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Design >> Mixed-Signal Design >> Should nwell shield be tied to vdd or gnd?
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Message started by ywguo on Jun 11^th, 2004, 1:29am

Title: Should nwell shield be tied to vdd or gnd?
Post by ywguo on Jun 11^th, 2004, 1:29am

Hello,

As well known, nwell is often used as shield for PIP cap, MIM cap, or resisitors. But should the nwell shield be tied to vdd or gnd?

In Chapter 2, Analog Integrated Circuit Design, by David Johns and Ken Martin, it shows some examples that nwell shield is tied to ground.

From my point, I always tie the nwell shield to VDD to assure the p-n junction is reverse biased.

Who can analyze the two method for me?

Thanks in advance.

Yawei Guo

Title: Re: Should nwell shield be tied to vdd or gnd?
Post by Paul on Jun 25^th, 2004, 8:18am

Hello Yawei,

if you tie it to VDD, the PN junction is reverse biased. If you tie it to GND, the PN junction has 0V bias, which is fine too. As it is supposed to serve as a shield, you must select your biasing node carefully. In single-ended circuits GND is usually used as the reference, a good reason to use it for shielding. But don't take the same GND node than the one for the surrounding substrate contacts, it makes shieling useless.

Paul

Title: Re: Should nwell shield be tied to vdd or gnd?
Post by ywguo on Jun 27^th, 2004, 9:39pm

Hello, Paul,

Thank you. Normally, I tie shielding, or guardring to a pair of vdd and vss which is separate from other powers and grounds.

Yawei