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A CMOS Interface for a Gas-Sensor Array with a 0.5%-Linearity over 500k/spl Omega/-to-1G/spl Omega/ Range and 2.5/spl deg/C Temperature Control Accuracy
Abstract
The interface IC includes 8 read-out channels and 2 closed-loop temperature control circuits, is fabricated in 0.35mum 2P4M CMOS and dissipates 27mW from a 3.3V supply. The read-out structure, based on a controlled oscillator, achieves a 0.5% linearity and a SNR >48dB over the 500kOmega-1GOmega sensor resistance range with a 114dB DR. The temperature control systems maintain a 100degC gradient in the range 100 to 400degC with plusmn2.5degC accuracy
... Some common methods of measuring a wide range of resistances are shown in [14][15][16][17][18][19][20][21][22][23][24][25][26][27]. Reference [16] presents a resistance to frequency (RTF) circuit. ...
... References [22][23][24] propose a method for the measurement of a wide range of resistances using a current mirror and a capacitor. The method converts resistance into current by applying a fixed voltage across the resistor. ...
... By measuring the voltage across this capacitor, the resistance value can be obtained. Due to the copy error of the current mirror and the limited gain of the operational amplifier, it is difficult to maintain a measurement error of less than 1% with a resistance variation range of 10 8 [24]. If we want to measure a resistance with a range of 10 8 variations and require an error of less than 1%, then we need a 34-bit ADC (log 2 (10 10 ) = 33.2). ...
In this work, an interface circuit applied to resistive gas or chemical sensors is proposed. The interface circuit includes a detection front-end, a single-end to differential circuit, a successive approximation analog-to-digital converter (SAR ADC), and some reference auxiliary circuits. In detection front-end circuits, mirrored currents in a current mirror usually differ by several orders of magnitude. In order to ensure that the current mirror can be copied accurately, this work uses a negative feedback structure consisting of an operational amplifier and an NMOS tube to ensure that the VDS of the current mirroring tube remains consistent. Simulation results show that the replication error of the current mirror is 0.015%. The proposed interface circuit has a detection range of 10 Ω to 1 GΩ with a relative error of 0.55%. The current multiplication or divided technique allows the interface circuit to have a high sampling frequency of up to 10 kHz. The proposed circuit is based on a 180 nm CMOS process with a chip area of 0.308 mm2 (723 μm ∗ 426 μm). The power consumption of the whole interface circuit is 3.66 mW when the power supply voltage is 1.8 V.
... An interesting thing to notice is that the large dynamic requirement is common to most resistive sensor interfaces. The work by Grassi [4] In another implementation of the resistive gas sensor interface [5], an OPAMP is wrapped around a NMOS as shown in Figure 22 to generate a constant voltage bias across the sensor, which generates a constant current based on the value of the resistor. The generated current is integrated on a known capacitor, and by measuring how long it takes to charge and discharge the capacitor of a known value, the current is converted into a digital word. ...
... The systems that were studied in Section 2.2 can be viewed from a different perspective in light of Figure 23. The interfaces introduced in [4] and [5] both actuates the sensor with a voltage source, and measures the voltage signal from the sensor. In our design, we opt to actuate the directly with a variable current source, ...
... The architecture is also sampling-rate scalable, and by duty-cycling the system, a linear power scaling is achieved. This resulted in the worst-case power consumption of 32µW at 1.7kS/s, a two orders of magnitude power reduction at higher sampling rate and lower accuracy compared with the state-of-the-art sensor interface circuits ( [4] and [5].) The majority of the power is consumed from the DAC, and it can be reduced by integrating CNT sensors on-chip to reduce the parasitic capacitance at the signal node. ...
Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2007. Includes bibliographical references (p. 95-98). A carbon nanotube is considered as a candidate for a next-generation chemical sensor. CNT sensors are attractive as they allow room-temperature sensing of chemicals. From the system perspective, this signifies that the sensor system does not require any micro hotplates, which are one of the major sources of power dissipation in other types of sensor systems. Nevertheless, a poor control of the CNT resistance poses a constraint on the attainable energy efficiency of the sensor platform. An investigation on the CNT sensors shows that the dynamic range of the interface should be 17 bits, while the resolution at each base resistance should be 7 bits. The proposed CMOS interface extends upon the previously published work to optimize the energy performance through both the architecture and circuit level innovations. The 17-bit dynamic range is attained by distributing the requirement into a 10-bit Analog-to-Digital Converter (ADC) and a 8-bit Digital-to-Analog Converter (DAC). An extra 1-bit leaves room for any unaccounted subblock performance error. Several system-level all-digital calibration schemes are proposed to account for DAC nonlinearity, ADC offset voltage, and a large variation in CNT base resistance. Circuit level techniques are employed to decrease the leakage current in the sensitive frontend node, to decrease the energy consumption of the ADC, and to efficiently control the DAC. The interface circuit is fabricated in 0.18 /m CMOS technology, and can operate at 1.83 kS/s sampling rate at 32 pW worst case power. The resistance measurement error across the whole dynamic range is less than 1.34% after calibration. A functionality of the full chemical sensor system has been demonstrated to validate the concepts introduced in this thesis. by Taeg Sang Cho. S.M.
... The temperature cycle was selected by switching between two reference resistors (specific to two temperatures 150 C and 400 C, as shown in Fig. 3), according to the control logic signal. Malfatti et al. [26] ...
... ). Fig. 4. Wheatstone bridge temperature control schematic (adapted from [26]). Fig. 5. Schematic of proportional temperature controller circuit (adapted from [27]). ...
Modern gas sensor technology is becoming an important part of our lives. It has been applied within the home (monitoring CO levels from boilers), the workplace (e.g., checking levels of toxic gases) to healthcare (monitoring gases in hospitals). However, historically the high price of gas sensors has limited market penetration to niche applications, such as safety in mines or petrochemical plants. The high price may be attributed to several different components: (1) cost of a predominantly manual manufacturing process; (2) need for interface circuitry in the form of discrete components on a PCB; and (3) fireproof packaging, making the cost of gas detection instruments typically many hundreds of dollars. Consequently, there has been a considerable effort over the past 20 years, towards the goal of low-cost ($1-$5) gas sensors, employing modern microelectronics technology to manufacture both the sensing element and the signal conditioning circuitry on a single silicon chip. In this paper, we review the emerging field of CMOS gas sensors and focus upon the integration of two main gas-sensing principles, namely, resistive and electrochemical and associated circuitry by CMOS technology. We believe that the combination of CMOS technology with more recent MEMS processing is now mature enough to deliver the exacting demands required to make low-power, low-cost smart gas sensors in high volume and this should result in a new generation of CMOS gas sensors. These new integrated, mass-produced gas sensors could open up mass markets and affect our everyday lives through application in cars, cell phones, watches, etc.
... The emergence of nanotechnology has also accelerated such research efforts by proposing new reliable sensing materials [1]. To reflect this trend, a number of sensor-integrated CMOS platforms have been introduced, and demonstrated a stable and accurate operation [2] [3]. The power consumption of these systems exceeds several milliwatts, primarily due to the use of a microhotplate which heats up the sensors to achieve high sensitivity, and also due to the use of OPAMPs to accommodate a large dynamic range. ...
... Readout Rate Power Consumption Malfatti et al.[2] ...
This paper presents a hybrid CNT/CMOS chemical sensor system that comprises of a carbon nanotube sensor array and a CMOS interface chip. The full system, including the sensor, consumes 32 muW at 1.83 kS/s readout rate, accomplished through an extensive use of CAD tools and a model-based architecture optimization. A redundant use of CNT sensors in the frontend increases the reliability of the system.
... On-off controlled loops can also be combined with both a Wheatstone bridge, where one of the branches is the heater resistance, as well as with a window comparator, like Malfatti et. al showed in 2006 [14]. Amos and Segee implemented the described on-off control loop using a microcontroller [15]. ...
The choice of suitable semiconducting metal oxide (MOX) gas sensors for the detection of a specific gas or gas mixture is time-consuming since the sensor’s sensitivity needs to be characterized at multiple temperatures to find its optimal operating conditions. To obtain reliable measurement results, it is very important that the power for the sensor’s integrated heater is stable, regulated and error-free (or error-tolerant). Especially the error-free requirement can be only be achieved if the power supply implements failure-avoiding and failure-detection methods. The biggest challenge is deriving multiple different voltages from a common supply in an efficient way while keeping the system as small and lightweight as possible. This work presents a reliable, compact, embedded system that addresses the power supply requirements for fully automated simultaneous sensor
characterization for up to 16 sensors at multiple temperatures. The system implements efficient (avg. 83.3% efficiency) voltage conversion with low ripple output (<32 mV) and supports static or temperature-cycled heating modes. Voltage and current of each channel are constantly monitored and regulated to guarantee reliable operation. To evaluate the proposed design, 16 sensors were screened. The results are shown in the experimental part of this work.
... The splitting ratio was chosen considering internal diameter and length of the column, as well as the requirement of the two detectors. Sensors bias and data acquisition have been performed with dedicated electronics; details of the acquisition system can be found in [10]. Details of chromatograms obtained with this system are reported in [11]. ...
In this work, a novel data analysis method for the exploitation of semiconductor metal-oxide-based detectors in chromatographic systems is presented and evaluated. The method exploits the properties of the detector response in the time domain for increasing the time resolution of chromatograms measured with gas sensors. The performances in terms of sensitivity and response speed of a microfabricated Cr-doped WO3 sensor array have been compared against a state-of-the-art mass spectrometer detector in order to validate the method in a demanding application such as the determination of the content of volatile organic compounds in wines.
... In this work, an integrated interface circuit for the array of MOX gas sensors was employed in order to satisfy the system requirements and properly read the sensing device. The developed interface, realized in 0.35um CMOS standard technology, is composed by eight readout channels and two temperature control circuits [5]. The single read-out channel presents a straightforward calibration procedure in order to adjust the resistance variations of the sensor baseline value and allows the sensor resistance measurement in the 500kΩ-1GΩ range with a resolution better than 0.5%. ...
In this work, the performances of a Cr-doped WO<sub>3</sub> sensor array are investigated for the application of this device as detector in gas chromatographic systems. Chromatographic separation modules combined with detectors based on semiconducting metal oxides enable the implementation of systems with both high selectivity and high sensitivity. Comparison of results of the sensor array and mass spectrometer detectors validates the use of this class of devices for low-cost systems for the determination of the volatile organic compounds contents in wines for quality assessment of alcoholic beverages.
... A first arrangement has been performed by biasing the sensor with a voltage that is constant and, as a consequence, independent from the oscillator circuit, by means of a voltage buffer. The sensor current drives a push-pull CMOS current mirror circuit that feeds a capacitor C, which integrates the current, followed by a Schmitt trigger that actually bounds the voltage swing across C. The maximum accuracy reached by this solution is about 0.5% for sensor resistances in the 500kΩ to 1GΩ range, but the read-out channel still needs to be calibrated controlling the voltage reference value V REF and re-configuring the value of C [5], thus leading to a circuit complexity and cost comparable with the one reported in the first example, which exploited a reconfigurable transresistance input stage. ...
In this paper a low-cost CMOS wide-dynamic-range interface for resistive gas sensors is presented. The circuit is based on resistance-to-time conversion. The state of art of this measurement method has been improved first by separating the oscillator circuit from the sensing device, thus leading to higher linearity performance, and then by embedding a novel digital measurement control system. Measurements on a silicon prototype, designed in 0.35mum CMOS technology, show that the circuit achieves, without calibration, a precision in resistance measurement of about 0.4% over a range of 4 decades and better than 0.8% over 5 decades (dynamic range, DR=141dB). Furthermore, after calibration, it reaches a precision of 0.4% ranging in [1kOmega-1GOmega], thus leading to a DR of 166dB. Finally, in fast-mode, the circuit achieves an accuracy of 0.2% over 2 decades on a single scale with a throughput higher than 100Hz consuming 15mW from 3.3V supply
... A number of resistive sensor interface architectures have been introduced in literature using an OP-AMP to widen the dynamic range [3] or enhance the accuracy of the measurement [4]. Since high gain is necessary in the OP-AMP, the power consumption of these interfaces is not suited for ultra-low power applications. ...
This paper presents an energy efficient chemical sensor system that uses carbon nanotubes (CNT) as the sensor. The room-temperature operation of CNT sensors eliminates the need for micro hot-plate arrays, which enables the low energy operation of the system. The sensor interface chip is designed in a 0.18 mum CMOS process and consumes, at maximum, 32 muW at 1.83 kS/s conversion rate. The designed interface achieves 1.34% measurement accuracy over 10 kOmega -9 MOmega dynamic range. The functionality of the full system, including CNT sensors, has been successfully demonstrated.
With the development of science and technology, gas sensors have gradually expanded to many vertical fields. However, each application requires different technical requirements for gas sensors, such as low cost, small size, high sensitivity, high precision, and low power consumption. To create gas sensors that meet the performance requirements, researchers worldwide are working on solutions in fields such as sensitive materials, sensor arrays, and interface circuits. However, there are bottlenecks in the research of interface circuits, which limit the volume, power consumption, and intelligent design development of gas sensors. Therefore, this article reviews various types of gas sensors and interface circuits implemented with different technologies, analyzes their research significance in various areas, and provides ideas for future gas sensor research directions.
The paper presents a sensor system with temperature control of platinum microheater using a combined control strategy of open loop and closed loop control. The method proposed in this paper modifies the Proportional Integral control for faster response under sampling time constraint of large sampling interval, as a large sampling interval between two control commands slow the response. The initial quick temperature rising open loop response of microheater contributes to faster rise time and then it is switched to closed loop control when the quick initial rise has taken place, hence termed as Open Loop to Closed Loop Switched Proportional Integral (OLCLS PI ) control. The settling time obtained by OLCLS PI and PI are 3 s and 14 s respectively for 2% accuracy, thereby providing faster response than PI. The work is studied with elaborate modelling and simulation using MATLAB and practical implementation on embedded platform is presented using in-house developed MOX based gas sensor which utilizes a platinum microheater.
Within this work the development of an integrated gas sensor as System-On Chip (SOC) in a 0.35μm standard CMOS process plus CMOS compatible SnO2-deposition and Si-release steps is presented. The SnO2 layer provides high gas sensitivity to 10ppm for CO in humid air. An optimized Micro-Hotplate (μHP) consisting of a fully released membrane with a poly-Si heater in the oxide stack is designed. Due to the small area of AμHP=100×100μm2 and the switched capacitor temperature controller, low power consumption Pel=24mW at high temperatures T=400°C and short rise time of inîe=11.8ms are achieved. The differential setup contains sense and dummy sensors in order to compensate drift and tolerances. The readout stage consists of a gain adjustable amplifier with digital offset compensation and shows a relative error e
Within this work the development of integrated Micro-HotPlates (μHPs) for gas sensing applications as a System-On Chip (SOC) is presented. As gas sensors exploit resistance variations of sensing materials like SnO2 at high operating temperatures, integrated μHPs are required for the dynamic and low power operation of these sensors. The optimized μHP structures consist of fully released membranes with polysilicon heaters in the oxide stack and suspension arms to the bulk silicon. Thanks to the optimized μHP design very low power consumption of Pel ~20mW at high temperatures up to T=400°C together with a thermal uniformity of only ΔT~1K across the active area ending up in the highest reported efficiency of =26-20K/mW for standard CMOS hotplates is achieved. Further a rise/fall time trise/tfall =4.5/5.4ms was measured. Long term stability of the μHP has been proven applying ten million measurement cycles. Thermography confirmed the temperature distribution and functionality. The realized hotplates cover a heating area of AμHP=100×100μm2/70×70μm2 at arm lengths of larm=70μm/50μm respectively. The chips have been realized in 0.35μm standard CMOS technology and released in a post process MEMS-etching step.
This paper presents a low-power resistive sensor interface circuit with correlated double sampling which reduces the effect of amplifier offset and enables time-interleaved single-to-differential sampling. The proposed sampling scheme, used with a 12b SAR-type analog-to-digital converter, effectively doubles the input signal and improves linearity. The fabricated chip in 0.13μm CMOS demonstrates a sampling rate of 1-kS/s and a dynamic range of 117dB with a maximum conversion error of 0.32-percent while consuming only 11.3-μW from single supply voltage of 0.5V.
We present an integrated analog front-end (AFE) for the read-channel of a parallel scanning-probe storage device. The read/write element is based on an array of microfabricated silicon cantilevers equipped with heating elements to form nanometer-sized indentations in a polymer surface using integral atomic-force microscope (AFM) tips. An accurate cantilever model based on the combination of a thermal/electrical lumped-element model and a behavioral model of the electrostatic/mechanical part are introduced. The behavioral model of the electrostatic/mechanical part is automatically generated from a full finite-element model (FEM). The model is completely implemented in Verilog-A and was used to co-develop the integrated analog front-end circuitry together with the read/write cantilever. The cantilever model and the analog front-end were simulated together and the results were experimentally verified. The approach chosen is well suited for system-level simulation and verification/extraction in a design environment based on standard EDA tools.
In this paper, we present the design and the characterization of a wide-dynamic-range interface circuit for resistive gas-sensors able to operate without calibration. The circuit is based on resistance-to-frequency conversion, which guarantees low complexity. The state-of-the-art of this measurement method has been improved first by separating the resistance value controlled oscillator circuit (RCO) from the sensing device, thus leading to higher linearity performance, and then by exploiting a novel digital frequency measurement system. Measurement results on a silicon prototype, designed in a 0.35-mum CMOS technology, show that the circuit achieves, without calibration, a precision in resistance measurement of 0.4% over a range of 4 decades and better than 0.8% over 5 decades (dynamic range, DR = 141 dB). Furthermore, after calibration, it reaches a precision of 0.4% for resistance values ranging between 1 kOmega and 1 GOmega, thus leading to a DR of 168 dB. The prototype chip consumes less than 15 mW from a 3.3-V supply.
In this work, we describe the design implementation, validated by experimental results, of an innovative gas sensor array for wine quality monitoring. The main innovation of this integrated array deals with the simultaneous outputs, from a single chip on TO-12 socket, of 8 different signals coming from a WO3 thin film structure heated in a linear temperature gradient mode, allowing an overall evaluation of gas sensing properties of the material in a 100°C-wide window, typically from 300 to 400°C. The implemented sensitive layer is a WO3 film deposed by RF-sputtering. Preliminary tests of gas sensing showed good responses to the target analytes for the specific application (1-heptanol, 3-methyl butanol, benzaldehyde and ethyl-hexanoate).
This paper presents a low-cost interface for high-value resistive sensors varying over a wide range, from kΩ to GΩ. The proposed circuit that acts as a "resistance to period converter" is suitable to be interfaced to a microcontroller or a counting device. The behavior of real electronic components that produce estimation errors, is taken into account. Experimental results obtained by the realized prototype show a good resolution and repeatability.