Contents lists available at ScienceDirect Measurement journal homepage: www.elsevier.com/locate/measurement A synergy of voltage-to-frequency converter and continued-fraction algorithm for processing thermocouple signals Aven Murmu a , Biswajit Bhattacharyya b , Sugata Munshi b, ⁎ a West Bengal State Electricity Distribution Company Limited, India b Dept. of Electrical Engg., Jadavpur University, Kolkata 700032, India ARTICLE INFO Keywords: Thermocouples Linearization Cold junction compensation Thermistor Voltage-to-frequency converter PC sound card 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 tem- peratures 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 pro- grammable 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. 1. Introduction Thermocouples are in use for a long time as temperature transdu- cers. They have been popular due to their ability to cover wide ranges of temperature and also because they are available in varieties that can be selected to suit different types of environmental conditions. Two important problems that need attention in conditioning thermocouple signals are linearization and reference junction compensation. The first refers to the need for achieving a linear transfer (output versus input) characteristic, the second addresses the fact that the circuit should emulate a condition similar to that when the reference junction of the thermocouple is maintained at 0 °C. The task of reference junction compensation of thermocouples by electronic hardware has been addressed adequately by the investigators for a long time and several methods have been developed [1–4]. On the other hand, an overwhelming majority of the techniques developed for linearization, have been read-only memory (ROM) based look-up table (LUT) methods and software methods involving piecewise linear or polynomial interpolations [3,5–7], those representing the inverse transfer characteristics by appropriate polynomials [8], other numerical techniques and also methods employing soft-computing tools [9–12]. Hardware based linearization schemes, in particular analog linearizing circuits, have rarely been reported, with a few exceptions [13,14]. The plentiful use of computer based data acquisition systems in practice, and with the easy availability of microcontrollers, the software methods can be readily implemented. However, the fact remains that the signal conditioning circuits may still offer low-cost solutions in dedicated temperature measurement systems, particularly if the accu- racy requirement is not very stringent. The hardwared linearizers can also serve as first-stage linearizers, and further improvement in the linearity may be achieved by LUT method or by software methods. In the present work a voltage controlled oscillator (VCO) has been deployed to process the thermocouple signal. A thermistor with nega- tive temperature coefficient (NTC) is used to monitor the reference junction temperature (i.e., the ambient temperature), and it constitutes a timing resistor of the VCO. The output of the VCO is a train of rec- tangular pulses (a quasi-digital output), which can be easily taken to a personal computer (PC) or microcontroller without digitization. This signal carries information about both reference junction temperature as well as the difference between the temperatures of two junctions of the thermocouple. Thus, the VCO serves as a low cost interface of the thermocouple with a PC or microcontroller based system. https://doi.org/10.1016/j.measurement.2017.11.047 Received 12 October 2015; Received in revised form 10 November 2017; Accepted 20 November 2017 ⁎ Corresponding author. E-mail addresses: avenmurmu@gmail.com (A. Murmu), bbhattacharyya@ee.jdvu.ac.in (B. Bhattacharyya), smunshi@ee.jdvu.ac.in (S. Munshi). Measurement 116 (2018) 514–522 Available online 22 November 2017 0263-2241/ © 2017 Elsevier Ltd. All rights reserved. T