Capacitive Sensor Interface Circuits

Abstract

This thesis focuses on simple capacitive measurement techniques suitable for integration in CMOS technologies. The main motivation being: to realize simple frontends for capacitive sensors and microsystems for integration in high-density sensing applications, for example, in arrays of sensors for highresolution ultrasound imaging. In addition, there are many applications where a high accuracy in sensing is not essential; in such cases, a simple interface circuit can not only save the design time, but may also offer area and power advantages over the more complex circuits. Therefore, one of the main aims in this research has been to realize simple circuit topologies that may benefit such applications.

Two different kinds of sensing circuits form the highlight of this thesis. The first interface circuit is the realization of a current-mode approach that has the main advantage of being able to produce a fully-differential output also from a single-ended sensor by using just a fixed reference capacitor. The circuit, prototyped in a commercial 0.8-µm CMOS process, was estimated capable of achieving an accuracy of around 0.2% relative to full-scale which may be sufficient in many applications. In the second prototype, the feedback biasing technique is rediscovered for nanoscale CMOS technologies. It is shown that some of the classic limitations imposed by the use of feedback biasing in CMOS circuits are removed in nanoscale technologies, and when using MOSFET as feedback resistor; it is possible to realize extremely compact amplifiers. Such feedback-biased cascaded CS amplifiers, designed in a commercially available CMOS technology, achieved a voltage gain of 28 dB, an output noise power spectral density of 0.11 (µV )2/Hz at center-frequency, and a total harmonic distortion of —30 dB at full-scale output. These specifications are acceptable for application of such amplifiers as CMUT frontends. By using subthreshold MOSFETs as feedback resistors, extremely compact amplifiers (measruring just 20 µm x 10µm) were obtained. However, by using the MOSFET feedback resistor, the linearity of the amplifier is affected by the non-linearity in the resistance of the MOSFET. A simple remedy is proposed that recovers a large part of the linearity degradation by sacrificing slightly on the input resistance. The linearity improvement was observed to be more than 100% in the best case. The area overhead due to the additional device is very small