Electronic – A confusion on amplifiers

You cannot just connect the probe across inputs to the inamp and expect good results.

Operational amplifiers need a path to ground from each input. So the second circuit is better.

the 741 is totally unsuitable, but the LMC6041 in the notes should work well for this circuit. (2 fA is typical rating though, use the guaranteed value of 4 pA for design unless you want your circuit to only mostly work... 4pA through 300Mohm sensor is 1.6mV offset.)

LMC6041 will be happy with a single supply, so if the -5V supply is inconvenient have a look at this circuit: What is the purpose of this op amp?

I suggest you stop simulating and actually build the circuit. LMC6041 is $2 from a distributor or ask National, they'll give free samples for school projects. You don't need to hook up the ADC yet, just get the probe and signal conditioning amplifier working. Read the output with a voltmeter. Just like software, breaking the problem into small pieces is the way to make progress.

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.