All-Digital CMOS On-Chip Temperature Sensor with Time-Assisted Analytical Model

Abstract

This paper proposes a Time-to-Digital Converter (TDC) as an on-chip temperature sensor. An all-digital TDC, with 12-bit coarse and 5-bit fine output bits, has been implemented in 0.18 \(\upmu \)m CMOS with a time-resolution of 92.71 ps, time range upto 12.151 \(\upmu \)s and power consumption of 353.8 \(\upmu \)W. The temperature dependence of the clock period, of ring oscillator in the TDC, has been carried out to develop an analytical model. The proposed analytical model is used to estimate the temperature directly from the digital counts obtained from the TDC. The time to digital conversion achieves a temperature resolution <0.01 \(^\circ \)C and temperature range \((-55\sim 125)\) \(^\circ \)C, sampled at 125 KHz frequency. The results of the temperature sensor are first validated with temperature independent inputs to the TDC. Further, a Proportional To Absolute Temperature (PTAT) current reference circuit for time inputs is integrated with the TDC. The PTAT current reference circuit has a 12.21 ppm/\(^\circ \)C drift in time input with the temperature. The proposed TDC based temperature sensor with current reference circuit restricts the error within −0.4 \(^\circ \)C \(\sim \) 1.24 \(^\circ \)C and compares favourably with the reported literature.