Cesare Buffa

An energy-efficient 17-bit noise-shaping Dual-Slope Capacitance-to-Digital Converter for MEMS sensors

A noise-shaping Dual-Slope (DS) Capacitance-to-Digital Converter (CDC), specifically designed for interfacing capacitive MEMS sensors, is presented. In particular, this work proposes a design with a MEMS sensor built with a bridge of capacitors. In this bridge, some capacitors are function of the pressure in order to obtain a variation in the output of the bridge related with the change of pressure. Then, the capacitive to digital conversion is realized using two steps. First, a Switched-Capacitor (SC) preamplifier is used to make the capacitive to voltage (C-V) conversion. Second, a time domain noise-shaping Dual-Slope ADC is used to digitalize the magnitude of the capacitive bridge. The use of time instead of amplitude resolution leads to the following strengths: 1) intrinsically small sensitivity to temperature and process variations; 2) simplicity of trimming offset and gain to correct the sensor parameter spread; and 3) area and energy efficient implementation. The effectiveness of the method is demonstrated by measurements performed on a prototype, designed and fabricated using standard digital 0.13μm CMOS technology. Experimental results show that it achieves a resolution of 17-bit, which corresponds to a capacitive resolution of 5.4aF, while consuming only 146μA from a 1.5V power supply, with an effective area of 0.317mm2.

A High-Resolution Self-Oscillating Integrating Dual-Slope CDC for MEMS Sensors

An integrating dual-slope (DS) capacitance-to-digital converter (CDC), specifically designed for interfacing capacitive MEMS sensors, is presented. In particular, this work proposes a CDC that interfaces a MEMS sensor built with a bridge of capacitors. In this bridge, some capacitances are pressure sensitive, causing pressure-related changes in the bridge output. The voltage to digital conversion is then realized in two steps. First, a voltage amplifier boosts the output of the bridge. Second, an integrating DS ADC digitizes the output of the amplifier. The proposed ADC uses time instead of amplitude resolution to generate a multi-bit digital output stream. In addition, it performs noise shaping of the quantization error to reduce measurement time. These characteristics lead to the following properties: intrinsically low sensitivity to temperature and process variations, simplicity of trimming offset and gain to correct for sensor parameter spread, and an energy-efficient implementation. The effectiveness of the proposed architecture is demonstrated by measurements performed on a prototype, designed, and fabricated using standard 0.13 μm CMOS technology. Experimental results show that the proposed CDC achieves a maximum resolution of 17 bits, which corresponds to a capacitive resolution of 5.4aF, while consuming only 146 μA from a 1.5 V power supply, with an effective area of 0.317mm2.

A MOX gas sensors resistance-to-digital CMOS interface with 8-bits resolution and 128dB dynamic range for low-power consumer applications

In this paper an interface circuit for MOX gas sensor is presented. It is based on a resistance-to-frequency converter and improves existing solutions in term of performance (offset) and power efficiency. The resistive range covered is 100Ω-1MΩ, with an equivalent 8-bit precision in a total measurement time of 1 second. This corresponds to a dynamic range of about 128dB. Power consumption and design strategy are optimized for mass production targeting consumer applications. The interface is implemented in a standard CMOS 130nm technology with an area of 125000 μm2 and 450μA of current consumption.

A Capacitance-To-Digital Converter for MEMS Sensors for Smart Applications

The use of MEMS sensors has been increasing in recent years. To cover all the applications, many different readout circuits are needed. To reduce the cost and time to market, a generic capacitance-to-digital converter (CDC) seems to be the logical next step. This work presents a configurable CDC designed for capacitive MEMS sensors. The sensor is built with a bridge of MEMS, where some of them function with pressure. Then, the capacitive to digital conversion is realized using two steps. First, a switched-capacitor (SC) preamplifier is used to make the capacitive to voltage (C-V) conversion. Second, a self-oscillated noise-shaping integrating dual-slope (DS) converter is used to digitize this magnitude. The proposed converter uses time instead of amplitude resolution to generate a multibit digital output stream. In addition it performs noise shaping of the quantization error to reduce measurement time. This article shows the effectiveness of this method by measurements performed on a prototype, designed and fabricated using standard 0.13 µm CMOS technology. Experimental measurements show that the CDC achieves a resolution of 17 bits, with an effective area of 0.317 mm2, which means a pressure resolution of 1 Pa, while consuming 146 µA from a 1.5 V power supply.

SNDR Limits of Oscillator-Based Sensor Readout Circuits

This paper analyzes the influence of phase noise and distortion on the performance of oscillator-based sensor data acquisition systems. Circuit noise inherent to the oscillator circuit manifests as phase noise and limits the SNR. Moreover, oscillator nonlinearity generates distortion for large input signals. Phase noise analysis of oscillators is well known in the literature, but the relationship between phase noise and the SNR of an oscillator-based sensor is not straightforward. This paper proposes a model to estimate the influence of phase noise in the performance of an oscillator-based system by reflecting the phase noise to the oscillator input. The proposed model is based on periodic steady-state analysis tools to predict the SNR of the oscillator. The accuracy of this model has been validated by both simulation and experiment in a 130 nm CMOS prototype. We also propose a method to estimate the SNDR and the dynamic range of an oscillator-based readout circuit that improves by more than one order of magnitude the simulation time compared to standard time domain simulations. This speed up enables the optimization and verification of this kind of systems with iterative algorithms.

9.5 A 1.8V true-differential 140dB SPL full-scale standard CMOS MEMS digital microphone exhibiting 67dB SNR

Over the last few years, robust MEMS microphones have gained significant market share in consumer applications such as mobile phones and hearing aids. The consumer market is demanding improved quality for audio recording, while also being capable of suppressing high-energy disturbers, e.g., wind noise. The challenge is to achieve both high DR and SNR at low supply voltages that limit signal swing. Signal levels are expressed in dB Sound-Pressure-Level (dB SPL) with respect to the human hearing threshold at a sound pressure of 20µPa corresponding to 0dB SPL. Todays mass-market MEMS digital microphone solutions typically process sound up to 120dB SPL [1,2].