Analog and mixed signal design: Measuring temperature with thermisters

Measuring temperature fairly accurately can be done using a number of methods. The sensors that are available to do this are the RTD, the pn junction, the positive temperature coefficient thermister ( PTC) and the negative temperature coeffcient (NTC)thermister. Among these options the NTC thermister seems to be used more and more in applications where the temperature rate of change is fast. The advantages and disadvantages of using this type of device are: Advantages, fast reaction time, small size, two wire connections and relatively inexpensive. The disadvantages are: the temperature versus resistance characteristic is very non linear, some kind of excitation is required, the temperature range is limited, subject to self heating and relatively fragile. In spite of the disadavantages the thermister is a choice many design engineers are making. The web has a number of good articles that are very helpful in the understanding of the thermister. Articles from Betatherm, Microchip technology and National Instruments to mention a few. The challenge does not lie in understanding the thermister itself. It is very easy to undertstand, at least as far as the users perspective goes. The challenge is in coming up with analog and mixed signal circuitry that interfaces with the thermister and allows for accurate measurement of the temperature. Signal Processing Group Inc., has developed a number of circuits which can be used with varying accuracies to measure temperature with thermisters. Interested users may contact SPG at http://www.signalpro.biz> contact.