Grounded Deep Nwell purpose?

[mult]

Does anybody know the purpose, why sometimes Deep Nwell is connected to Ground not to Power?

(No PMOS inside DNW;
DNW only acts as isolation of some Ptubs from global Psub)

Thanks.

 

[fire_bolt]

Hi mult,

Grounded DNW serves ESD purpose by collecting more unwanted charge from p-sub or p-epi and grounding it. This advantage is mainly due to its large area and depth in CMOS process (cross-sectional view).

For more information, please refer http://article.sapub.org/10.5923.j.eee.20120205.13.html

regards,
fire_bolt

 

[dick_freebird]

Figure the handle (Psub) is electrically noisy and hard to
quantify / predict due to many return-current loops. A
DNW can be used to return Ptub body noise currents to
a quiet ground, equipotential but without the activity,
instead. Provided you have a sensible impedance from
the DNW to "wherever". Especially if there is a lot of digital
in the mix, putting the analog / RF / jitter-sensitive digital
in a "clean pocket" can pay of in detail spec performance.