A +0.66/−0.73 °C Inaccuracy, 1.99-μW Time-Domain CMOS Temperature Sensor With Second-Order ΔΣ Modulator and On-Chip Reference Clock

This paper presents a compact and low-power time-domain CMOS temperature sensor intended for Internet of Things. To eliminate the off-chip reference clock that is commonly needed for time-domain CMOS temperature sensors, a precise on-chip reference clock is designed to measure the temperature-dependent delay generated by an inverter chain with even stages. In the reference clock circuit, a relaxation oscillator showing discharging phases with negative temperature coefficient (TC) is designed, which are compensated by 2 identical inverter-chains with positive-TC delay, resulting in a reference clock with nearly temperature-independent pulse width and period. In addition, a 2nd-order hybrid time-voltage delta-sigma (ΔΣ) modulator with feedforward path is proposed to quantize the temperature-dependent delay of the main inverter chain, achieving 100-mK resolution only in about 25-ms conversion time. Fabricated in 0.18-μm CMOS, measurement results show that the best (worst)-case temperature accuracy of the clock's reference time is ±0.015% (±0.055%) and the temperature sensor achieves a maximum inaccuracy of +0.66/-0.73°C from -20°C to 80°C. The prototype occupies 0.45-mm
²
chip area and consumes 1.99-μW from a 1.8-V supply at room temperature.