Integrated front-end analog circuits for mems sensors in ultrasound imaging and optical grating based microphone
Qureshi, Muhammad Shakeel
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The objective of this research is to develop and design front-end analog circuits for Capacitive Micromachined Ultrasound Transducers (CMUTs) and optical grating MEMS microphone. This work is motivated by the fact that with micro-scaling, MEMS sense capacitance gets smaller in a CMUT array element for intravascular ultrasound imaging, which has dimensions of 70um x 70um and sub pico-farad capacitance. Smaller sensors lead to a lower active-to-parasitic ratio and thus, degrads sensitivity. Area and power requirements are also very stringent, such as the case of intravascular catheter implementations with CMOS-First CMUT fabrication approach. In this implementation, capacitive feedback charge amplifier is an alternative approach to resistive feedback amplifiers. Capacitive feedback charge amplifier provides high sensitivity, small area, low distortion and saving power. This approach of charge amplifiers is also suitable in capacitive microphones where it provides low power and high sensitivity. Another approach to overcome capacitive detection challenges is to implement optical detection. In the case of biomimetic microphone structure, optical detection overcomes capacitive detection's thermal noise issues. Also with micro-scaling, optical detection overcomes the increased parasitics without any sensitivity degradation, unlike capacitive detection. For hearing aids, along with sensitivity, battery life is another challenge. We propose the use of 1-bit front-end sigma-delta ADC for overall improved hearing aid power efficiency. Front-end interface based on envelope detection and synchronous detection schemes have also been designed. These interface circuits consume currents in microampere range from a 1.5V battery. Circuit techniques are used for maximizing linear range and signal handling with low supplies. The entire front end signal processing with Vertical Cavity Surface Emitting Laser (VCSEL) drivers, photodiodes, filters and detectors is implemented on a single chip in 0.35um CMOS process.