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A CMOS fully-integrated humidity sensor for a RFID sensor platform has been realized in 0.16μm CMOS technology. It consists of a top-metal finger capacitor, covered by a humidity-sensitive polyimide layer, and an energy-efficient inverter-based capacitance-to-digital converter (CDC). Measurements show that the CDC performs a 12.5-bit conversion in 0.8ms while consuming only 8.6μA from a 1.2V supply. Together with the co-integrated humidity sensor, this translates into a resolution of 0.05% RH in the range of 30% RH to 90% RH, at an energy consumption of only 8.3nJ per measurement.