Study On Low-Power Integrated Temperature Sensor On A Single Chip

With the expanding of the human exploration, detection equipments often operate in environments with extreme temperature. How to ensure the equipments to work reliably in such environments is one of the issues that people care about. Moreover, with the improvement of living standards, people’s health has become a topic of increasing concern. Thus, the real-time monitoring of body temperature which is a basic indicator of human physical condition has drawn much attention. To deal with the above situations, temperature sensors are critical devices. However, the growth of demand for miniaturization, portability and low-cost of devices reduces the applicability of discrete temperature sensor made by special material or thin films. In such condition, integrated temperature sensors are invented, which rapidly attract much attention due to their feature of great implantability.Before 2005, all integrated temperature sensors with relatively high resolution and accuracy were designed to operate in voltage-domain mode. Since the analog-to-digital converter(ADC) is one of the most important modules in this type of temperature sensor, it is much difficult to design such temperature sensor. Besides, the power consumption of such sensor system cannot be decreased significantly. In the year of 2005, another type of integrated temperature sensor with a new operation mode was developed. This type of temperature sensor obtains the temperature information by quantizing the delay time related to temperature. Therefore, it is named as time-domain integrated temperature sensor. The employment of the time-to-digital converter(TDC) to obtain digital codes makes the sensor be able to be integrated with full digitalization, which can effectively reduce the difficulty of design and costs of application. However, the delay cells used as temperature sensing circuits are sensitive to supply voltage and process variation. Besides, the nonlinearity of delay time changing with temperature variation will induce large temperature error in measurements. Given the above drawbacks, the traditional time-domain integrated temperature sensor is not suitable for application with demand for high resolution and accuracy. This dissertation makes an in-depth study on the temperature sensor. A temperature sensor combining with operating characteristics of voltage-domain and based on time-domain quantization is proposed. The proposed sensor with tunable resolution consumes low power and occupies small chip area.The contents of this dissertation include design and implementation of a low-power single-chip integrated temperature sensor with tunable resolution. The dissertation also analyzes and designs temperature sensing circuits, reference clock generators and analog-to-frequency converters(AFCs):1. A low-power single-chip integrated temperature sensor with tunable resolution based on time-domain quantizer. The analog circuit with output current possessing proportional-to-absolute-temperature(PTAT) characteristics is employed as the temperature sensing circuit, which solves the problem that the resolution of traditional time-domain temperature sensor is limited by the temperature sensing circuit with digital “gate” architecture. Digital temperature readings are obtained by quantizing the frequency related to temperature in the proposed sensor. Thus, the tunable resolution is realized by tuning the frequency of reference clock, which makes the proposed temperature sensor be suitable for many applications. Given that the off-chip reference clock generator is employed by all available integrated temperature sensors, the architecture of integrated temperature sensor with on-chip reference clock generator is proposed to implement such a sensor on single chip. In order to mitigate the impact of devices’ mismatch and modules’ offset on the sensor, relaxation oscillators with the same structure are employed to make up the reference clock generator and AFC.2. Temperature sensing circuit based on temperature compensation by subthreshold leakage current of MOSFET. Given that the available curvature temperature compensation suffers from complex circuit design and large power dissipation, a temperature compensation technique using subthreshold leakage current of MOSFET is proposed. The subthreshold leakage current of MOSFET in terms of temperature is studied, and thus, the leakage current is utilized to compensate the output current of temperature sensing circuit. On the basis of the technique, a dynamic threshold-voltage MOSFET(DTMOS) based self-biased circuit with active resistor is proposed to be used as temperature sensing circuit. The aim of the proposed temperature sensing circuit is further reducing the chip area and improving the temperature linearity of the output signal.3. Design of the on-chip reference clock generator and AFC based on relaxation oscillator. Compared to relaxation oscillators, ring oscillators are more sensitive to process, supply voltage and temperature variation(PVT variation). Therefore, relaxation oscillators are more suitable for the integrated temperature sensor studied in this dissertation. Given that the PVT sensitivity of traditional relaxation oscillator is dependent on that of the reference voltage in the oscillator, a bandgap reference circuit based on operational amplifier with asymmetric differential input pairs is proposed to provide reference voltage to the relaxation oscillator. Since the proposed reference circuit dose not introduce so many devices or clock signals to mitigate the influence of offset voltage, the proposed circuit, compared to the circuit using dynamic offset cancellation(DOC) technique, can not only effectively reduce the impact of offset voltage, but also output a reference voltage with low noise. In order to decrease the chip area and simplify the design of relaxation oscillator with high stability against supply voltage and temperature variation, dynamic-threshold(DT) and switched-resistors(SR) techniques are proposed. The DT and SR techniques make the relaxation oscillator possess favorable supply voltage and temperature stability without employing bandgap reference.