When initially introduced fifty years ago, all LVDT linear position sensors were AC-operated and required external oscillators, carrier amplifiers, demodulators and filers to operate.

The introduction of high-density microelectronics enabled the incorporation of signal conditioning and processing functions inside the LVDT housing rather than requiring an external box. The DC-operated LVDT maintains all the desirable characteristics of the AC-operated LVDT, but has the simplicity of DC operation.  It is comprised of an AC-operated LVDT and a carrier generator/signal conditioning module.

So why haven't DC-operated linear position sensors replaced the original AC-operated versions? 

Read the entire article to find out.