Input Impedance Question

Nothing wrong about the spec as far as I can see - it sounds like you want it couched in terms that are meaningful to your limited knowledge and application requirements.

• 1 Mohm minimum input impedance is fine - it's 1 Mohm guaranteed or greater - most folk want high input impedances so a guarantee is a good thing.
• Common mode input voltage range applies to differential amplifiers and it basically means, if you want to measure a small AC signal (say 100 mVp-p) it will do so even if both input wires are raised by a potentially interfering voltage of +/- 10V. Good spec I reckon.
• Common mode rejection - this tells you how good the input amplifier is when it is connected to a source voltage with different resistance connections. 70 dB at 60 kHz is pretty good for this sort of thing - you have got to envisage that some folk may not be able to adequately control the impedances of their signals that are to be measured by this device.
• Input imbalance adjustment - no amplifier is perfect and most will produce a small dc error at their output - this can be nulled-out by the adjuster.
• Linear output is telling you that it won't produce significant distortion (maybe due to clipping) for output peaks up to 10V. I also read this as meaning that the nominal rated full scale output level is +/-10V.
• Corner frequency is the point where the output starts to naturally fall when the input frequency gets above a certain value and for the default amp you are using this is 10kHz. It's adjustable.
• Cross talk of 80 dB RTI - referred to input is what RTI means. Cross talk applies to amplifiers with multiple channels and interference between one channel's signal and another channel.
• Gain resolution - if you want to adjust the gain there is a finite limit to how exact you may want it and this is the resolution.

Your hyperphysics link is correct, and so is your conclusion about the input impedance of the voltage follower. The math follows from the basic control system diagram. You can see a nice presentation on it here:

http://slideplayer.com/slide/1496732/