# Amplify voltage controlled current source in a instrumentation 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. 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:

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

• what resistance are you modelling for the body. Commented Oct 30, 2016 at 17:43
• I think you don't need two current sources, just one will do. Electrode 2 should be connected to ground. Also, the human body is an antenna for RF signals, so you should have input filters on Electrodes 3 and 4. There could also be DC offsets caused by differing skin-electrode electrolytic potentials, so I would AC-couple the amplifier inputs (unless you want to measure these DC offsets too!). Commented Oct 30, 2016 at 17:45
• The amplifier's gain-setting resistor, R7, will probably need to be adjusted in real-world use, to keep the output signal within the range of your power supplies. This is usually done by replacing R7 with a digipot (digital potentiometer). The digipot is controlled by your computer device (Arduino, etc.). Commented Oct 30, 2016 at 17:50
• I'm not modeling any resistance to the body. I'm connecting the current source outputs directly to the instrumentation amplifier inputs. As if the body had no resistance. Commented Oct 30, 2016 at 19:39
• That makes no sense. Those inputs have giaohm impedences Commented Oct 30, 2016 at 22:29