I am trying to develop a system for bioimpedance measurement.

I will use a frequency equivalent to 50Khz, using this project to support me: http://www.instructables.com/id/Body-Composition-using-BIA/?ALLSTEPS

The project is used an instrumentation amplifier, in my city, just found the INA111, but it is a very expensive component, so i decided to make my own instrumentation amplifier using three Op Amps (TL084CN). I'm using this article to support me: Design of an Instrumentation Amplifier - Justin Bauer

My doubt is: Since the AD5933 can only read positive voltages, a input of 2.5V is connected into the Vref terminal of instrumentation amplifier (INA118P). This is done to 'convert' the AC voltage to DC (I think).  

How do I do that? If I do not have terminal Vref on my "home Instrumentation Amplifier"??

Sorry my english is bad (i'm not American), and i'm starting with the electronics.

I am trying to develop a system for bioimpedance measurement.

I will use a frequency equivalent to 50KHz, using this project to support me.

The project uses an instrumentation amplifier. In my city I just found the INA111, but it is a very expensive component, so I decided to make my own instrumentation amplifier using three Op Amps (TL084CN).

I'm using this article to support me Design of an Instrumentation Amplifier - Justin Bauer.

My doubt is:

Since the AD5933 can only read positive voltages, an input of 2.5V is connected into the Vref terminal of instrumentation amplifier (INA118P). This is done to 'convert' the AC voltage to DC (I think). 

How do I do that? If I do not have terminal Vref on my "home Instrumentation Amplifier"?

Sorry, my English is bad (I'm not American), and I'm starting with electronics.