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A major concern when designing mixed-signal integrated circuits today, is to avoid unwanted signal interaction between digital and analog circuit blocks present on the same silicon substrate. Part of this interference is conducted through the substrate. To minimize this substrate noise coupling, simple models are needed which can predict the noise and guide the IC designer in the layout of the mixed-signal chip. In this paper we present simple models for the resistance and surface potential of rectangular contacts based on approximating the rectangular contacts with elliptic contacts. The accuracy of the approximate model is validated with finite element and boundary element calculations.