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LINEAR_READOUT_CIRCUIT

2017-03-06 05:01  
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LINEAR_READOUT_CIRCUIT
LINEAR_READOUT_CIRCUIT
Thermistors have found their way into a number of temperature-sensing applications. However, their resistance is a nonlinear function of temperature. In many applications, a digital readout of the thermistor also is desired. It is possible to produce a digital signal with a frequency that is linearly proportional to the temperature. The entire active portion of the circuitry can be implemented using 3/4 of an LM339. Comparator A is configured to operate as an op amp that generates voltage (Vt) which is inversely proportional to the thermistor's resistance, Rt. The voltage Vt is used as the thresh-old for the relation oscillator formed by comparators B and C. Comparator B monitors the voltage on the integrating capacitor (Vc). When it crosses Vt, it triggers the one-shot formed by comparator C via the signal Vtrig The output of comparator C resets the integrating capacitor for a duration, to, where to= (R2+3 kΩ)C2. Then the cycle begins again. The circuit achieves linear readout because the exponential nonlinearity of the thermistor is compensated by the exponential decay of the voltage in the relaxation oscillator.