Fully differential electro-mechanical phase locked loop sensor circuit

Embedding a micro-machined sensing element in a closed loop, force feedback system is a technique commonly used to realize high performance MEMS (micro-electro-mechanical systems) sensors due to the advantages of better linearity, increased dynamic range and reduced parameter sensitivity. Electro-mechanical sigma delta modulators (EMΣΔ) have been proposed for this reason and high order loops have been shown to enjoy a good signal to noise ratio (SNR) of more than 100 dB. It is also well known that achieving stability in high order EMΣΔs is a challenging task and in practice stability can be lost with large input signals or due to non-ideal effects in the circuits implemented. In this work we propose a novel differential frequency domain technique for closed loop control of micro-machined sensors. This method, called the electro-mechanical phase locked loop (EMPLL), uses a differential electro-mechanical phase locked loop to control and measure the deflection of micro-machined sensors. We believe that EMPLLs have the potential to have significant advantages over EMΣΔs for high performance MEMS sensors. Preliminary research suggests that this novel approach will lead to significant benefits in signal to noise ratio, parameter sensitivity, and input signal range.