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A new frequency to voltage closed-loop integrated sensor circuit suitable for the read-out module of a monolithically integrated SAW sensor on Si is proposed. This closed-loop system consists of a voltage controlled oscillator, a peak detecting comparator, a finite state machine, and the monolithically integrated SAW sensor device. The output of the system is forced to oscillate within a narrow voltage range that correlates with the SAW pass-band response. The period of oscillation is of the order of the SAW phase delay. The output voltage range varies with changes in SAW center frequency, thus tracking mass sensing events in real time. The analysis and simulation of the system are presented for two SAW devices operating at 374 and 140 MHz, respectively. Experimental results for a test chip fabricated with AMIC5N 0.5 μm process are presented for comparison with simulation data of the 140 MHz design. Because of this frequency to voltage conversion, this approach is uniquely suitable for full monolithic integration of autonomous sensor systems and tags.