The focus of the research work presented in this thesis was to design a low-noise accelerometer utilising a microfabricated, capacitive sensor element, which had a high quality factor, Q. The sensor element with high Q leads to a low Brownian noise floor. However, the high Q of the sensor element also leads to a high settling time and it may also lead to saturation of the sensor element displacement or the interfacing electronics. Thus, the high Q of the sensor element needs to be damped to achieve a reliable, low-noise operation. In this thesis, first, we go through the different interface topologies present in the existing literature to summarise both the advantages and disadvantages of the open- and closed-loop capacitive interfaces. Then, the building blocks of a capacitive accelerometer, a capacitive sensor element, different charge sensitive amplifiers (CSA) and first-order controllers, are studied. Later, we present the implementation of two accelerometer interfaces. The first implementation utilises a closed-loop, switched-capacitor analog interface. The measurements showed that the accelerometer is able to achieve both a low-noise and reliable operation, i.e. high Q of the sensor element is damped. Due to the closed-loop interface, the interface output was inversely proportional to the supply voltage. When the interface output was fed to an analog-to-digital converter (ADC), which uses supply voltage as reference voltage, the accelerometer output was dependent on the supply voltage meaning that the noise at the supply voltage is seen at the accelerometer output. In order to overcome this issue, a second interface was implemented. The second implementation utilised a novel hybrid interface, which consisted of a ratiometric, open-loop interface and an AC force-feedback. The AC force-feedback was utilised only to damp the high Q of the sensor element, whereas the ratiometric output of the interface was fed to an ADC, which uses the supply voltage as reference voltage, in order to decrease the dependency of the output to the supply voltage. The measurement results of the second implementation has shown that the hybrid interface is able to damp the high Q of the sensor element and to decrease the output dependency to the supply.
|Translated title of the contribution||Korkean Q-arvon mikromekaanisia anturielementtejä käyttävät kapasitiiviset kiihtyvyysanturirajapinnat|
|Publication status||Published - 2013|
|MoE publication type||G4 Doctoral dissertation (monograph)|
- sensor interface
- capacitive sensor