A Dual Slope LVDT-to-Digital Converter

This paper presents a simple, new LVDT-to-digital converter that does not require a precise sine-wave excitation or a phase-matching circuit, like the traditional methods. The scheme relies on a triangular current excitation to the primary coil, derived from a couple of dc reference voltages. A dual-slope conversion of the secondary voltages in a ratiometric manner yields a digital output that is automatically de-sensitized to extraneous variations and directly proportional to the measurand (displacement). A detailed analysis of the effect of circuit parameters on the performance of the scheme is also presented. A prototype of the proposed circuit built and tested in the laboratory gave an effective number of bits of up to 12.7 bits and a speed of 10 conversions/s. The non-linearity error noted from the tests conducted was less than 0.15%.