An all-digital charge to digital converter

PhD ThesisDuring the last two decades, the topic of the Internet of Things (IoT) has become very popular. It provides an idea that everything in the real world should be connected with the internet in the future. Integrating sensors into small wireless networked nodes is a huge challenge due to the low power/energy budget in wireless sensor systems. An integrated sensor normally consumes significant power and has complex design which increases the cost. The core part of the sensor is the sensor interface which consumes major power especially for a capacitor-based sensor. Capacitive sensors and voltage sensors are two frequently used sensor types in the wireless sensor family. Capacitive sensors, that transform capacitance values into digital outputs, can be used in areas such as biomedical, environmental, and mobile applications. Voltage sensors are also widely used in many modern areas such as Energy Harvesting (EH) systems. Both of these sensors may make use of sensor interfaces to transform a measured analogue signal into a frequency output or a digital code for use in a digital system. Existing sensor interfaces normally use complex analog-to-digital converter (ADC) techniques that consume high power and suffers from slow sensing response. This thesis proposes a smart all-digital dual-use capacitorbased sensor interface called charge to digital converter (QDC). This QDC is capable of not only sensing capacitance but also sensing voltages by using fully digital solutions based on iterative delay chain discharge. Unlike the conventional methods vii that only treats the sensed capacitance only as the input signal, this thesis proposes a method that can directly use the stored energy from the sensed capacitance as well to power parts of the circuit, which simplifies the design and saves power. By playing with the capacitance and input voltage, it can be used as a capacitance-to-digital converter (CDC) to sense capacitance under fixed input voltage and it also can be used as a capacitorbased voltage sensor interface to measure voltage level under fixed capacitance. The new method achieves the same accuracy with less than half the circuit size, and 25% and 33% savings on power and energy consumption compared with the state of art benchmark. The method has been validated by experimenting with a chip fabricated in 350nm process, in addition to extensive simulation analysis

Voltage and capacitance sensing using time comparison

PhD ThesisWith the rapid advancement of electronic and mechanical system miniaturisation, new application types such as portable systems, internet of things (IoT) and wireless sensor networks (WSNs) have become promising areas of growth for industry. In these areas, the limits on battery life have opened opportunities for energy harvesting to become a commonplace choice as the system power source, which brings its own problems. One of these problems is that energy harvesting is in general a much more variable energy source than batteries and mains power supply, because of the unpredictable and intermittent nature of the external energy environment [1]. This implies that both energy harvesters and the loads they support require significantly more control, tuning and management than if the energy was supplied by traditional means. On the other hand, sensing is also an important aspect for such systems as many of these systems are sensors used to monitor physical parameters in the environment. Another reason is that the control, tuning and management of energy harvesting requires the support of energy/power sensing. It is therefore inevitable that sensing methods need to be developed targeting an environment where energy supply is volatile. However, sensing under a variable energy supply faces numerous problems. One such problem is the energy consumption of the sensing itself. In this regard, the capacitive sensor is widely used for sensing a physical parameter, such as pressure, position, and humidity, as it is suitable for low-power applications with limited energy budgets [2–4]. Another problem faced by sensing under energy supply variability is the difficulty of maintaining stable voltage and/or current references. This thesis is motivated by these issues. In this thesis, a new sensing method is developed based on time domain techniques, which will be shown to be 1) suitable for capacitive sensing of environmental physical parameters, 2) suitable for sensing voltage, from which power and energy information can be derived, supporting energy harvesting management uses, and 3) robust to voltage and power volatility, making sensors derived from this method useful for miniaturised and energy autonomous systems. At the centre of this work is a novel reference-free voltage level-crossing sensor, realised through time comparison techniques. By working in the time domain, it avoids the need for voltage or current references. Two more sophisticated sensors are then developed around this level-crossing sensing engine. The first is a voltage monitor which is capable of sensing the crossing of multiple predefined voltage boundaries within a range, targeting energy harvesting system management uses. The second is a capacitance-to-digital converter which senses and converts the value of a target capacitance to digital value. This could be used to support the monitoring of physical vi parameters in the environment including pressure, temperature, moisture, etc. as these might be made to directly affect the values of capacitances. This thesis describes detailed design theory and reasoning, implementation, and validation of the presented sensors. Circuits are implemented in very-large-scale integration and investigated in the Cadence Analog Design Environment. In addition to analogue simulations, experiments were also conducted on a fabricated chip. Data collected from these simulation and physical experiments show that the time-domain method developed in this work has quantitative and qualitative advantages over existing designs