Simple, wide-range, linear temperature-to-frequency converters using standard thermistors
Bertil Sundqvist · Journal of Physics E Scientific Instruments · 1983
The exponential voltage-time relationship of a charging RC circuit is used to linearise the approximately exponential resistance (R) against temperature (T) behaviour of a thermistor. The resulting output signal is a frequency, linear in and directly proportional to absolute temperature. Using standard, inexpensive thermistors a temperature error of less than 0.5K over a 75K range is obtained, the error being due to deviations from the thermistor's nominally exponential R(T) relationship. The circuit can be used to linearise any transducer having an exponential relationship between input and output signals, or to obtain an output signal proportional to the logarithm of any input voltage signal (that is, proportional to the input signal in dB).