An eddy-current displacement-to-digital converter based on a ratio-metric delta-sigma ADC

This paper describes a smart Eddy-current displacement sensor for use in precision industrial applications. A novel readout scheme based on ratio-metric delta-sigma analog-to-digital conversion is proposed. The system employs two sensing coils incorporated in a low-power front-end oscillator. This produces two anti-phase outputs whose amplitudes are proportional to the inductances of the coils, and are thus a differential function of displacement. After synchronous down-conversion, these signals are fed into a second-order continuous-time delta-sigma modulator that directly produces a digital output that is a ratio-metric function of the coil inductances. This approach eliminates the need for a stable voltage reference, suppresses the oscillator's multiplicative noise contributions, and effectively filters the ripple associated with the down-conversion. The sensors are excited at 15 MHz, which reduces the eddy-current penetration depth to only a few tens of μm. The interface has been realized in a 0.35 μm BiCMOS technology and consumes 18 mW from a 3.3 V supply. In a measurement time of 1 ms, it digitizes the inductance ratio with a resolution of 15 bits, and thus achieves a displacement resolution of 135 nm on a range of 3 mm.