Photodiode IC giving non-intuitive outputs
I will outline some general considerations that would help clarify the problem.
What I see inside the circuit are six transimpedance amplifiers with photodiodes connected to the op-amp inverting inputs. The current generated by the photodiode during illumination should enter its cathode so the output voltage of the op-amp should change in a positive direction.
The voltage Vc = 1.5 V applied to the non-inverting inputs &quot;shifts up&quot; the op-amp output zero level; so the output voltage will vary from 1.5 V to 5 V.
The reference voltage of the Arduino ADC should be 5 V and the Arduino ground should be connected to common ground.