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This paper presents a design methodology for power and area minimization of flexible FFT processors. The methodology is based on the power-area tradeoff space obtained by adjusting algorithm, architecture, and circuit variables. Radix factorization is the main technique for achieving high energy efficiency with flexibility, followed by architecture parallelism and delay line circuits. The flexibility is provided by reconfigurable processing units that support radix-2/4/8/16 factorizations. As a proof of concept, a 128- to 2048-point FFT processor for 3GPP-LTE standard has been implemented in a 65-nm CMOS process. The processor designed for minimum power-area product is integrated in 1.25 × 1.1 mm2 and dissipates 4.05 mW at 0.45 V for the 20 MHz LTE bandwidth. The energy dissipation ranging from 2.5 to 103.7 nJ/FFT for 128 to 2048 points makes it the lowest energy flexible FFT.