Current-Based High-Sensitivity Differential Detection of Light Power Using Si Photodiodes in Bridge Configuration for Chemical/Biological Optical Sensing☆

Abstract We present a new optoelectronic technique based on the differential measurement of currents for the detection of the variations of low concentrations of chemical and biological substances by measuring light power absorption through two Si-photodiodes (SiPD) in a bridge configuration. The solution exhibits high sensitivity, linear response and allows the compensation of the initial bridge unbalance without changing its elements so optimising signal amplification gain and detection resolution. The technique shows unique performances with respect to voltage amplitude measurements performed by lock-in amplifiers. Moreover, the experimental apparatus is simple and suitable for portable integrated sensor systems. Its main performances have been evaluated through a prototype PCB demonstrating the capability to detect light power variations with a settable maximum sensitivity of 30mV/nW and a resolution of 33pW

Uncalibrated operational amplifier-based sensor interface for capacitive/resistive sensor applications

In this paper, a new configuration of operational amplifier -based square-wave oscillator is proposed. The circuit performs an impedance-to-period (Z–T) conversion that, instead of a voltage integration typically performed by other solutions presented in the literature, is based on a voltage differentiation. This solution is suitable as first analogue uncalibrated front-end for capacitive and resistive (e.g. relative humidity and gas) sensors, working also, in the case of capacitive devices, for wide variation ranges (up to six capacitive variation decades). Moreover, through the setting of passive components, its sensitivity can be easily regulated. Experimental measurements, conducted on a prototype printed circuit board, with sample passive components and using the commercial capacitive humidity sensor Honeywell HCH-1000, have shown good linearity and accuracy in the estimation of capacitances, having a baseline or reaching a value ranging in a wide interval [picofarads–microfarads], as well as, with a lower accuracy, in the evaluation of more reduced variations of resistances, ranging from kiloohms to megaohms, also when compared with other solutions presented in the literature

A Novel Uncalibrated Read-Out Circuit for Floating Capacitive and Grounded/Floating Resistive Sensors Measurement

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On The Sensitivity Characteristics in Novel Automatic Wheatstone Bridge-Based Interfaces

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Low-cost Discrete Off-the-shelf Components 1MHz Analogue Lock-in Amplifier for Fast Detection of Organic Compounds through Pulsed Lasers

Abstract We report on a low-cost analogue Lock-In Amplifier (LIA) designed to measure amplitude variations of 100 ns pulsed signals at operating frequencies f 0 up to 1MHz. The fabricated prototype PCB, implemented through discrete off-the-shelf components, allowed to validate the solution and to perform circuit testing and characterisations. The LIA architecture is simple and based on the classic phase-sensitive synchronous demodulation technique including two different amplification stages together with suitable filtering blocks that allow setting the instrument gain, sensitivity and resolution. With respect to conventional LIAs typically working at lower operating frequencies, the reported solution provides also high-speed DC output of about 1ms. By employing short voltage pulses, the LIA is capable to detect fast and small variations of the signal amplitude envisaging its use in sensor applications to measure reduced variations of chemical and physical phenomena through high-speed systems with very small time constants

Metasurface based on cross-shaped plasmonic nanoantennas as chemical sensor for surface-enhanced infrared absorption spectroscopy

Infrared spectroscopy is an effective technique extensively used in research and industry for the label-free and unambiguous identification of molecular species. However, the sensitivity of this technique is severely limited as a result of Beer's law and, the small infrared absorption cross-section that make prohibitively weak the absorption signals, of minute amounts of analyte as those present in monolayers. This limitation can be overcome by enhancing the infrared vibration of molecules through the enhancement of the electromagnetic (EM) field. Surface Enhanced InfraRed Absorption (SEIRA) using resonant metal Nano-scale Antennas (NAs) can provide huge electromagnetic fields on the nanometer scale featuring localized collective oscillations of electrons, an effect named Localized Surface Plasmonic Resonances (LSPRsWe here report on a series of 2D arrays of cross-shaped NAs having several mm 2 area coverage (metasurface) as SEIRA optimized antennas, which can be used in practical applications such as the vibrational sensing of chemical and biological analytes. Cross-shape designed NAs are insensitive to the polarization of the electromagnetic radiation impinging the active area. Due to the random orientation of the dipole moments of molecules they are particularly suitable for the construction of bio-molecular sensors. At the same time, the 2D-array configuration ensures a good near-field signal enhancement arising from the coupling between neighbour NAs Moreover, SEIRA NAs can be easily integrated with micrometre-sized channels and be suitable for the high sensitivity, real time analysis of IR emitting samples, in matrices where IR spectroscopy is severely limited due to absorption bands of liquid water. We present the design, fabrication and experimental characterization of large-area metasurfaces based on cross-shaped plasmonic NAs for the spectroscopic characterization of various types of compounds and for sensing applications in the mid-infrared range. The cross-shaped NAs we have designed exhibit SEIRA phenomena which are very sensitive to both refractive index changes in the surrounding medium and to the specific molecular vibration band emerging from surface adsorbed molecules. To test this effect on our device, we have used as model compounds small molecules (molecular weight (MW) < 500 g/mol) containing triple bond groups resonating at about 2100 cm −1 and a large polymer (MW ˜ 950,000 g/mol) containing carbonyl groups resonating at wavenumbers of about 1700 cm −1 . We show a sensitivity of 600 nm/RIU at different wavelengths at a maximum amount of immobilized small molecule of 0.7 fmoles and a SEIRA enhancement factor of 48,000. We also show the device potential to reveal chemical reactions, occurring on the sensor surface at the same scale, where the nitrile group is converted to a triazole ring

A Capacitance-to-Time Converter-Based Electronic Interface for Differential Capacitive Sensors

In this paper we present an oscillating conditioning circuit, operating a capacitance-to-time conversion, which is suitable for the readout of differential capacitive sensors. The simple architecture, based on a multiple-feedbacks structure that avoids ground noise disturbs and system calibrations, employs only three Operational Amplifiers (OAs) and a mixer implementing a square wave oscillator that provides an AC sensor excitation voltage. It performs a Period Modulation (PM) and a Pulse Width Modulation (PWM) of the output signal proportionally to the sensor differential capacitance values. The sensor variation range and the detection sensitivity can be easily set through the additional resistors. Preliminary PSpice simulation results have shown a good agreement with theoretical calculations as well as a linear response with a high detection sensitivity of differential capacitive sensors having a baseline in the range [2.2 ÷ 180 pF]. Moreover, different experimental measurements have been also performed by implementing the circuit on a laboratory breadboard using commercial discrete components so validating the idea and providing the circuit performances with different kind of differential capacitive sensors achieving detection resolutions of about 0.1 fF in an overall differential capacitive variation range that is equal to ±15.8 pF. The achieved results demonstrate that the proposed interface solution is suitable for on-chip integration with different kinds of differential capacitive sensing devices, such as Micro-Electro-Mechanical-System (MEMS), force/position, and humidity sensors in biomedical and robotics applications

A Current-Mode TransImpedance Amplifier for Capacitive Sensors

A Current-Mode (CM) TransImpedance Amplifier (TIA) based on Second Generation Current Conveyors (CCIIs) for capacitive microsensor measurements is presented. The designed electronic interface performs a capacitance-to-voltage conversion using 3 CCIIs and 3 resistors exploiting a synchronous-demodulation technique to improve the overall detection sensitivity and resolution of the system. A CM-TIA solution designed at transistor level in AMS0.35 µm integrated CMOS technology with a power consumption lower than 900 µW is proposed. Experimental results obtained with a board-level prototype show linear behavior of the proposed interface circuit with a resolution up to 34.5 fF and a sensitivity up to 223 mV/nF, confirming the theoretical expectations