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A supply voltage monitor (SVM) with self-controlled dual-oscillator-based architecture is proposed herein for biosensing systems combined with a biofuel cell (BFC) in this paper. The output of the BFCs can be used to monitor the biological signals while powering the BFC-combined biosensing systems. Thus, the SVM is designed to convert the change in the supply voltage (V DD ) into a code. The architecture of the proposed SVM allows self-controlled periodic operation without external signals. Furthermore, the frequency subtraction technique that uses two oscillators employing gate-leakage-based architecture with different frequency sensitivities to V DD allows accurate code generation with low power consumption and a small circuit area for supply voltage monitoring. The proposed SVM is fabricated using two different CMOS process technologies, including 65-nm CMOS and 55-nm deeply depleted channel (DDC) CMOS. The implementation of the 65-nm CMOS obtains an operating V DD range of 250 mV (0.75-1 V), draws a standby power consumption of 1.4 nW at 0.75-V V DD , exhibits a resolution of 2.4 mV with a nonlinearity error of -8.4/ +12.1 mV, and occupies a circuit area of 0.0047 mm 2 . Meanwhile, the implementation of the 55-nm DDC CMOS for low-voltage operation achieves an operating V DD range of 300 mV (0.225-0.525 V), draws a standby power consumption of 32.5 nW at 0.25-V V DD , exhibits a resolution of 0.94 mV with a nonlinearity error of -15.2/ +14 mV, and occupies a circuit area of 0.0032 mm 2 .