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Subthreshold operation of digital circuits has emerged as a promising approach to achieve ultralow power dissipation. However, extensive application of subthreshold logic is limited due to low performance and high susceptibility to process variation (PV). This paper proposes a PV-tolerant ultradynamic voltage scaling (UDVS) system where performance requirements dictate whether the devices will work in the subthreshold or superthreshold region. Due to different mechanisms of current conduction, it is necessary to use different P/N ratios for different regions of operation to improve circuit robustness, performance, and power. With an analytical model of circuit robustness, we present an adaptive body-biasing technique to dynamically adjust the ??-ratio depending on the operating region. Measurements show that our methodology improves the dynamic range of operation the circuits-from 1.2 V all the way down to 85 mV consuming 40 nW (at 85 mV) of power for an 8 × 8 finite-impulse response filter fabricated in a 0.13-??m technology, and can salvage circuits which otherwise would fail to operate due to device mismatches and skewed P/N ratios.