A Coupled Inductive Bridge for Magnetic Sensing Applications

Abstract

A highly-sensitive magnetic sensor with excellent long-term stability is presented. We modify a conventional all-inductor AC Wheatstone Bridge by coupling two inductor pairs in a cross-coupled configuration which halves its size and doubles its sensitivity, while maintaining a fully differential output that reduces common-mode induced offset and drift. The sensor was fabricated with integrated excitation and receiver circuitry in a 65nm bulk CMOS process. It operates between 770MHz and 1.45GHz, has an effective sensing area of 200µm × 200µm, and reliably and continuously detects single 4.5µm magnetic label beads without significant drift over time periods notably longer than previously reported works. To our best knowledge, this is the first demonstration of a magnetic sensor using a fully symmetric, gain enhanced, and all-inductor coupled bridge circuit.