A Linear Temperature-to-Frequency Converter Using an Integrable Colpitts Oscillator

A novel temperature-to-frequency (T/F) converter using a thermistor has been developed. The resistance of the thermistor is converted into its equivalent inductance to form the resonant circuit of the Colpitts oscillator. To linearize the T/F conversion, two resistors, one in series and the other in parallel with the thermistor, are used and their values are determined so that the resultant resistance, and hence the equivalent inductance, is proportional to the square of a temperature in a specified temperature range. The conversion sensitivity of 59 Hz/°K and the residual nonlinearity less than 2.85 percent over the temperature range from 253 to 333°K are obtained by the prototype converter which oscillates at 6 kHz at 298°K. The minimum detectable temperature variation is estimated to be +8.5 × 10-3°K. The power dissipation in the thermistor is quite low, being 28 μW at 273°K and 23 μW at 323°K. The proposed converter, except for the thermistor, can be fabricated in monolithic IC form.