This paper describes the design of a temperature sensor based on integrated poly-silicon thermistors. The thermistors are incorporated in a Wien-bridge RC filter, which, in turn, is embedded in a frequency-locked loop. The loop's output frequency is then determined by the filter's temperature-dependent phase shift, thus realizing an energy-efficient and high resolution temperature sensor. After a 3-point calibration, the sensor achieves an inaccuracy of less than 0.12 C (min-max) from 40 C to 85 C. This translates into a frequency stability of better than 2 ppm from 40 C to 85 C when the sensor is used to temperature compensate the quartz-crystal oscillator of a 32 kHz real-time clock. The 0.09 mm sensor also achieves 2.8 mK (rms) resolution in a 32 ms conversion time while dissipating only 31 W.
A temperature sensor based on the temperaturedependence of integrated poly-silicon resistors is used to stabilize the quartz-crystal of a 131kHz real-time clock (RTC). After a 3-point calibration, the sensor achieves an inaccuracy of less than ±0.12°C (min-max) from -40°C to 85°C, which translates into a frequency stability of less than ±2ppm. The sensor also achieves 2.8mK (rms) resolution in a 32ms conversion time, which corresponds to a state-of-the-art resolution FOM of 8pJK 2 .
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