Ultra-low-noise readout circuits for magnetoresistive sensors

Abstract

The continuous search for highly sensitive, agile and cost-effective sensors for magnetic biosensing applications has been met with high performance magnetoresistive (MR) sensors. While the MR effect has been discovered 150 years ago, there is a growing trend of improving the sensitivity of MR sensors while keeping their noise performance as low as possible. However, such improvements have to be complemented with high performance frontends that can effectively amplify the minute MR sensor's signals while keeping the system's noise floor unaltered. More importantly, the designed frontends have to be equipped with offset compensation peripheral circuits that can efficiently handle the large spread of the base resistance in MR sensors with high MR ratios such as in tunnel magnetoresistive (TMR) sensors. In this thesis, we developed multiple frontend electronics that successfully interfaced MR sensors while, simultaneously, achieving competitive noise performance compared to state-of-the-art (SoA) designs tailored for MR sensor readout. The first variant of chips are specically designed for high performance and high linearity designs thanks to a novel implementation of an ultra-low-noise current bias achieving SoA current noise floor of 2.2 pA/sqrt(Hz) and chopped voltage-mode amplification stages resulting in a total voltage noise floor of 8 nV/sqrt(Hz), including a TMR sensor and a reference resistor with base resistance of 1 kOhm. In order to integrate an analog-to-digital converter (ADC) without substantial additional power and/or area, we show in this work a continuous-time current-mode Sigma-Delta modulator (CT C-SDM) that can directly interface MR sensors without additional amplifiers. Our proposed design does not only show a competitive noise floor of 8.1 pA/sqrt(Hz), but also features a novel DC servo loop (DSL) around the modulator that maximizes the useful dynamic range (DR) of the modulator while successfully rejecting the undesired DC offsets of MR sensors. Both design variants shown in this thesis, pave the way to designing high performance point-of-care (PoC) systems for in-vitro diagnostics while keeping their costs low compared to alternative bulky and expensive systems.