Resonant forward-biased guard rings for suppression of substrate noise in mixed-mode CMOS circuits

Previous work at Stanford University has demonstrated that inductance in the substrate connection is the principal problem underlying the coupling of digital switching noise into analog circuits. The low impedance substrate can be treated as a single node over a local area. Switching in the digital circuits produces current transients in the substrate. These transients are subsequently amplified in the analog portions of the overall mixed-mode circuit. Various guard rings and other techniques, including the use of new logic circuit families, have been proposed to suppress this noise. This work demonstrates that by using the capacitance of a forward biased guard ring(s), the substrate noise at a specific frequency(ies) can be reduced by resonating the guard ring capacitance with the substrate lead inductance to provide a very low substrate-to-ground impedance. In this manner, noise at particular frequencies, which are problematic to the analog circuit, can be suppressed. Tuning can be accomplished by varying the current in the forward-biased guard ring diodes