Well Grounded, Digital Is Analog

文章摘要：One can now ask why the quiet analog signals do not harm the digital circuits? Oh, but they do damage the digital signals, just as the noise from one digital circuit interferes with other digital circuits. As we have discussed, analog and digital circuits are actually the same in this regard. The on

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

One can now ask why the quiet analog signals do not harm the digital circuits? Oh, but they do damage the digital signals, just as the noise from one digital circuit interferes with other digital circuits. As we have discussed, analog and digital circuits are actually the same in this regard. The only difference is the presence of thresholds in digital. Really all noise in digital circuits is bad. Even if it is below the threshold, the noise contributes to errors and jitter. In analog every noise is bad, because there is no threshold to hide behind.

Figure 5b compounds the problem by adding a brushed motor with arcing brushes. These sparks are large enough in amplitude to cause errors in the digital circuits despite the thresholds.


Figure 5b. Adding an arcing , sparking brushed motor to the ground plane.

In both Figures 5a and 5b the ground areas are labeled as analog and digital. While one hopes that the signals stay in their regions, nothing keeps them separate. So in Figure 6a we have erected a fence.


Figure 6a. A fence is intended to corral the electrons (signals).

The fence is really a slot cut into the ground plane, shown here as a dotted line under the fence. The slot separates the ground plane into two sections only allowing a connection near the star ground point. Now the electrons are forced to remain only on their side of the fence.

Return now to arcing sparking motor in Figure 5b. Figure 6b addresses this challenge with two fences.


Figure 6b. Two fences isolate the circuits.

In the case of the brushed motor, the garbage created by the motor would be suppressed as close to the motor as possible. This is why it is common to find a capacitor soldered directly on the motor as well as diodes arranged to short out the back electromotive force (EMF). The EMF is the reverse voltage spike created by the motor's collapsing magnetic field. In a brushed motor that happens as the brushes bounce and as they commutate (switch) the motor current.

In Figure 6a the digital and analog circuits must be protected from each other as well as from the arcing and sparking motor. Two fences do the duty with connections again only near the ground star point.

Conclusion

Do ground star points fix all the grounding problems? No. Grounds are never perfect and each circuit board is different. A good ground star point is an excellent starting point for the board design. Then experience and thinking like an electron can provide the insight to help corral the electrons. Experience shows that it is best to start with a system plan and modify it to meet local circuit requirements. Quiet grounds, well-disciplined signals, and happy circuits do not just happen. They are designed.

上一页  [1] [2] [3] [4]