Wow. Great explanation! Thanks to you, that's the track I was attempting to go down (I had a very similar LTSpice going), but I was confused about the reference. Now I am much further, and I can do some more tweaking on my own as I do parts selection. Thank you!
If that pic was what clued you in about the reference, then it was a misleading clue.
Actually it was probably misleading all around considering where I placed the voltage source. The pic was means to illustrate only the concept of the 3-resistor divider, and what would happen if 1,000V were applied to it.
Let me start over here...
In the pic I placed the GND on one of the signal wires to the Instrumentation Amp.
I did that because LTSpice forces you to put a GND somewhere in the circuit, and depending where you put it, affects the simulated waveforms.
I was lazy and put the GND on that wire because I wanted to see the waveform from top and bottom of the middle resistor.
I should have put the GND at the bottom where it goes, and set up a 2nd reference point.
Here's what it should look like:
The Instrumentation Amp circuit can be powered by an isolated (NO reference) source like you already have:
it can be referenced to "Earth"
or it can be referenced to "neutral":
it doesn't matter where or if you tie it in, because the ins. amp amplifies only the
difference between its two inputs.
Hopefully I clarified my clarification and didn't just make it worse.
(sorry
)