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
