An LVDT is an electromechanical transducer that converts linear mechanical displacement into a proportional AC voltage. It's the workhorse of industrial position measurement — found in hydraulic cylinders, turbine valve actuators, materials testing machines, and aerospace control surfaces — because it offers frictionless operation, infinite mechanical life, and sub-micrometer resolution.

The structure is three coils wound coaxially on a hollow bobbin: one primary in the center, and two secondaries wound symmetrically on either side, connected in series-opposition. A movable ferromagnetic core (the "armature") slides through the bore, attached to whatever you're measuring.

Drive the primary with a sine wave — typically 1–10 kHz at 3–15 V RMS. The core couples flux from the primary to each secondary. When the core sits centered, both secondaries see equal flux, their voltages cancel, and output is ideally zero (this is the null position). Move the core toward secondary A, and V_A grows while V_B shrinks. The differential output magnitude is proportional to displacement, and the phase (0° or 180° relative to the primary) tells you which direction.

Real-world example: A Schaevitz E100 LVDT has a ±2.5 mm range and sensitivity of 2.4 mV/V/mm. Excite it with 3 V RMS at 2.5 kHz, and at full stroke you get: 2.4 × 3 × 2.5 = 18 mV RMS output. Tiny, but the noise floor of a good synchronous demodulator is in the nanovolt range, giving you better than 0.1 µm resolution.

Signal conditioning is the trick. You can't just rectify the output, because you lose direction information and null becomes noisy. Instead, use a synchronous demodulator (phase-sensitive detector): multiply the secondary output by a square wave locked to the primary excitation, then low-pass filter. This gives you a clean bipolar DC voltage proportional to position, with full direction sensing and excellent noise rejection at all frequencies except the carrier. Chips like the AD698 or AD598 integrate excitation oscillator, demodulator, and output amplifier in one package.

Rule of thumb: Set excitation frequency to 10× your highest mechanical bandwidth. Measuring vibration at 200 Hz? Drive the primary at 2 kHz minimum. The carrier must be high enough that demodulation and filtering preserve your signal bandwidth — typically the output filter cutoff sits at about 1/10 the carrier frequency.

Watch out for: quadrature error (residual null voltage 90° out of phase from imperfect coil symmetry), temperature drift of coil resistance, and cable capacitance on long runs which can phase-shift the secondary signals enough to corrupt synchronous detection.