A high-sensitivity reconfigurable integrating dual-slope CDC for MEMS capacitive sensors

The high-resolution and low power consumption Capacitance-to-Digital-Converters (CDC) demand in read-out circuits for MEMS capacitive sensor applications has become more and more important in the last few years. Traditionally, most of the sensor interfaces must be tailored towards a specific application. This approach results in a high recurrent design cost and time to market. On the other hand, generic sensor interface design reduces the costs and offers a handy solution for multisensor applications. This paper presents a generic sensor interface architecture which can read out a broad range of capacitive sensors. The proposed architecture contains a switch capacitor (SC) capacitance-to-voltage (C-V) stage followed by a programmable noise-shaping ADC. The ADC of the proposed CDC is implemented using an integrating Dual-Slope converter. As a difference with standard solutions, the ADC of the proposed CDC does not need SC integrators, leading to a very low-power and area design. In addition the current consumption can be traded-off according to the accuracy and speed requirements of the application. Finally, by adding a Reconfigurable Digital Filter (RDF), a reconfigurable CDC is achieved without any cost in the analog circuitry. System level simulation using a pressure MEMS capacitive sensor model reveals that with low clock frequencies high resolutions can be achieved. As an example using a clock frequency of 160kHz and measuring 10 capacitive values per second, 20bits of resolution is obtained in each measurement. This corresponds an equivalent capacitance resolution of 1aF assuming a differential capacitive value of the sensor of 1pF.