Fabiano Fruett

Temperature sensors and voltage references implemented in CMOS technology

This paper reviews the concepts, opportunities and limitations of temperature sensors and voltage references realized in CMOS technology. It is shown that bipolar substrate transis- tors are very suited to be applied to generate the basic and PTAT voltages. Furthermore, it is shown that dynamic element matching and auto-calibration can solve the problems related to mismatching of components and noise. The effects of mechan- ical stress are a major source of inaccuracy. In CMOS technology, the mechanical-stress effects are small, as compared to those in bipolar technology. It is concluded that, with low-cost CMOS tech- nolog, rather accurate voltage references and temperature sensors can be realized.

The piezojunction effect in silicon sensors and circuits and its relation to piezoresistance

Mechanical stress has a significant effect on the elec- trical characteristics of silicon transistors through the piezojunc- tion effect. This effect is physically related to the piezoresistive ef- fect, but its results are quite different. Both effects can be applied to sensor devices. They can also have a negative effect on circuit performance and this may need to be minimized. This paper gives a overview of the physical causes and shows how understanding of the piezojunction effect can be used to maximize it for sensors and minimize it for circuits.

Minimization of the mechanical-stress-induced inaccuracy in bandgap voltage references

This paper shows a systematic approach to calculate and to minimize mechanical-stress-related problems in bandgap voltage references. Mechanical stress is the main cause of long-term drift and packaging-induced inaccuracy in bandgap voltage references. Especially, low-cost epoxy packaging can cause a large mechanical stress, being in the range from -200 to +200 MPa. This stress shows local variations over the chip area and changes over time or during thermal cycling. Due to the piezojunction effect, this stress causes changes and drift in the base-emitter voltages of bipolar transistors and consequently in the output voltage of bandgap references. In this paper, it is shown that, even when using low-cost IC and packaging technology, a bandgap reference with a high immunity for the effects of mechanical stress can be realized.

The piezojunction effect in NPN and PNP vertical transistors and its influence on silicon temperature sensors

Abstract This paper describes a test structure to characterize the piezojunction effect for the base-emitter voltage V BE and the PTAT voltage Δ V BE . The piezojunction effect directly affects the accuracy of temperature sensors and special types of pressure sensors. Packaging is a source of mechanical stress in electronics circuits. Measurements have been performed for two types of vertical bipolar transistors. Firstly, an NPN transistor of a BiCMOS technology and secondly, a PNP substrate transistor of a CMOS technology, both of them using a [100] silicon wafer geometry. It has been found that the sensitivity for the uniaxial stress of the V BE of the PNP is four times less than that of the NPN transistor. In both cases, the PTAT voltage appears to be hardly stress-sensitive.

Embedded sensor system and techniques to evaluate the comfort in public transportation

This paper shows an embedded measurement system based in tri-axial accelerometers, global positioning systems (GPS) and a temperature sensor supported by wireless communication and SD Card storage and beyond, the application of three sensor system algorithms to measure the comfort in public transportation. The first algorithm is based on the detection of accelerations peaks over threshold values produced by reckless driving, holes, speed bumps or others defects. The second one adds the detection of acceleration changes (Jerk) peaks over 0.4m/s3; whereas, the third one adds the RMS measurement from frequency-weighted accelerations following the standard ISO2631–1. Finally, all the algorithms report the time, type, magnitude, and geographic position of the event in a LabVIEW™ interface using a Google™ online map.

A Multi-Terminal Pressure Sensor with enhanced sensitivity

This paper describes the design, microfabrication and characterization of a CMOS compatible Multi-Terminal Pressure Sensor (MTPS). This sensor is an alternative to the pressure sensors based on the conventional silicon Wheatstone piezoresistive bridge (WB) or four-terminal piezotransducers. The layout of the MTPS is designed in such a way that the sensor sensitivity is effectively improved and the short-circuit effects, which are modeled by the Geometrical Correction Factor (G), can be minimized. The sensor design was supported by Finite Element Method (FEM). The MTPS sensitivity amounts to 4,8 mV/psi.

Real-time distributed fiber microphone based on phase-OTDR

The use of an optical fiber as a real-time distributed microphone is demonstrated employing a phase-OTDR with direct detection. The method comprises a sample-and-hold circuit capable of both tuning the receiver to an arbitrary section of the fiber considered of interest and to recover in real-time the detected acoustic wave. The system allows listening to the sound of a sinusoidal disturbance with variable frequency, music and human voice with ~60 cm of spatial resolution through a 300 m long optical fiber.

Instrument based on MEMS accelerometer for vibration and unbalance analysis in rotating machines

The present work shows a low-cost instrument, based on MEMS accelerometer, specially designed to analyse unbalance in rotating machines. The acceleration signals, acquired using a sample rate of 817 Hz in two perpendicular axes at a free wheel shaft, were used to analyze the shaft vibration in the frequency domain by its Fast Fourier Transform (FFT). These signals from orthogonal accelerometers are also used to calculate the shaft unbalance. The instrument prototype was mounted on the shaft of a bicycle wheel, freely rotating at 120 rpm. Composing the characterization apparatus, some weights were introduced at the endings of a rim spokes to simulate a mechanical wheel unbalance. The experimental results show that both: vibration and unbalance can be measured successfully.

Theoretical and experimental comparison between power and degree of polarization based optical fiber current sensors

This paper presents a theoretical and experimental comparison of two techniques of optical fiber current sensors (OFCS) based on the Faraday Effect: the power and the degree of polarization (DOP) methods. The DOP method is shown to be insensitive to state of polarization (SOP) changes induced by environmental conditions. A maximum variation of 0.4% in the DOP value in the entire range of applied current from 0 to 100 A was measured. It is the first time to our knowledge that this fact was experimentally verified. Also, for the first time known it is reported that zero current response, DOP = 0.996, is in agreement with the theoretical expected value of unitary DOP. Besides, the theoretical and experimental normalized DOP is shown to have a maximum difference of 0.33% in the entire range of applied current. On the other hand, the power method OFCS is greatly affected by SOP changes. In order to be used in a practical application, its complexity must be increased by the use, for example, of polarization maintaining fibers instead of standard fibers used in the DOP method, increasing the cost of the sensor element.

On-field distributed first-order PMD measurement based on pOTDR and optical pulse width sweep.

A method for PMD distributed localization and estimation based on polarization optical time domain reflectometer technique, pOTDR and pulse width sweep is used on-field for the first time. The method consists in launching light pulses with variable widths in an optical fiber under test and then analyzes the Rayleigh backscattered signal spatial power distribution after passing through a polarizer. Both localization and PMD magnitude are function of OTDR pulse width and can be obtained from the ripple analysis, enabling the characterization of the fiber links.