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In resume it reduce the leakage current on board surface. This is generally usefull when you're using very high input impedance components, with same impedance order of the board. This helps to prevent EMI on this inputs pins too.
Guard rings, or “guarding,” is another common type of shielding used with op amps; it is used to prevent stray currents from entering sensitive nodes. The principle is straightforward—completely surround the sensitive node with a guard conductor that is kept at, or driven to (at low impedance) the same potential as the sensitive node, and thus sinks stray currents away from the sensitive node.
Guard ring is a method through which you can reduce the substrate coupling noise.
Think of a circuit with analog and digital signals. The digital signal is the aggressor since it changes rapidly and the analog signal is the victim. Now as the digital signal signal changes state, the "parasitic" capacitors between the changed node and the substrate become affected. Thus this voltage transfers the changes voltage into the substrate, (since the voltage across a capacitor cannot change instantly) This voltage finally reaches the victim affecting its output. Now if around the victim, you insert an artificial ground, you will be grounding your noise. This means that since the ground has less resistance than the analog section, the noise will choose to move toward the ground, thus the victim will remain unaffected. This shield is called a guard ring.
It might be interesting to know that a guard ring is a ring worn on the finger to prevent another from slipping off.
Is it out there any existing literature dealing with the topic of guard rings? I have made a preliminary search on the net and I was unable to find a single reference on the theory behind the design and operation of these by-no-means-trivial though extremely important IC elements. I only found random advice/explanations from people in forums here and there, but nobody pointing to any books from which this knowledge can be learned.
Thanks in advance for any clues to existing references!
Thanks for the references. Hastings' treatment is good but kind of brief, and doesn't give much examples or practical considerations. ...Maybe there are any other references with more examples/practical considerations, like for instance:
-Which types of blocks need to be protected with rings and which doesn't
-Criteria to decide when it makes sense to protect and when it doesn't
-How to position and dimension the rings
-How to best place the analog and digital blocks with relation to each other
Any recommended IEEE papers would also help a lot.
Guard rings are used to reduce the substrate coupling noise. The basic idea is this:
If there are two paths for the noise to go to; well it will choose the one with the lower resistance like any other electrical signal. So if we make a ring around our "sensitive"device, noise will not even get there. this is the idea.