If the pH sensor only has two wires which I will connect to INP and VCOM what do I connect the guard to? There is not a third conductor in the pH sensor cable.
Lauren -
The guard ring needs to be connected to the guard pins (GUARD1 and GUARD2), which are at the same potential as that of the INP pin. Solder mask should not cover the input and the guard area, including guard traces on either side of the PCB.
The LMP91200 datasheet shows this in Figure 38 and the LMP91200 EVM users guide shows this on pages 6 and 7 ==> https://www.ti.com/lit/ml/snau122a/snau122a.pdf, where a BNC connector was used and a triaxial BNC can replace it
Like this (screenshot from a Keysight video on the topic):
I found this pH probe, the only one i could find with a Triax connection
https://www.mt.com/dam/MTF_PROF/52000527_les_typesdeconnexionsdeselectrodesdephredox.pdf
They have stated that the inner shield is connected to their reference and outer to earth. So i think you need to be careful when selecting pH probe. I don't which one is the best solution but my spontaneous thought before reading the datasheet in detail was to connect the outer shield to earth.
