An Ultralow Power Time-Domain Temperature Sensor With Time-Domain Delta–Sigma TDC

Without using bipolar transistors, serially connected inverter cells generate clock delay according to temperature. The delay is compared with a reference clock to estimate the temperature. The proposed time-to-digital converter (TDC) structure is using a time-domain delta-sigma (ΔΣ) modulator. This type of TDC with ΔΣ modulator can achieve higher resolution by increasing the oversampling ratio, with the advantages of low area and low power consumption. To increase the accuracy by producing true temperature-independent time delay, an external reference clock is utilized, instead of temperature-independent inverter cells, for robust operations. The measured temperature sensors demonstrated a minimum power consumption of 480 nW and a resolution under 0.1 °C. The 3σ error of the sensor is ±0.99 °C over -20 °C-80 °C temperature range from ten-sample measurement results. The chip area is 0.089 mm
²
using a Dongbu 0.18-μm CMOS process. The conversion rate is 1.25 samples/s.