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Dummies for Resistors

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krrao

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Hi All,
Why do we use dummies for Resistors? For poly resistor we use dummies at the ends to avoid poly etching.
Is it necessary to use dummies for N-well resistors also? if yes, then why?
--
warm regards,
krrao.
 

good question
i dont think N-well resistors have this etching problem.
in what circumstances will you choose N-well res. then?
there is a parasitic capacitance formed between N-well to psub when you use Nwell kind of resistor.
 

krrao said:
Hi All,
Why do we use dummies for Resistors? For poly resistor we use dummies at the ends to avoid poly etching.
Is it necessary to use dummies for N-well resistors also? if yes, then why?
--
warm regards,
krrao.
Click to expand...

Just to clarify, poly resistor dummies are used to ensure that the etch environment for the outside edges of the resistor array is the same as the etch environment between each resistor body. This can be a factor for critical resistors that must be matched to a high degree of precision.
 

I think you need dummy resistors for diffused resistors as well, to make sure your working devices receive the same diffusion inteactions with their neighbours, as clarified in the book " The art of analog design, 2nd Edition" section 7.2.7. But i think poly resistor are better suited for matchings.
 
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In case you need to match only 2 n-well resistors, I think you need no dummies, if they are adjacent and have the same environment at both their sides.
 

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