Achieving Low-Noise, High-Bandwidth, Nanoscale-Resolution Position Sensing with GMR Sensors

Abstract

<p> Nanometer-resolution position sensing is a critical requirement in several precision applications such as Atomic Force Microscopy (AFM), especially over large bandwidths. Giant magnetoresistance-based (GMR) sen?sors have shown great promise for this; however, they exhibit significantly higher 1/f noise even over large band?widths, which limits the achievable SNR. Therefore, the sensing system needs to be designed at the system-level in order to minimize excess intrinsic noise generation and transmission, as well as external noise pickup. This article, both analytically and experimentally, investigates the noise characteristics of the GMR sensor and its readout circuit components, and their effect on SNR. A general process for designing a low-noise high-bandwidth readout circuit, applicable not just to GMR sensors but also to other high?1/f-noise sensors, is presented. A resolution of 2.5 nm over a bandwidth of 100 kHz is demonstrated on an AFM nanopositioner, a ten-fold improvement over previously reported performance and comparable to other state-of?the-art, but more complex, position sensing schemes. The readout scheme is simple to implement using COTS com?ponents and easily extendable to higher bandwidths, unlike other schemes, such as modulation/demodulation, where the requirement for increasingly higher carrier frequencies renders further improvements in bandwidth impractical. </p>