The schematic seems to be incorrect.
Pins 2,7 are guard drivers IIRC so you are not supposed to connect them to any input, I hope it's a mistake.
It seems to be unity gain, is it how it's supposed to be?
The input supposedly goes below ground. The input of the opamp may or may not work (see specs), but the output certainly won't go below the negative supply.
You don't need to match resistances seen by the inputs; bias current is too low to matter with such low values of resistance.
Hi!
* pin 2,7 are guards, and are driven on purpose (per spec: A "Guard" is a driven trace or shield that physically surrounds the input trace and feedback circuitry that is held at
a potential equal to the average input signal potential)
* Yes, I configured it as unity gain. I suppose ONE of the probes could have been configured otherwise, but the ORP probe will swing from just above ground to ~4V.
* The input is always above ground, as the reference for the probes makes it so (for PH probe, it +0.5V, for ORP, + 2.048).
I hope this additional info clarifies things a little.