try to find something about project bellow:
PSoC Today: Impedance Meter information and project
Here is the project. Its a couple of years old so will need updating
by Creator. Consider contacting the author.
The project uses sync detection and discusses that.
And my 2ct. : There was a project called "Synchronous Detection" which was very sophisticated.
Have a look here: secure.cypress.com/search.cfm there are some videos about.