Electrical – Amplify voltage controlled current source in a instrumentation amplifier

current-sourceinstrumentation-amplifieroperational-amplifier

I need to apply a current in the body by two electrodes and then analyze this signal, I will receive the signal by other 2 electrodes and amplify using a instrumentation amplifier using three Op Amps (TL084CN).
But, I can not to simulate the offset by voltage controlled current source in a instrumentation amplifier.

I am using the topology described in the article "Design of an Instrumentation Amplifier – Justin Bauer", where the gain is 10.
Example:

With Vin = 500mV peak to peak, then Vout = 5V peak to peak. (Gain = 10).
Putting 5V DC on Vref, then Vout is oscillating between + 10V and 0V.
All right.

My problem is:
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.
enter image description here
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.
I am using Proteus 8.5.

What am I doing wrong?

Sorry my poor english, i don't speak english.

Bibliography:

http://www.egr.msu.edu/classes/ece480/capstone/spring15/group05/uploads/4/7/5/1/47515639/ece_480_app_note_justin_bauer.pdf

https://www.researchgate.net/publication/258327515_High_Accurate_Howland_Current_Source_Output_Constraints_Analysis

Best Answer

I'm not modeling any resistance to the body. I'm connecting the current source outputs directly to the instrumentation amplifier inputs.

That looks like the problem - you are trying to inject a mA into a giga ohm (the input impedance of the InAmp) and not surprisingly the current driver cannot force 1 mA into 1 Gohm. It would be able to if the power rails were +/- 1 million volts but they aren't and cannot be.

Try putting 100 ohm across the output of the two drivers. This then forces a voltage of 100 mV and, with a gain of ten you will get 1V out. Your expectations of 20V out are unreasonable given that the InAmp power rail is 12 volt.

One more thing - you will need a galvanic connection between the driver circuit 0 V and the InAmp 0 V. The human body (a real one) might facilitate this via other "connections" but for your simulation you'll need one.

Try 100 kohm.