Nonius based time-to-digital converter for low-power RFID sensory systems

This paper presents a nonius-based time-to-digital converter (TDC) for its use as part of a RFID tag sensor. The TDC can digitize any physical magnitude previously converted to time delay and it exploits the benefits of time domain conversion: high resolution with reduced power consumption and low voltage operation. A high level study is carried out to analyze the feasibility of the nonius-based TDC for RFID sensing applications. A calibration strategy tailored for RFID sensing applications is proposed that makes the TDC very robust against process and mismatch variations. Subgate resolution is analyzed to overcome the technology gate delay limits. An implementation of the converter using a 0.18-μm CMOS standard process has been analyzed for human body temperature sensing applications. A resolution of 0.035°C and accuracy value of 0.053°C are achieved in the range from 35°C to 43°C. The average power consumption of the converter is only 3.9nW at 10 samples per second from a 1.8 V voltage supply.