Design of Read Circuitry for Nonlinear Smart Sensors

Abstract

The purpose of this paper is comparative study, design and simulation of two stage, analog to digital convertor to convert the properly conditioned stepwise linearized analog output signal from a ratio metric NTC thermistor circuit. The design part consists of linearization of thermistor temperature- resistance characteristics in analog domain. A CMOS sample and hold circuit is used to bridge linearizing circuit in analog domain and the flash ADC circuit in digital domain. Ratio of the voltage is sensed rather than absolute voltage across the thermistor, as part of linearization. Further linearization is done in two stage flash ADC consisting of a reference voltage divider resistance, comparators, analog multiplexers and 8bit differential ADC. A graph is plotted between temperature and the corresponding resistance of NTC thermistor NTCLE100E10 [1], having a resistance of 10kΩ at 25°C. The graph is an exponential curve and it is linearized [2] around 50°C by drawing a curve between temperature and voltage ratio across a resistor connected in series to the thermistor. This curve is still nonlinear at extreme values of the considered range -20 °C to 100 °C. The full range is divided into four sub ranges and piecewise linearization is done simultaneously with analog to digital conversion. Analog and digital integrated circuits are generally low powered and the voltage limit is 5V. Thermistor is compatible in this range. A number of signal conditioning circuits are devised in the past. Here the linearizing steps involved in simple circuit elements and simple design. Signal conditioning of thermistor response has a number of design circuits already devised, still there is room for research to make it is small in size, low powered, high sensitivity and cost effective.