This thesis reports the results of research on features, options and limitations of low-cost, high-performance, universal integrated interface for capacitive sensors. It concerns development-driven research, where the objectives of the research focus upon possible realization and application of the interface with optimum results at the level of multi-disciplinary systems. Moreover, the objectives of this research include maximization of the performance in relation to the cost. The interface supports different groups of capacitive sensors such as floating high-quality capacitive sensors, floating leaky capacitive sensors, and grounded capacitive sensors. The interface can be adjusted for both: different ranges of capacitances and different signal bandwidths. To check the validity of the proposed ideas and the results of the analysis, three different types of integrated circuits have been designed, fabricated and tested. These designs and test results are presented in chapter 5, 7 and 8. The material found in the various chapters can be summarized as follows: Chapter 2 gives an introduction of capacitive sensors. The three main categories of capacitive sensor –A type, D type and ? type– are explained. In this chapter, some important existing concepts, such as those for two or three-terminal capacitors, guarding and shielding, are summarized. The concept of segmented and differential capacitive sensors and their benefit have also been discussed. In chapter 3, two types of suited A/D converters are compared: the sigma-delta converter and the oscillator-based converter. The arguments for selecting the oscillator-based converter, such as simplicity, spread of power consumption, and compatibility with Smartec’s UTI, have been discussed. Three important measurement techniques necessary to achieve a high measurement performance are briefly described in this chapter. These techniques are: auto-calibration, two-port measurement and chopping. Some important characteristics of the interface are explained as well. In chapter 4 we first show how an interface as introduced in chapter 3 can be made flexible, so that it can be used in a wide variety of applications. Then, a detailed analysis of important interface features, such as noise, linearity, and immunity for the effects of parasitic capacitances are presented. At the end of this chapter, the challenges of characterizing and measuring very small nonlinearities are presented, together with a practical method to solve the identified problems. Chapter 5 presents a very flexible, high-performance, universal interface design for which full advantage has been taken of the knowledge and techniques presented in the previous chapters. This interface is suitable for measuring high-quality floating capacitive sensor in various ranges up to 220 pF. The measurement time can be set from about 100 µs up to 50 ms, which corresponds to a data acquisition rate of 20 samples/s up to 104 samples/s. With a measurement time of 1 s, the measured resolution is as high as 20 bits, which corresponds to 1 aF in a 1 pF range. This resolution can easily compete with that of other state-of-the art designs and is at least three bit higher than the resolution of Smartec’s UTI, which is available in the. Moreover, our novel interface circuit has much more flexibility with respect to the selection of the input capacitance range, parasitic-input-capacitance range, and the measurement time. Furthermore, the nonlinearity of this interface is about two bits better than Smartec’s UTI. To help the user in making proper choices when optimizing the interface system for particular applications, a users guide has been included. Chapter 6 introduces a novel interface in which the principle of negative feedback for the front-end circuit has been applied. Another difference with the design presented in chapter 5 is that the input capacitance-to-voltage converter (CVC) has been removed, so that the relaxation oscillator works as a capacitance-to-period (CPC) converter. The resulting structure is more power-efficient. In addition, since the main source of nonlinearity is found to be in the CVC, removing this converter yields a higher linearity. Furthermore, we showed that applying negative feedback can also increase the resolution. The conditions to be met for proper operation are explained and measurement results have been presented. Chapter 7 introduces an interface with a modified CVC, which in first order is immune to the leakage of the capacitive sensing elements. Measurement results show that the error in measuring a capacitor with the value of 100 pF caused by a shunting resistor of 100 k? is less than 0.6 pF. Finally, chapter 8 presents an interface for grounded capacitive sensors that are equipped with active guarding, using feedforward instead of feedback. Since there is no possibility of instability in this method, there is more freedom to optimize the immunity of interface for of the effects of parasitic cable capacitances. The measurement results show that when a connection cable with a length of 30 m is used to measure a capacitor in the range of 10 pF to 330 pF, the cable capacitance of 3 nF causes an error of less than 0.3 pF.