Low-power MEMS pressure sensor for wireless biomedical applications

Abstract

Reducing power consumption leads to improve wireless sensor autonomy, increase battery life, and reduce radiated power. State-of-the-art blood pressure sensors based on piezoresistive transducers in a full Wheatstone bridge configura tion uses low ohmic values because high sensitivity and low noise approach. In this work, the piezoresistance values are increased in order to reduce one order of magnitude the power consumption.The noise introduced by this improvement was proved that does not limit the accuracy for 8-bit applications. Therefore, a low power consumption pressure sensor with high sensitivity and low noise is proposed. Power consumption versus sensitivity tradeoff is analyzed in detail. I. IN T RODUCTIONWireless blood pressure sensors are especially suitable for surgery rooms, intensive care or post-anesthetic recovery units, even small laboratory animals. Low power consump tion is a critical point to improve the distance between the wireless sensor and monitoring equipments and improve the battery life of the sensors, reducing at the same time the power radiated to establish the wireless communication. State of-the-art blood pressure transducers, based on four resis tors in a full Wheatstone bridge configuration, are usually optimized for sensitivity [1]-[3] and linearity. Temperature effect on sensitivity in silicon piezoresistive transducer has been studied in detail in [3]- [5]. Analysis of the noise in piezoresistive transducer has been presented in [5]. Most of the piezoresisitive transducer are ion-implanted into a thin silicon monocrystalline membrane. Ty pical values are in the range between 100 fl and 3 kfl, powered between 3 V and 5 V. This means a power consumption between 3 m W and 250 m W, typically 20 mW, only for the full Wheatstone bridge without the required signal conditioning circuit -a signal conditioning circuit with at least one operational amplifier is required.In this work, the piezoresistance ohmic values are increased in order to reduce the power consumption. It was proved that the noise introduced by this improvement does not limit the accuracy for 8-bit applications. The achieved power con sumption results are below 322 /-l W, including both the full Wheatstone bridge (62/-lW) and the signal conditioning sys tem, for a mixed signal technology of 1.0 /-lm CMOS process, including integrated sensors -XC 10 technology process from XFAB. This work proposes optimal tradeoff design points for piezoreslstlve transducer and sensor in terms of sensitivity, power consumption and noise. This paper is organized as follows. In Section II mechanical behavior of a diaphragm -pressure sensor-is analyzed in detail. In addition, the layout dimensions and positions of several piezoresistances -250, 500 and 1000 kfl-are calculated. The analysis of both sensitivity and power consumption of a full Wheatstone bridge and differential amplifier circuit to achieve optimum power and sensitivity design points is presented in Section m. Finally, conclusions are presented in Section IV. II. ELECTROMECHAN...

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