The Research And Design Of Capacitance Detection Circuits Used In Digital Accelerometers

Currently accelerometers are widely used in automotive, industry automation,aerospace and many other areas. Compared to piezo-resistive accelerometers, capacitive accelerometers are popular because of its low temperature sensitivity. In addition,compared with analog accelerometers, the digital accelerometers have the advantages of performing error correction and compensation directly without going through another ADC stage.Conventional capacitance detection circuits for digital accelerometers have two important modules, the capacitance-to-voltage (C/V) converters followed by the voltage-to-digital (V/D) converters. However, there are three shortcomings in this combination. Firstly, converting to voltage method is very susceptible to environmental factors like temperature change and circuit inherent noise. Secondly, the C/N converters considerably consume a substantial amount of power in most capacitance detection circuit because they need to be of high bandwidth and high gain in order to achieve low-noise output; hence it is not desirable for lower-power design. Thirdly, due to the large process variation in MEMS technology, the sensors nominal capacitances may vary greatly; this will impose large input common-mode problems in the C/V front-end amplifier. Therefore, this thesis proposed a new sensing system to combine the sensing structure with a Sigma-Delta modulator to solve the above problems.In this new sensing system, the differential capacitors of sensing structure are designed as sampling capacitors in a Sigma-Delta modulator. The change in capacitance of the sensing structure is directly converted to the required error signal in the Sigma-Delta Loop. Hence, there is not intermediate voltage conversion stage. Another advantage is that the front-end amplifier is just a simple integrating amplifier, the high power consumption C/N converter is therefore not needed in the front-end. Thus, it also reduces the power consumption at the same time. To address the input common-mode problems in the front-end amplifier, a proprietary input-common mode compensation feedback circuit is proposed.The new sensing system can be of open-loop sensing system or closed-loop sensing system. The two types of system have been integrated in the standard CMOS process. The simulation results show that the signal-to-noise ratio (SNR) of the open-loop modulation circuit is of 101 dB, while the SNR of closed-loop modulation is of 102dB, representing an accuracy of 16.6 bit.