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This paper presents a passive UHF RFID ASK demodulator that operates over a +24 dBm to -14 dBm RF input power range. The demodulator automatically adjusts between high sensitivity mode for weak RF signal power and overvoltage protection mode for high RF power. The input overvoltage protection circuit is designed to protect the IC from high input power while not impacting the sensitivity at weak input power. The demodulator is comprised of a RF rectifier, a variable gain attenuator with automatic threshold adjustment and a nano-power data slicer. The demodulator handles demodulating signals with a minimum to maximum envelope ratio of 0.8 over the entire input power range, and the data slicer consumes only 160 nA from a 0.9 to 1.25 V rectified supply. The RFID chip is fabricated in a 0.13 μm analog-CMOS technology and the entire chip occupies an area of 0.55 mm2.