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!!!

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.