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Analog circuit sizing under process/parameter variations is formulated as a mini-max geometric programming problem. To tackle such problem, we present a new method that combines the ellipsoid method and affine arithmetic. Affine arithmetic is not only used for keeping tracks of variations and correlations, but also helps to determine the sub-gradient at each iteration of the ellipsoid method. An example of designing a CMOS operational amplifier is given to demonstrate the effectiveness of the proposed method. Finally numerical results are verified by SPICE simulation.