A 300°C, SOI transimpedance amplifier with application to capacitive temperature sensing

This paper reports a high-temperature, wide gain-bandwidth SOI-CMOS transimpedance (R_m) amplifier that is well suited for application to MEMS-based impedance sensors. The amplifier was fabricated using a fully depleted 0.5-μm technology and achieves a gain-bandwidth of 8 MΩ by 1.2 MHz at room temperature while drawing 0.66 mW from a 3.3-V supply. Gain and bandwidth remain above 2 MΩ and 0.3 MHz, respectively, at all temperatures up to 300^oC, while power consumption remains under 1 mW. Measured input-referred noise current is just 3.0 pA/√Hz at 100 kHz; consistent with simulation. The amplifier requires one off-chip capacitor but otherwise employs only CMOS transistors; die area is 8500 μm². A novel temperature sensor was constructed using a MEMS-based capacitive strain sensor, which was glued to a 2-mil thick stainless steel (17-4-PH) base using high-temperature epoxy. The mismatch in coefficient of thermal expansion (CTE) between the two materials (~8.7 ppm/°C) generates a sensor displacement when temperature is varied, which results in a capacitance change. An FEA model predicted that the structure can deliver a sensitivity of 10.7 μ-strain/^oC, which produces a -1.12 fF/^oC capacitance change in the strain sensor. The transimpedance amplifier was packaged in the same DIP as the capacitive temperature sensor; and this active sensor was stimulated using a 100 kHz, differential sinusoid and heated to temperatures as high as 250 °C. The amplified sensor output was measured using a spectrum analyzer, and the differential capacitance of the sensor was found to be +0.28 pF at room temperature, with a temperature coefficient of approximately -1 fF/^oC, in good agreement with the predicted model.