Properties and modeling of ground structures for reducing substrate noise coupling in ICs

Substrate noise can seriously degrade the performance of system-on-chip designs containing sensitive analog circuits together with noisy digital circuits. To reduce the coupling guard bands or guard rings are often used for protecting the analog circuitry. However, the efficiency of these ground structures is often not known on beforehand. Therefore, in this paper we investigate the noise reducing properties of different ground structures. It is shown that distributed ground structures are typically more efficient than localized ground structures due to smaller inter-contact influences. We also introduce two convenient measures of grounding efficiency, the global and local substrate potentials, and we present compact analytical models for these. The models are verified with numerical calculations and the accuracy is shown to be very good