pH serves for amplifying signal from pH probe. It is compatible with usual pH probe with BNC connector. Signal from probe lays in range -420mV - 420mV. It is needed to transfer it to range 0 - 5V, so it can be meassured on input analog pin of arduino. In first step it is amplified by operational amplifier in non-inverting mode. Level of amplifying is set by trimr marked as GAIN in range 130% - 1100% of input signal strength. In second step so-called offset is set, its level is set by trimr marked as OFFSET and its purpose is obvious from picture.
In last step there is linear optocoupler which along with isolating switching source provides galvanic isolation of measuring circuit. Before RJ connector is placed protecting Zener diod.
Modul is primarily intended for Sensuino Shieldu which is given by its pin distribution in RJ connector. But it is also possible to use it with another A/D converter. On input pins is necessary supply voltage +5V for switching power source which supply galvanic isolated part. Also supply voltage witch is in minimal over 2V higher then maximum output voltage. This voltage surves for supply of operational amplifier placed after opto-isolating part.