"Open Wiring" vs. Ratiometric LVDTs

So open wiring consists of connecting the output windings in series opposing? What is the advantage of ratiometric operation? I can imagine that for large displacement the sum of A and B might vary. Does the ratiometric connection extend the linear range?

(I have never gotten good performance at low displacements from an LVDT without using a synchronous detector to eliminate unwanted quadrature that creates a soft null. Quadrature injection is just too touchy.

Jerry

Ratiometric leaves your measurement insensitive to changes in excitation such as may happen with long cable runs and/or temperature variations.

Synchronous detection is definitely the way to go (and it's aided by ratiometric detection -- you use the phase of the sum to time the synchronous detection, thereby rendering your application insensitive to phase shift within the detector or on long cable runs).

If you check the Analog Devices web site for LVDT signal conditioners you'll find white papers extolling the virtues of ratiometric detection.

I did that by using the open wire connection (with synchronous detector) and dividing the signal by the excitation voltage. (There's no need to actually divide in analog implementations. Using a controlable-gain amplifier -- an OTA, for example -- simply normalize the excitation reading before reading the displacement. Maybe that will work for you?

Jerry