Design of a Wide Temperature Range, High Linearity Time Domain CMOS-Based Temperature Sensor for Wearable IOT Applications

Abstract

The proliferation of conformable devices with embedded wireless capability has propelled the expansion of interconnected devices into a swarm called the Internet of Things (IOT). This allowed the localized sensing while processing is normally done remotely into the cloud. The local sensors need to dissipate low power while providing acceptable performance based on the application. One such sensor is the ubiquitous temperature sensor. Temperature sensing has become a pivotal component in most smart systems for maintaining the device performance at optimum, thereby preventing degradation. Amongst such sensors, solid-state based temperature sensors have proven to provide the widest sensing range as well as promotes integration into a complete system-on-chip. There have been several approaches for the readout circuit either based on MOS or BJT, with either analog or digital outputs. In this work, a low voltage and low power temperature sensor with digital time-based output is presented. The circuit uses a BJT - less current-mode bandgap core incorporating sub-threshold MOS. Temperature dependent output voltages are derived from the core, that drives the source and sink currents of a voltage-frequency converter. The circuit has achieved high linearity of (r2 = 0.99) over the temperature range of −40 to 110 deg C, a power dissipation of just around 30 I-lW at a single supply rail of 1.0 V. The circuit has been designed using TSMC 0.18um technology obtained from MOSIS wafer test runs and was verified using SPICE.