A 6.6-μW Wheatstone-Bridge Temperature Sensor for Biomedical Applications

Abstract

This letter presents a compact, energy-efficient, and low-power Wheatstone-bridge temperature sensor for biomedical applications. To maximize sensitivity and reduce power dissipation, the sensor employs a high-resistance (600 $\text{k}\Omega $ ) bridge that consists of resistors with positive (silicided-poly) and negative ( $n$ -poly) temperature coefficients. Resistor spread is then mitigated by trimming the $n$ -poly arms with a 12-bit DAC, which consists of a 5-bit series DAC whose LSB is trimmed by a 7-bit PWM generator. The bridge is readout by a second-order delta–sigma modulator, which dynamically balances the bridge by tuning the resistance of the silicided-poly arms via a 1-bit series DAC. As a result, the modulator’s bitstream average is an accurate and near-linear function of temperature, which does not require further correction in the digital domain. Fabricated in a 180-nm CMOS technology, the sensor occupies 0.12 mm2. After a 1-point trim, it achieves +0.2 °C/−0.1 °C ( $3{\sigma }$ ) inaccuracy in a ±10 °C range around body temperature (37.5 °C). It consumes 6.6 ${\mu }\text{W}$ from a 1.6-V supply, and achieves 200- ${\mu }\text{K}$ resolution in a 40-ms conversion time, which corresponds to a state-of-the-art resolution FoM of 11 fJ $\cdot \text{K}^{2}$ . Duty cycling the sensor results in even lower average power: 700 nW at 10 conversions/s.