A thermistor-based temperature sensor for a real-time clock with ±2 ppm frequency stability

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 ±2ppm from -40°C to 85°C when the sensor is used to temperature compensate the quartz-crystal oscillator of a 32kHz real-time clock. The 0.09mm2 sensor also achieves 2.8mK (rms) resolution in a 32ms conversion time while dissipating only 31μW.