A ratio metric analog-to-digital converter for eddy current displacement sensors

Abstract

Displacement sensors are widely employed in industry for measuring position and movement of objects as well as for measuring other physical quantities like pressure, acceleration, etc., which can first be converted into movement. This thesis describes the implementation of a ratio metric analog-to-digital converter (ADC) for an eddy current displacement sensor interface. The interface which has been implemented by Dr. Nabavi as a PhD project, consists of a low-power front-end oscillator and a synchronous demodulator. An excitation frequency of 20 MHz enables the accurate sensing of targets with only a few tens of ?m thick conductive surfaces. The ADC sampling frequency is 1.25 MHz, which provides an over-sampling ratio (OSR) of 312. The sampling frequency is obtained from the excitation frequency, which synchronizes the ADC and the interface. A ratio-metric measurement approach is applied to suppress the oscillator's noise contribution. The ratio-metric ADC has been realized in a 0.35?m BiCMOS technology and consumes 1.8mW. Measurement results demonstrate a total harmonic distortion of about -85dB, and a resolution of 16 bits within 1 kHz signal bandwidth.