The guardring and the doped substrate

[chang830]

Hi,
In Razavi's book "Design of CMOS Analog Integrated Circuit",page 663,it asserts that the "In circuits fabricated on lightly doped substrae ,"guard rings" can be employed to isolate the sensitive sections from the substarte noise produced by other sections."

My question is for the heavily doped substarte, the guard ring can also reduce the substarte coupling in mixed signal circuit design?

Thanks

 

[pfd001]

For hearvily doped substrate, the guard ring becomes less effective because of the noise coupling by substrate directly

 

[laglead]

Lightly-doped substrate have high resistance. So, noise current will tend to go through a low resistance path--guard ring to power/ground.