Higher order sigma-delta modulator interfaces for capacitive sensors

MEMS capacitive sensors, such as pressure sensors, accelerometers and gyroscopes have been one of the most successful examples of micro-system technology. They are routinely used in many applications; the most important being automotive safety systems. These sensors are typically open-loop and have low to medium performance specifications. Recently, there has been an emerging requirement for more high performance sensors for more advanced automotive applications and many others. To fulfill this requirement it appears a very promising approach to apply some more advanced control and electronic interface techniques to existing sensing elements. Incorporation of a capacitive sensing element in a sigma-delta modulator (SDM) control architecture is an elegant way to improve the sensors´ performance, especially concerning linearity, dynamic range and bandwidth. Additionally, this type of control system yields a digital output signal that can directly interface to a standard digital signal processor. So far, MEMS capacitive sensors have mainly been incorporated in lower order SDM architectures, where the sensing element acts as the loop filter, and provides only noise-shaping up to order two. However, to provide higher order noise-shaping it is necessary to add additional electronic integrators. In this presentation, higher order SDM architectures are discussed to be used with MEMS capacitive sensors. Various higher order SDM topologies will be presented; all have been extensively simulated at system level, and the most promising ones also implemented in hardware. As will be seen in the presentation, both simulation and measurement results agree very well.