A synergy of voltage-to-frequency converter and continued-fraction algorithm for processing thermocouple signals

Abstract

A low cost signal conditioning circuit has been devised for thermocouple temperature transducers, in which the thermocouple and the NTC thermistor for reference junction compensation have been introduced in a simple voltage-to-frequency converter circuit using LM331 IC. From the time domain parameters of the output pulse train, the reference junction temperature is obtained using the well known Steinhart-Hart equation, and the measuring junction temperature is calculated via a two-dimensional continued-fraction based algorithm. This algorithm calculates the temperature under measurement and also performs the dual task of cold junction compensation and minimization of the effects of the drifts of the signal conditioning circuit components. The performance of the system has been verified experimentally with a T-type thermocouple. The output of the voltage-to-frequency converter is acquired by a personal computer (PC) through its sound card, and the temperatures are calculated and compared with Pt-100 standard. A decent accuracy of approximately ±1.4 °C has been achieved over 45–100 °C. The possibility of replacing the PC with a microcontroller unit or field programmable gate array (FPGA) system by virtue of the simplicity of the numerical algorithm used, makes the scheme a very good choice for embedded system applications.