Checking LVDT Demodulation Phase Around Mechanical Null

An LVDT interface must recover both displacement magnitude and direction from its transformer signals. Near mechanical null, the secondary signal can become small, making phase interpretation and offset especially important. A circuit that gives a plausible output at one end of travel can still behave incorrectly around the center.

Confirm the transducer connection

Identify the primary, secondary windings, winding polarity, and supported connection arrangement from the LVDT documentation. Record excitation frequency and amplitude at the transducer, not only at the driver output. Include cable effects when the transducer is remote.

The Analog Devices CN0301 LVDT interface note describes synchronous demodulation and explains that primary to secondary phase shift may require compensation in the illustrated AD698 arrangement. The correct treatment depends on the transducer and interface topology.

Map position to raw signals

Move the core through several known positions on both sides of null. Observe primary and relevant secondary signals with a common time reference. Record amplitude and phase behavior before looking only at the final DC output.

Label the physical direction corresponding to increasing output. Swapping a winding connection can reverse that relationship while leaving the signal amplitude apparently reasonable. Resolve the intended polarity in the electrical design and documentation rather than relying on an unexplained software sign change.

Examine the small signal region

Take closer position steps around null and inspect repeatability, offset, noise, and continuity of the output. Check whether the demodulation reference remains valid as the measured secondary amplitude approaches its minimum. Follow the selected conditioner’s guidance for reference selection and phase compensation.

Separate an electrical offset from a mechanical zero reference. A fixture’s nominal center position may not coincide exactly with the transducer’s electrical null. Record how the product defines zero and how that point is established during calibration. Retain the fixture reference and approach direction when repeating this check to expose mechanical hysteresis.

Verify the complete motion range

Check the intended travel, temperature, excitation tolerance, and relevant motion rate. Filtering that produces a quiet static reading may introduce delay during movement. Compare dynamic position response with the application’s required timing, not only its steady accuracy.

Save transducer wiring, excitation conditions, phase measurements, compensation components, and position calibration data. Revisit them when the LVDT or cable changes. A useful validation shows a continuous, correctly directed, repeatable position output through null and across the required range; agreement at two endpoints alone can conceal the most sensitive region of the interface.