Hello,

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:
http://www.egr.msu.edu/classes/ece4.../1/47515639/ece_480_app_note_justin_bauer.pdf

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.

 

The ground terminal for R2 in Figure 5 of your reference circuit (see below) is the same as the Vref terminal of the INA118.
Remove R2 from ground and apply the Vref voltage there.
That will offset the output voltage by the value of Vref.

Where can you buy components where you live?

 

The ground terminal for R2 in Figure 5 of your reference circuit (see below) is the same as the Vref terminal of the INA118.
Remove R2 from ground and apply the Vref voltage there.
That will offset the output voltage by the value of Vref.

Where can you buy components where you live?

View attachment 114414

There are some parts stores, but only found the INA111.
Thanks for the answer,
I'm trying to simulate in Proteus, and I can not view the offset in the virtual oscilloscope, but I think it is a software error.

 

Below is an LTspice simulation of the circuit with a Ref input of 1v to 5V.

 

Below is an LTspice simulation of the circuit with a Ref input of 1v to 5V.

View attachment 114450

I'm using a current source (Mirrored Modified Howland Current Source (MMHCS)) where +Iout = 1mA and -Iout = -1mA.
With a initial voltage source where amplitude = 2V and the frequency = 1kHz.
I can not to simulate the offset.
Vout is always between + 10.5V and -10.5V.

I tried to apply in Vref 10V, 5V, 2V, but nothing works. The Vout still varies from -10.5V to + 10.5V. The Vref only distorts the voltage signal. I want the Vout varies between (say) + 20V and 0V, it will use the AD converter Arduino Uno, that just read positive signs.

Source:
https://www.researchgate.net/public...nd_Current_Source_Output_Constraints_Analysis

 

Sounds like you need to reduce the gain of the instrumentation amp.