This is the last installment of the grounding series articles I have posted.
In Part II of this series, we have explained that star ground provides a single or common reference point to a circuit in an effort to avoid sharing of paths that could cause errors and degrade the integrity of the signals. This is accomplished by selecting a single "mecca" point where all ground return paths meet, and that this point should be a low impedance node. But this scheme is not always practical and doesn't solve all the woes of circuit grounding. Long routes going to the mecca point can itself be a source for error as it becomes inductive to high frequency signals. The rule of keeping the routes or wires as short as possible applies to grounding as well.
The other scheme that we have described is the use of ground plane, because it presents much lesser resistance and inductance compared to the individual wires and traces. But we have cautioned that there are two paths the signal takes, namely the least resistance path for the DC, and the least impedance path for the AC. Figure 3 in the part 2 of the series illustrates this concept. Designers have to be aware of these considerations in order to be effective in designing the ground scheme for his circuit.
As to the question whether which of the two is more effective to use, the answer is there really is not a cure-all for all grounding problems. Mecca will be effective in cases where the longer routes will not present an inductive parasitic to the current path. Examples are sense lines and signal lines which are high impedance or don't expect to have high frequency signals travelling through it. Ground planes on other hand presents a better alternative to minimizing the resistance and impedance of the return path. But again this doesn't solve everything because no two points in the ground plane are exactly equal, but this is minimized by using ground plane. It can be very effective when used with careful planning and design of the circuit, being mindful of the paths of the current flow.
This brings us to the topic of handling grounding in mixed-signal circuits.
Mixed-signal systems present a unique challenge to circuit designers in terms of what grounding scheme is best applied. Mistakes and bad practices can lead to a non-functional system, despite efforts and clever innovations in circuit topologies. Best practice calls for keeping the analog circuits away from the digital circuits where possible, and making the interface only on points where necessary, which means separating both the power rail and the ground, using split planes. Digital circuits are known to be "noisy" because of the logic transitions which draw large current glitches from the supplies. These noise when coupled to analog circuits can very well mess up its performance. Logic circuits on the other hand have higher noise immunity than analog circuits.
Applying ground planes, as well as power planes, on printed circuit boards has increasingly become the practice and is a necessity when high speed signal processing is involved. The ground plane also has the added benefits of conveniently serving as noise shield and reducing noise interference.The analog and digital grounds still has to meet at some points, and it can be accomplished by small and short trace commonly known as "sense" lines.
So the ground plane provides low impedance nodes for the return currents, and separating the "quite" analog ground from the digital "noisy" ground prevents interaction between the two. Combining the techniques of mecca in the analog circuit, the use of ground plane, and careful placement of components being aware of how much and what current will flow, offer the best solution and benefits to circuit designers.
For other Parts of the series:
Grounding Series Part I
Grounding Series Part II
Recommended Additional Readings:
Staying Well Grounded by Hank Zumbahlen, Analog Dialogue Vol. 46
Chapter 1 Grounding and Wiring, The Circuit Designer's Companion 3rd Edition, Peter Wilson
The other scheme that we have described is the use of ground plane, because it presents much lesser resistance and inductance compared to the individual wires and traces. But we have cautioned that there are two paths the signal takes, namely the least resistance path for the DC, and the least impedance path for the AC. Figure 3 in the part 2 of the series illustrates this concept. Designers have to be aware of these considerations in order to be effective in designing the ground scheme for his circuit.
As to the question whether which of the two is more effective to use, the answer is there really is not a cure-all for all grounding problems. Mecca will be effective in cases where the longer routes will not present an inductive parasitic to the current path. Examples are sense lines and signal lines which are high impedance or don't expect to have high frequency signals travelling through it. Ground planes on other hand presents a better alternative to minimizing the resistance and impedance of the return path. But again this doesn't solve everything because no two points in the ground plane are exactly equal, but this is minimized by using ground plane. It can be very effective when used with careful planning and design of the circuit, being mindful of the paths of the current flow.
This brings us to the topic of handling grounding in mixed-signal circuits.
Mixed-signal systems present a unique challenge to circuit designers in terms of what grounding scheme is best applied. Mistakes and bad practices can lead to a non-functional system, despite efforts and clever innovations in circuit topologies. Best practice calls for keeping the analog circuits away from the digital circuits where possible, and making the interface only on points where necessary, which means separating both the power rail and the ground, using split planes. Digital circuits are known to be "noisy" because of the logic transitions which draw large current glitches from the supplies. These noise when coupled to analog circuits can very well mess up its performance. Logic circuits on the other hand have higher noise immunity than analog circuits.
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| Figure 1 Separate Analog and Digital Circuits |
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| Figure 2 Separated Analog and Digital Ground Planes |
So the ground plane provides low impedance nodes for the return currents, and separating the "quite" analog ground from the digital "noisy" ground prevents interaction between the two. Combining the techniques of mecca in the analog circuit, the use of ground plane, and careful placement of components being aware of how much and what current will flow, offer the best solution and benefits to circuit designers.
For other Parts of the series:
Grounding Series Part I
Grounding Series Part II
Recommended Additional Readings:
Staying Well Grounded by Hank Zumbahlen, Analog Dialogue Vol. 46
Chapter 1 Grounding and Wiring, The Circuit Designer's Companion 3rd Edition, Peter Wilson
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