guard ring enclosure pattern

[babaduredi]

Hi,
Once we are done with the layout of a some block, we enclose it in two guard rings to shield it from surrounding noise when instantiated at upper level hierarchy. In this case there are 2 possibilities:

1. Enclose the block in n-well guard ring and than whole structure(block in n-well guard ring) in p-tap guard ring,
2. Enclose the block in p-tap guard ring and than whole structure(block in p-tap guard ring) in n-well guard ring.

Out of above 2, which one provides better shielding? Please tell me if sufficient shielding can be achieved using n-well guard ring alone. I am talking about n-well process and block contains both analog and digital circuitry.
Thanks.

 

[erikl]

If your blocks don't already use an outer p-tap guard ring, use such one as a 1^st inner guard ring around the block. Another 2^nd n-well guard ring (n⁺taps connected to VDD) could be a further shield against impinging electric fields, virtually grounding them (same effect as with the p-tap guard ring), additionally extracting straying charge carriers intruding into its reverse diode field.

A well GNDed (i.e. connected to GND by metal) p⁺ on substrate guard ring is the best you can do against substrate noise. Another such p⁺ on substrate guard ring (or an n⁺ on n-well guard ring connected to VDD) may improve the noise shielding -- depending on what you define as sufficient.

You also might want to study this thread.

 

[jarillak]

Hi.................


First enclose your circuit with P-TAP(GND connection) and then draw the N-TAP(VDD) over it.
It will give your better protection to your circuit.

 

[babaduredi]

Hi jarillak,
thanks for your reply. Could you please justify your answer, i mean the logic behind this pattern providing better protection.