Re: guard rings
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.
Cheers