A Novel Single-Element Inductance-to-Digital Converter with Automatic Offset Eliminator

A novel Inductance-to-Digital Converter (LDC) that outputs a digital value corresponding to the change in inductance (
$\Delta L$
) of a single element inductive sensor is presented. In most cases, the offset inductance
$L_{0}$
offered by the sensor will be large as compared to
$\Delta L$
(due to the measurand). Thus, the existing architectures that measure the sensor inductance
$L_{X}(= L_{0}\pm\Delta L)$
lead to underutilization of the output range and hence such systems cannot focus on sensing the measurand alone. The proposed LDC automatically eliminates the offset inductance in the final output and utilizes the entire digital scale only for representing
$\Delta L$
. Also, the digital output of the proposed LDC is made independent of the coil resistance
$R_{C}$
of the inductive sensor. The proposed LDC employs DC source for digitization, thereby eliminates the need for AC excitation sources that demands repeatable sinusoidal amplitude for high accuracy measurements. The direct digital conversion is performed by modifying the dual-slope conversion technique. A prototype of the LDC was developed and verified the automatic offset elimination operation of the proposed converter. The prototype exhibited a worst-case linearity error of 0.5% for
$\Delta L$
ranging from −9 mH to +9 mH.