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Why do we use floating wells instead of connecting them to the power supply?

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road_0506

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in layout designing, why do we use floating wells instead of connecting them to the power supply?
 

Re: floating well

that floating well might be used for isolating noise...and power supplies are noisy...
hence, connecting the well to the power supply will just make the well noisy...
 

    road_0506

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Re: floating well

sxunxs said:
that floating well might be used for isolating noise...and power supplies are noisy...
hence, connecting the well to the power supply will just make the well noisy...


thanks sxunxs... i think you are right...
 

Re: floating well

When do we use floating nwell? nwell + pwell become diode, but nwell is floating. I think it will cause some problem.
 

floating well

Sometimes floating wells help you to avoid the body effect of PMOS transistors by connecting the source and the bulk tie together. The body effect increases the threshold of th transitor which impacts the performance of the transistor.
 

floating well

decrease the body effect on threshold voltage
 

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