This paper presents a power-efficient 10/12 bit 40 kS/s SAR ADC for sensor applications. It supports resolutions of 10 and 12 bit and sample rates from DC up to 40 kS/s to accommodate a variety of sensor applications. A Data-Driven Noise-Reduction method is introduced to selectively enhance the comparator noise performance. In this way, a higher ADC resolution can be achieved with a small increase of the power consumption. A self-oscillating comparator is used to generate the bit-cycling clock internally. In this way, the ADC only requires an external clock at the sample-rate frequency. A segmented capacitive DAC with 250 aF unit elements is applied to save power and to reduce DNL errors at the same time. The implemented prototype in 65 nm CMOS occupies an area of 0.076 mm 2. For the two supported resolutions (10/12 bit), the ADC achieves an ENOB of 9.4 and 10.1 bit while consuming 72 and 97 nW from a 0.6 V supply at 40 kS/s. This leads to power efficiencies of 2.7 and 2.2 fJ/conversion-step for 10 bit and 12 bit resolution, respectively. Furthermore, the leakage power, which is below 0.4 nW, ensures that the efficiency can be maintained down to very low sample rates.

A 10b/12b 40 kS/s SAR ADC With Data-Driven Noise Reduction Achieving up to 10.1b ENOB at 2.2 fJ/Conversion-Step

A 20-bit incremental ADC for battery-powered sensor applications is presented. It is based on an energy-efficient zoom ADC architecture, which employs a coarse 6-bit SAR conversion followed by a fine 15-bit ΔΣ conversion. To further improve its energy efficiency, the ADC employs integrators based on cascoded dynamic inverters for extra gain and PVT tolerance. Dynamic error correction techniques such as auto-zeroing, chopping and dynamic element matching are used to achieve both low offset and high linearity. Measurements show that the ADC achieves 20-bit resolution, 6 ppm INL and 1 μV offset in a conversion time of 40 ms, while drawing only 3.5 μA current from a 1.8 V supply. This corresponds to a state-of-the-art figure-of-merit (FoM) of 182.7 dB. The 0.35 mm2 chip was fabricated in a standard 0.16 μm CMOS process.

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A 6.3 µW 20 bit Incremental Zoom-ADC with 6 ppm INL and 1 µV Offset

This paper presents an energy-efficient capacitive-sensor interface with a period-modulated output signal. This interface converts the sensor capacitance to a time interval, which can be easily digitized by a simple digital counter. It is based on a relaxation oscillator consisting of an integrator and a comparator. To enable the use of a current-efficient telescopic OTA in the integrator, negative feedback loops are applied to limit the integrator's output swing. To obtain an accurate ratiometric output signal, auto-calibration is applied. This eliminates errors due to comparator delay, thus enabling the use of a low-power comparator. Based on an analysis of the stability of the negative feedback loops, it is shown how the current consumption of the interface can be traded for its ability to handle parasitic capacitors. A prototype fabricated in 0.35 μm standard CMOS technology can handle parasitic capacitors up to five times larger than the sensor capacitance. Experimental results show that it achieves 15-bit resolution and 12-bit linearity within a measurement time of 7.6 ms for sensor capacitances up to 6.8 pF, while consuming only 64 μA from a 3.3 V power supply. Compared to prior work with similar performance, this represents a significant improvement in energy efficiency.

An Energy-Efficient 15-Bit Capacitive-Sensor Interface Based on Period Modulation

This paper presents a fully integrated CMOS humidity sensor for a smart RFID sensor platform. The sensing element is a CMOS-compatible capacitive humidity sensor, which consists of top-metal finger-structure electrodes covered by a humidity-sensitive polyimide layer. Its humidity-sensitive capacitance is digitized by an energy-efficient capacitance-to-digital converter (CDC) based on a third-order delta-sigma modulator. This CDC employs current-efficient operational transconductance amplifiers based on current-starved cascoded inverters, whose limited output swing is accommodated by employing a feedforward loop-filter topology. A programmable offset capacitor is included to remove the sensor's baseline capacitance and thus reduce the required dynamic range. To reduce offset errors due to charge injection of the switches, the entire system is auto-zeroed. The proposed humidity sensor has been realized in a 0.16- μm CMOS technology. Measurement results show that the CDC performs a 12.5-bit capacitance-to-digital conversion in a measurement time of 0.8 ms, while consuming only 8.6 μA from a 1.2-V supply. This corresponds to a state-of-the-art figure-of-merit of 1.4 pJ/conversion-step. Combined with the co-integrated humidity sensing element, it provides a resolution of 0.05% RH in the range from 30% RH to 100% RH while consuming only 8.3 nJ per measurement, which is an order-of-magnitude less energy than the state-of-the-art.

A 1.2-V 8.3-nJ CMOS Humidity Sensor for RFID Applications

In this paper, the flexible capacitive humidity sensors using the polyimide foils as the substrate together with the moisture sensitive materials had been investigated. A simple and low cost inkjet-printing method had been developed to form silver interdigitated electrodes on PI foils. Two kinds of capacitive humidity sensors with Ag IDEs were constructed. And the humidity sensor characteristics such as sensitivity and hysteresis were analyzed. The results indicated that the metallization of Ag on PI is potential for various future flexible applications.

Fabrication of silver interdigitated electrodes on polyimide films via surface modification and ion-exchange technique and its flexible humidity sensor application

This paper presents a review of the latest advances in the field of capacitive, inductive (eddy current), and magnetic sensors, for measurement of absolute displacement. The need for accurate displacement and position measurement in the micrometer, nanometer, and subnanometer scales has increased significantly over the last few years. Application examples can be found in high-tech industries, metrology, and space equipment. Besides measuring displacement as a primary quantity, absolute displacement sensors are also used when physical quantities such as pressure, acceleration, vibration, inertia, etc., have to be measured. A better understanding of the commonalities between capacitive, inductive, and magnetic displacement sensors, as well as the main performance differences and limitations, will help one make the best choice for a specific application. This review is based on both theoretical analysis and experimental results. The main performance criteria used are: sensitivity, resolution, compactness, long-term stability, thermal drift, and power efficiency.

Advances in Capacitive, Eddy Current, and Magnetic Displacement Sensors and Corresponding Interfaces

This paper introduces a clock-free self-timed incremental ΔΣ ADC. Unlike conventional ΔΣ ADCs, it does not require a dedicated clock signal, thus saving energy and reducing system complexity. As such, it has similar advantages as self-timed (or asynchronous) SAR ADCs [1]. It is particularly suited for use in energy-constrained sensor applications, in which conversions of a quasistatic input signal are triggered by infrequent and possibly irregular external events. As it autonomously powers down upon completion of a conversion, it can adapt to a wide range of conversion rates in an energy-efficient way.

A 1V 14b self-timed zero-crossing-based incremental ΔΣ ADC

A fully-integrated humidity sensor for a smart RFID sensor platform has been realized in 0.16μm standard CMOS technology. It consists of a top-metal finger-structure capacitor covered with a humidity-sensitive layer, combined with a micro-power flexible sensor interface based on a second-order incremental delta-sigma converter. The interface can be easily reconfigured to compensate for process variation of the sensing element. In a measurement time of 10.2 ms, the interface performs a 13-bits capacitance-to-digital conversion while consuming only 5.85 μA from 1.8 V supply. In combination with the co-integrated sensor capacitor, it thus provides a humidity-to-digital conversion with a resolution of 0.1% RH in the range of 20% to 90% RH at only 107 nJ per measurement. This represents a significant improvement in energy efficiency compared to existing capacitive-sensor interfaces with comparable performance.

Zhichao Tan

Papers

An Energy-Efficient 15-Bit Capacitive-Sensor Interface Based on Period Modulation

A 1.2-V 8.3-nJ CMOS Humidity Sensor for RFID Applications

Advances in Capacitive, Eddy Current, and Magnetic Displacement Sensors and Corresponding Interfaces

A 1V 14b self-timed zero-crossing-based incremental ΔΣ ADC

A 1.8V 11μW CMOS smart humidity sensor for RFID sensing applications