An Algorithmic Readout Approach for Thermal Conductivity Based CO2 Sensors

Abstract

This thesis presents a new approach to reading out thermal-conductivity-based gas sensors. This method is intended for the readout of a CMOS compatible resistive thermal-conductivity transducer for indoor CO2 sensing applications, without requiring precision off-chip components. Instead of accurately regulating the power dissipated in the transducer and measuring its temperature, the temperature and power dissipation are both measured using an algorithmic approach. A high-resolution ADC digitizes the voltage drop across the transducer and the current through it, measured using an on-chip reference resistor. Moreover, by digitizing several base-emitter voltages of an on-chip bipolar transistor, a precision bandgap voltage reference is constructed in the digital domain, and accurate information about the ambient temperature is obtained, which is used to temperature compensate the voltage reference and the reference resistor. Thus, all necessary ingredients are obtained to calculate the power dissipation and temperature of the transducer, from which the thermal conductivity of the surrounding air, and hence CO2 concentration, can be obtained. A prototype integrated circuit implementing this readout approach has been realized in 0.16um CMOS. It has been tested in a climate chamber in combination with a platinum resistor mimicking the transducer. The digitally-constructed voltage reference has a temperature coefficient of 9ppm/°C, while ambient temperature is sensed with accuracy of ±0.2°C, with a resolution of 0.15°C. The resistance readings have an inaccuracy ranging between -1mOhms to 4mOhms on a nominal resistance of about 100Ohms (-10ppm - 40ppm) with a resolution of around 2mOhms in the temperature range from 10°C to 40°C; for the power measurements, the circuit achieved an accuracy between -0.03% and 0.06%, with an 800nW of resolution (in the same temperature range) which is one order of magnitude better than results presented in previous work. Although no CO2 measurements have been performed, an estimated thermal resistance accuracy of around 2862ppm with a resolution of 155.64[K/W] should be possible, which would enable detection of the CO2 levels in the air with an accuracy of around 0.72% and a resolution of 7705ppm.