 100MHz range. Finally, a small capacitor in the pico-Farad range is used to limit the higher radio frequencies.

Table 1 shows why the above four capacitors are necessary. Here a single 0.1µF ceramic capacitor is not effective in reducing interfering noise and spurious components below 33kHz or above 15MHz. Power line and switching power-supply ripple below 33kHz would not be attenuated sufficiently. A larger value electrolytic would be better. Noise between two and fifteen Megahertz are shunted to ground properly by the 0.1µF capacitor. Observe that at 15.8MHz, the 0.1µF capacitor becomes self-resonant. This means that the unavoidable parasitic inductor in series with the capacitor dominates. Above that frequency the capacitor looks like an inductor and this noise and garbage is not able to pass through the capacitor to ground.

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