A CMOS Oscillators-Based Smart Temperature Sensor for Low-Power Low-Cost Systems

Abstract

This paper proposes a CMOS oscillators-based smart temperature sensor with a SAR (Successive Approximation Register) search algorithm. To reduce the cost and release the number of bits, a temperature-dependent delay circuit (TDDC) composed of a thermal ring oscillator and a fixed-gain time amplifier was used to generate a thermal sensing delay proportional to the test temperature. An adjustable reference delay circuit (ARDC) composed of another thermal compensation ring oscillator and an adjustable-gain time amplifier was used to program a reference set-point delay. For digital output coding, a SAR control logic was adopted for selecting the optimal reference delay of the ARDC to approximate the thermal delay of the TDDC through the help of a time comparator. The chip size of the proposed oscillators-based sensor with 11 output bits was 0.25 mm2, which is less than the 0.6 mm2 of its delay-line-based predecessor with a 10 output bits in the same 0.35-μm TSMC CMOS process [1]. The measurement errors were within ±0.6 °C in the temperature range of 0 °C to 90 °C after two-point calibration for eight packaged chips.