A Thermistor-Based Temperature Sensor for a Real-Time Clock With <formula formulatype="inline"><tex Notation="TeX">$\pm$</tex> </formula>2 ppm Frequency Stability

Abstract

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 $\pm \hbox{0.12 }^{\circ}\hbox{C}$ (min-max) from $-\hbox{40 }^{\circ}\hbox{C}$ to 85 $^{\circ}\hbox{C}$ . This translates into a frequency stability of better than $\pm$ 2 $~$ ppm from $-\hbox{40 }^{\circ}\hbox{C}$ to 85 $^{\circ}\hbox{C}$ when the sensor is used to temperature compensate the quartz-crystal oscillator of a 32 kHz real-time clock. The 0.09 $\hbox{mm}^{2}$ sensor also achieves 2.8 $~$ mK (rms) resolution in a 32 ms conversion time while dissipating only 31 $\mu\hbox{W}$ .