The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Design >> Mixed-Signal Design >> substrate connection in mixed signal circuit
https://designers-guide.org/forum/YaBB.pl?num=1158040305

Message started by john_xu on Sep 11^th, 2006, 10:51pm

Title: substrate connection in mixed signal circuit
Post by john_xu on Sep 11^th, 2006, 10:51pm

i have a question on the substrate connection for mixed signal circuit with 0.6um p-sub lightly-dopled submicron cmos process.There is no well for NMOS Tr and it is simply the intrinsic substrate.In my mixed block, we have seperate analog vdd/gnd and digital vdd/gnd for crosstalk reduction from digital part.

I want to know the substrate connection, in my cicruit, i connected it to the analog gnd for which we believe it is most approviate for our cicruit. But i wonder if we can assign the substrate with different gnd. i.e., for analog part, we connect its substrate to analog gnd, for digital part, we connect it to digital gnd. To reudce the crosstalk between them, I put guardings to surround the sensitive analog and digiatl part, respectively, and laso, I put some heavy substrate ties strip between the analog and digital part. Seperate the analog sub and digital sub can avoid the body effect of the analog/digital part both. Is any risk in it?

Pls. comment!!!

Title: Re: substrate connection in mixed signal circuit
Post by ACWWong on Sep 12^th, 2006, 3:38am

What you are suggesting is very sensible if you wish to avoid digital-analog crosstalk. What are you connecting the "heavy substrate ties strips" to ? one to each analog and digital ? If so then the gap inbetween will probably be left as pwell (not p- sub) if you do nothing else (check with your foundry). Probably good to put some lightly doped n or nwell inbetween for even better isolation.
Anyway the only issue i see in this is how you handle ESD. Even if your PDK allows you to seperate the substrate into domians, it is often the case for ESD robustness the two domains must be electrically connected... although back-to-back diodes will probably be ok, again your foundry will prbably have ESD guidelines. So you will add a tiny bit of extra capacitance between the two substrate domains with the diodes which is no a big problem i guess.

Title: Re: substrate connection in mixed signal circuit
Post by loose-electron on Sep 12^th, 2006, 10:55am

John:

Did you not contact me directly on this matter? I replied on email -

Anyhow:

The substrate is connected to both, you can not avoid it.

I feel that having a low impedance return ground path for the digital is valuable, but there will always be a resistive path between analog substrate and digital.

Suggest – go to my web site and see the section on help for IC designers:

http://www.effectiveelectrons.com/helpIC.html

Read the parts on noise reduction (2 articles, Electronic Design Magazine), substrate noise, and simulations vs. silicon. All very practical help with these issues.