There are a few posts about the Spartan controller and the issue with output impedance, someone had an issue and built a low pass filter to condition the signal. The wiki also doesn't state anything about the issue with the Spartan controller, I'll add something. Though from how quiet this thread is nowadays it seems people have moved on?
In the end it was a post I found from Venderbroeck saying that the tank pressure input had the best result for the Spartan controller as the non-linearity was non existent on that channel. Yeah I knew A19 had an offset (0.7V if I recall) so I was using the accelerometer ground as the connector was already wired in and my 850 doesn't have the accelerometer, then I tried A18, then I swapped the pin to the tank pressure input which worked.
Yes the accelerometer channel does work for logging, just needs some code adding to the binary. I've also got the barometric sensor channel working as some code is needed to switch the channel