A Linearization Scheme for Thermistor-Based Sensing in Biomedical Studies

Temperature is one of the basic biophysical quantity monitored for various biomedical systems. Moreover, the variation of temperature is also an important parameter which can be used for estimating other measurands, such as respiratory airflow. This paper proposes a simple operational amplifier-based astable multivibrator circuit for linearization of the characteristic of a negative temperature coefficient thermistor constituting one of the timing resistors. The circuit has been combined with a lookup table to get the unknown temperature value from multivibrator output. Moreover, the same system topology can be used as a linearizer for measurement of respiratory airflow. The performance of the composite system has been verified experimentally. A linearity of approximately +/- 0.75% has been achieved over 30 degrees C-110 degrees C in the case of temperature measurement and +/- 1.2% for airflow of 10-60 LPM. Better results can also be achieved with the introduction of interpolation algorithms, but at a higher computational and component cost. The compactness of the complete system makes it a good candidate for embedded sensing applications in biomedical systems, such as point-of-care monitoring or in sleep study.