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Titel: High-Precision Mixed-Signal Sensor Interface for a Wide Temperature - Range [0°-300°C]
Keywords: Sensor interface, time domain signal processing, high precision
The feasibility of analog integrated circuits with an operating temperature range above 200°C has been proven for different kinds of integrated circuits [1], [2]. However, high-precision sensor interfaces for Wheatstone bridge based pressure sensors in high temperature (up to 300°C) environments still impose a hard challenge to the designer. Especially in these harsh conditions, small input voltages less than 10mV, amplifier offsets in the same order of magnitude, temperature dependent offset drift, and drifting of external bridge resistor values contradict the goal of accuracy in a range below 5% full-scale. We present a novel concept to circumvent these severe issues without imposing the need for advanced calibration steps. By combining an analog operational amplifier based input stage, a pulse-width modulator (PWM) and an elegant switching scheme, the temperature dependent quantities can be eliminated from the measurement signal. The switching scheme realizes different measurements (like measuring bridge signal, offset, temperature of the die, etc.) being encoded in the PWM signal: After a synchronization step, the actual input signal is observed. We prove that two additional observations are sufficient to correct offset and gain errors of the ASIC input stage as well as the inherent drift of the reference voltage. The output is a PWM like single logic signal carrying the information encoded in edge timings that can be extracted by simple time measurements. This signal can be transferred over large distances to a signal processing unit in a low temperature environment. A further analog-to-digital conversion or further analog signal conditioning is not required. Our system uses a simple correction formula to extract the required sensor value from the signal. This equation can be executed e.g. by a standard microcontroller in the low temperature part of the overall system. Since we observe the actual values of temperature dependent quantities in our switching scheme, calibration tables or interpolation methods are not required. The sensor interface has been prototyped in a commercially available 1.0µ partially depleted SOI technology XI10 by X-FAB Semiconductor Foundries (die size 5x2.2mm²) [3]. This realization is currently capable of executing 10-20 measurements per second. The characterization of the sensor system is done using a special temperature-controlled setup and an emulated Wheatstone bridge. A PT1000 temperature sensor is used to ensure a high resolution for the temperature characteristics. While measuring the performance of the sensor interface, the system is slowly cooled down from 300°C. This results in a fine-grained temperature resolution of the regarded quantities. The results show the measurements of the bridge signal being accurate within 1% full-scale precision over the full temperature range. The details of the new sensor concept and the characterization results will be presented at the conference.
Georg Gläser,
Institut für Mikroelektronik- und Mechatronik-Systeme gemeinnützige GmbH (IMMS GmbH)
Ilmenau, Thuringia

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