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This paper describes the design of a CMOS smart temperature sensor intended for RFID applications. The PNP-based sensor uses a digitally-assisted readout scheme that reduces the complexity and area of the analog circuitry and simplifies trimming. A key feature of this scheme is an energy-efficient two-step zoom ADC that combines a coarse 5-bit SAR conversion with a fine 10-bit ΣΔ conversion. After a single trim at 30°C, the sensor achieves an inaccuracy of ±0.2°C (3σ) from -30°C to 125°C. It also achieves a resolution of 15 mK at a conversion rate of 10 Hz. The sensor occupies only 0.12 mm2 in a 0.16 μm CMOS process, and draws 4.6 μA from a 1.6 V to 2 V supply. This corresponds to a minimum power dissipation of 7.4 μW, the lowest ever reported for a precision temperature sensor.
Date of Publication: July 2011