A CMOS temperature to duty cycle converter using differential temperature sensing

In this work, an all MOST based temperature sensor has been developed in a standard 0.18 mu m CMOS technology. The sensed temperature is expressed in terms of the duty cycle for the temperature range of -30 degrees C to 70 degrees C. The proposed temperature sensing circuit is divided into three parts, the sensor core, the subtractor and the pulse width modulator (PWM). The sensor core consists of two MOST based temperature sensors which sense the temperature in terms of the voltages. These voltages are proportional (PTAT) and complementary (CTAT) to absolute temperature respectively. The mean inaccuracy (degrees C) and the non-linearity(%) errors of the PTAT sensor are, +/- 0.35 degrees C and +/- 0.25% respectively. Similarly, the mean inaccuracy and the non-linearity errors for CTAT sensor are in the range of +/- 1 degrees C and +/- 0.8% respectively. The measured mean thermal responsivities of PTAT and CTAT voltages are 2.20 mV/degrees C and 2.27 mV/degrees C respectively. The CTAT voltage is subtracted from PTAT voltage using a subtractor circuit. The measured thermal responsivity of the resultant voltage is 6.18 mV/degrees C with the temperature inaccuracy of +/- 1.3 degrees C. The temperature information is expressed in terms of duty cycle by using a PWM circuit. The measured maximum temperature error is less than +/- 1.5 degrees C, obtained after single point calibration.