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In the last decade, wireless sensor networks (WSN) have gained considerable importance and momentum. They are used with a growing diversity of sensors in a plethora of applications, e.g., from Ambient Intelligence (AmI), Assisted Living (AAL) to agriculture. WSN based measurement and instrumentation systems commonly have to operate under tight power consumption constraints. In particular, WSN benefiting from micro-electro-mechanical-system (MEMS) miniaturization have a very limited power budget due to poor energy density of storage elements. While highly optimized microcontrollers are available by now, today's sensors and sensor electronics are the predominant cause of power consumption in WSN. The standard approach tries to minimize standby currents by low-power sensor, bridge, and amplifier design. In contrast, this paper investigates an approach of duty-cycled, reconfigurable sensor electronics for resistive bridge sensors, that seamlessly integrates into microcontroller' sleep modes. The circuit is optimized for the particular case of anisotropic magneto-resistive (AMR) sensor based localization in autonomous WSN. In a systematic analysis amplifier currents, slew-rate, and read-out or on-time Ton are optimized for minimum power consumption of a tri-axial AMR sensor. At the required read-out rates, e.g., one measurement per minute, energy consumption can be reduced to a factor 1.5·10-6 of the continuous operation. A reconfigurable chip in a standard 0.35 μm bulk CMOS technology is under preparation adding flexibility for different sensor types and calibration needs.