Minimizing 1/ f Noise in Magnetic Sensors using an AC MEMS Flux Concentrator
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Minimizing 1/f Noise in Magnetic Sensors using an AC MEMS Flux Concentrator
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A.S. Edelstein, G.A. Fischer, M. Pedersen, E.R. Nowak, Shu Fan Cheng, and C.A. Nordman5
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1. U.S. Army Research Lab, Adelphi, MD, USA 2. MEMS Exchange, Reston, VA, USA 3. Physics Dept., University of Delaware, USA 4. Naval Research Lab, Washington, DC, USA 5. NVE Corp., Eden Prairie, MN ABSTRACT We are developing a device, the MEMS flux concentrator, that will greatly decrease the effect of 1/f noise in magnetic sensors. It does this by modulating the incoming signal and thus shifting the operating frequency of the sensor. This is accomplished by placing flux concentrators on MEMS structures that oscillate at kHz frequencies. Depending upon the sensor, shifting the operating frequency reduces the 1/f noise by one to three orders of magnitude at one Hz. We have succeeded in fabricating the necessary MEMS structures and observing the desired kHz normal mode resonant frequency. Only microwatts are required to drive the motion. We have used spin valves for our magnetic sensors. The measured field enhancement provided by the flux concentrators agrees to within 3% with the value estimated from finite element calculations. Noise measurements provide strong evidence that the device is likely to reduce the effect of 1/f noise. Flip chip bonding is likely to allow us to fabricate complete, fully functioning sensors. INTRODUCTION Low frequency or 1/f noise is found in such diverse places[1] as electronic devices, the stock market, emissions of quasars, highway traffic, the global temperature, and the flow of the river Nile. Magnetic materials[2, 3] also exhibit 1/f noise and 1/f noise limits the performance of magnetic sensors at low frequency. The 1/f noise in magnetic sensors can be either electronic or magnetic[4]. In addition, magnetic sensors also have Johnson, shot and magnetic white noise[4]. White-noise magnetization
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fluctuations in magnetoresistive heads are a fundamental limit on their signal-to-noise ratio[5]. Table I lists the type of noise sources that are found in some of the major types of magnetoresistive sensors. Table I. Select magnetoresistive materials and their corresponding low frequency noise sources delineated by their spectral dependence. 1/f noise white Anisotropic Magnetoresistance Giant Magnetoresistance Tunneling Magnetoresistance
Johnson1 √ √ √
Shot2 √ √
Magnetic3 √ √ √
Electronic4 √ √ √
Magnetic √ √ √
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sets ultimate noise floor may set ultimate noise floor, depending on current bias 3 may set ultimate noise floor, depending on sample volume and magnetic properties 4 charge trapping or defect motion, depending on materials system 2
Magnetoresistive sensor technology is one of the most promising routes for producing low cost magnetic sensors. The reasons for this is that magnetoresistive sensors can be fabricated by batch processing and the drive and read out electronics are relatively simple. The earliest type of magnetoresistance sensor was the anisotropic magnetor
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