Ultra-low power accurate temperature sensor for IoT

Abstract

The analog front-end of a proposed BJT-based temperature sensor has been designed. The design has been analysed using Monte-Carlo simulations with mismatch over all process corners. The design was implemented in a 90nm generic process design kit. The analog front-end achieves ultra-low power consumption with a current consumption of 2.3 mA at a temperature of 27 °C and can operate over the military temperature range of -55 °C - 125 °C. It uses a voltage supply of 2 V. An adaptive self-biasing operational amplifier was implemented in the bandgap reference circuit to ensure sufficiently high DC loop-gain. It operates on an ultra-low current consumption of 631 nA. To reduce errors due to mismatch and process spread two correction techniques have been employed, that is chopping of the input signals of the operational amplifier and dynamic element matching of the current sources in the bipolar core. The residual temperature reading error at the output of the analog front-end is large and results in significant errors. This is because no compensation technique was implemented in the analog front-end to compensate for process spread of the BJTs and should be implemented digitally. The temperature reading errors due to offset and mismatch in the current sources have been reduced to around 0.03 °C at a temperature of 27 °C. The noise in the circuit was analysed without the dynamic effects of the DEM because of its simple implementation and results in an equivalent temperature error of around 0.12 °C, which indicates the resolution that is achievable. The settling time of the circuit is in the range of 110 ms.