A temperature difference-to-frequency converter using resistance temperature detectors

A novel temperature difference-to-frequency (TD/F) converter using two platinum resistance temperature detectors (RTDs) has been developed. The resistance difference of two RTDs is converted into a DC current to control the simulated inductor, which is designed so that its inductance is inversely proportional to the square of the current. The simulated inductor forms the resonant circuit of a Colpitts oscillator and hence the oscillation frequency becomes directly proportional to the resistance difference. A conversion sensitivity of 16 Hz/ degrees C and a residual nonlinearity of less than 2% over the temperature difference range of 35 degrees C to 155 degrees C are obtained by the prototype converter which oscillates at 667 Hz at a 51 degrees C temperature difference. The minimum detectable temperature difference is estimated to be +or-0.013 degrees C. The power dissipations in the RTDs are about 2 mu W. The proposed converter, except for the RTDs, can be fabricated in a monolithic IC form by scaling the component values.