Characterization of the thermal transients of an Ultra Low Power micromachined sensor

This paper describes an embedded system for the simultaneous dynamic control and thermal characterization of the heating phase of an Ultra Low Power (ULP) micromachined sensor, featuring thermal characteristics quite similar to those of innovative ULP semiconducting metal oxide (MOX) sensors. A Pulse Width Modulated (PWM) powering system has been realized using a microcontroller featuring an ARM7 core to characterize the thermal behavior of a device formed by a Pt microheater and a Pt temperature sensor, over an insulating membrane. Two operating modes, namely constant target heater resistance and constant heating power, were implemented. The aim was to analyze the relation between heating period and operating temperature, to observe the thermal time constants of the device and the total thermal conductance. Repeatability of experimental results was assessed by guaranteeing the standard deviation of the controlled temperature was within +/- 5.5 degrees C in worst case conditions. Experimental results show quantitatively a unique time constant tau both for the heater and the T-sensor, that changes dependently on the temperature rise Delta T between the ambient and the operating temperature in a range from 2 ms to 2.4 ms. The dependence of the operating temperature of such ULP micromachined systems on the frequency and duty cycle of the PWM signal was also characterized and guidelines for minimizing the temperature ripple were defined. Finally, we observed that in the chosen operating temperature range the thermal conductance is a linear function of the heating power.