Evaluation of active cancellation of substrate noise in mixed-signal ICs

In mixed-signal integrated circuits interference between noisy and sensitive circuits is a serious problem. Active cancellation of this substrate coupled noise was proposed about a decade ago and has been claimed to be more efficient than passive decoupling (DC/capacitive grounding). Most of the previously presented papers have investigated active decoupling methods using simple, less accurate, resistive network models as the common substrate. In this paper two common active decoupling circuits (feedforward and feedback) have been investigated using an accurate substrate model based on z-parameters. Three different configurations of contacts have been investigated: 1. all the contacts are close to each other; 2. just guard bands are close to each other; and 3. when guard rings are implemented. Conclusions have been reached with the aid of theoretical and simulation based approaches. When distances between contacts are small, the level of suppressed noise highly depends on the type of active decoupling circuit. In this case, if backplane is floating there is an optimum op-amp gain which should be used in order to gain the minimum coupled noise level. Conversely, when distances are large, none of the mentioned effects have been noticed. When guard rings are in use instead of guard bands, regardless of the fact that overall noise level is better, simple DC grounding is preferred.