A Fully Integrated Single-BJT-Based Temperature-to-Digital Converter Achieving ±0.23 °C (3 σ ) Over 2.0–5.5 V for Duty-Cycled Operation

This paper presents a fully integrated temperature-to-digital converter (TDC) fabricated in a 0.18 μm CMOS process, employing a single bipolar junction transistor (BJT). The proposed front-end sequentially biases a single BJT at two current levels to generate a ΔVBE, thereby eliminating the need for matched BJT pairs and improving reproducibility with a one‑point calibration. All critical analog and digital blocks—including the sensing front-end, an incremental delta-sigma analog-to-digital converter, power management units, clock generators, e-fuse memory, digital processing logic, and an I²C interface—are fully integrated on-chip to enable compact and standalone operation. After one-point calibration at 30 °C, the TDC achieves an inaccuracy of ±0.23 °C (3σ) over a −40 °C to 125 °C temperature range. At a conversion time of 7.68 ms, it achieves a 6.71 mK rms resolution, corresponding to a system-level resolution figure of merit (FoM) of 75.3 pJ·K². For the sensing core, the resolution FoM is 11.8 pJ·K². A co-integrated dual-BJT TDC exhibits an inaccuracy of ±0.37 °C (3σ), validating the effectiveness of the single-BJT approach for comparison. The TDC operates from a 2.0–5.5 V supply with low supply sensitivity of 2.54 m°C/V and supports a low standby current of 420 nA at 2.0 V, enabling compact size, high accuracy, and energy efficiency for duty-cycled applications.