A Thermistor-Based Temperature Sensor for a Real-Time Clock With $\pm$ 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 ±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 mm² sensor also achieves 2.8 mK (rms) resolution in a 32 ms conversion time while dissipating only 31 μW.