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If you use a twin well technology or different P wells can be fabricated and isolated, then you can tie each bulk with the source .If the technology supplies only N wells on P substrate, then you have to tie all bulks of NMOS together to the lowest potential at the circuit .I'm not sure, but some isolation techniques may exist like guard rings or STI/DTI (shallow/deep trench isolation) but I'm not sure .
Generally DeepNwell is used to isolate NMOS from the substrate of other NMOS.
For example suppose all the NMOS in a amplifier is kept in PWELL (common substrate ) and that is connectet to gnd. and there is other NMOS whose substrate is connected to other potential not to gnd. In that case you need to isolate that NMOS from other NMOS, otherwise substrate will get short. So to avoid this DeepNwell is used.
So, for that you have to put that particular NMOS in a DeepNwell Layer and that MOS should sorround with a NWELL guardring.
Here DeepNwell is act as a bottom side and the NWELL guardring is act as a sidewall to seperate that particular NMOS from common P-type substrate.
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