Hi Lefteris,
Sorry for the late reply. Your thread got buried in the forum. I will look into your questions a bit more, but for the time being, is the ultimate goal of your study to measure the impedance of the PZT? Because if that is the case, we may have an single chip solution that might be handy for you, the AD5933. The part is an impedance converter, and it looks like all the main components of your system should already be included here. Have a look at it and let me know if it suits your application. I will get back to you regarding your questions shortly.
Thanks!
Kris