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

This article presents a low-noise high-bandwidth giant magnetoresistance (GMR) sensor-readout circuit for nanometer resolution position sensing, a critical requirement in several precision applications such as atomic force microscopy. The main impediment to performance is the GMR sensor’s significantly higher 1/
$f$
noise extending over large bandwidths, thereby limiting 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. The noise characteristics of the GMR sensor and its readout circuit components, and their effect on SNR, are investigated analytically and experimentally. A general process for designing the 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 atomic force microscope (AFM) nanopositioner, a ten-fold improvement over previously reported GMR sensor performance and comparable to the state-of-the-art, but more complex, capacitive sensors. The readout scheme is simple to implement using commercially available off-the-shelf (COTS) components and easily extendable to even higher bandwidths, unlike other schemes, such as modulation/demodulation, where the requirement for increasingly higher carrier frequencies renders further improvements in bandwidth impractical.