A 200-μW Interface for High-Resolution Eddy-Current Displacement Sensors
Matheus Pimenta (Infineon Technologies)
Cagri Gürleyük (TU Delft - Electronic Instrumentation)
Paul Walsh (Infineon Technologies)
Daniel O’Keeffe (Infineon Technologies)
Masoud Babaie (TU Delft - Electronics)
K. A.A. Makinwa (TU Delft - Microelectronics)
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Abstract
This article presents a low-power eddy-current sensor interface for touch applications. It is based on a bang-bang digital phase-locked loop (DPLL) that converts the displacement of a metal target into digital information. The PLL consists of a digitally controlled oscillator (DCO) built around a sensing coil and a capacitive DAC, a comparator-based bang-bang phase/frequency detector (PFD), and a digital loop filter (DLF). The PLL locks the DCO to a reference frequency, making its digital input a direct representation of the sensing coil inductance. To compensate for the coil inductance tolerances, the DCO's center frequency can be trimmed by a second capacitive DAC. This approach obviates the need for a reference coil. When combined with a 5-mm-diameter sensing coil located 500μm from a metal target, the interface achieves a displacement resolution of 6.7 nm (rms) in a 3-kHz bandwidth. It consumes 200μW from a 1.8-V power supply, which represents the best-reported tradeoff between power consumption, bandwidth, and resolution.