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Design and Modelling of Smart Sensor Dedicated for Water Pollution Monitoring

Department of Microelectronics and Computer Science,Technical University of Lodz, PL

Keywords: ISFET, CHEMFET, SEWING, Water monitoring project, smart sensor

In this paper the smart sensor of chemical signal is presented. It reflects the current state of research performed in the System for European Water Monitoring (SEWING) project. The aim of the paper is to present the smart sensor dedicated for water pollution monitoring. It consists of the ISFET (H+ ion sensitive field effect transistor) sensor, operating circuit and analogue–digital converter. The need of suitable model creation as well as proper physical parameters identification is stressed. Two real semiconductor ISFET structures have been taken into account. The first one is developed by ITE (Warsaw, Poland) whereas the second one is developed by LAAS (Toulouse, France). The laboratory measurements of both structures were performed. Two models implemented in the hardware description languages HDL-A as well as VHDL–AMS are presented, and the comparison between them was made. Comparison of the implemented physical models with extracted parameters to the measurements was done too. The analysis of the whole microsystem is performed taking into account the presence in the measured solution of other than hydrogen ions – so called disturbing ion. The analysis of the possibilities of implementation of the best possible kind of the analogue–digital converter was undertaken too. The second part of designed silicon microsystem consists of data processing circuits. They consist of compensation circuits of the sensors, memories, registers and analogue to digital converter. The most important circuits of data processing unit is analogue to digital converter because it is responsible for speed and quality of analogue data conversion from chemical sensors. Authors tested two circuits of the a/d converters. The first one was the flash analogue to digital converter whereas the second was Sigma-Delta converter. The first converter characterized very high speed but quite large size while the second one was quite slower but occupied very small area. In this paper advantages and disadvantages of both converters will be presented and the optimal solution will be submitted to use in designed silicon microsystem. In the Department of Microelectronics and Computer Science the first approach to the ion identification from the matrix of ISFET/CHEMFET sensors was performed and considered. It uses the neural network for disturbing ion identification and estimation of theirconcentration.

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