Energy-efficient capacitive-sensor interface based on an incremental Delta-Sigma modulator employing current-starved inverter-based OTAs

Abstract

An interface circuit for a fully-integrated capacitive humidity-sensor is implemented using 0.16?m standard CMOS technology. This interface circuit is intended to be used in a smart RFID sensor platform. This thesis focuses on improving the energy-efficiency of the interface circuit. A third-order incremental delta-sigma converter based on switched-capacitor integrators is presented. Due to the fact that the operational trans-conductance amplifier (OTA), as a major building block for an integrator, consumes most of the power in the whole interface circuit, this work mainly concentrates on improving the energy-efficiency of the OTAs in the three integrators. A fully-differential current-starved inverter-based OTA structure is proposed for the integrator. A dynamic power-down scenario is applied to reduce the power consumption by approximately 20%. Detailed analysis and design optimizations are also provided. The interface achieves 13-bit capacitance-to-digital conversion while consuming 6.8?W from a 1.2V supply, resulting in a figure-of-merit (F.o.M) around 0.17pJ/Step. The circuit design and layout of a test chip are presented. Simulation results confirm the expected improvement in energy-efficiency.