[hvalentim]

(...) I'll try to explain why Davis (for example) do it slightly differently (using the negative input of an Op-Amp), and to answer your questions above...

Thanks a lot for the explanations. Although grasping the details is hard from me I am glad to see that a degree of consensus seems to emerge as for the need for different designs and why whether you are using a small PV panel or a micro diode.

No doubt, one can get there satisfactorily both ways. The sole advantage of diodes seems to be that particular models - so called "blue enhanced", such as Osram's B model - should have, at least in the spec.s, a spectral response slightly closer to a pyranometer.

It would be a very interesting exercise having a BPW34B and a solar cell, each with it's suitable circuit design, side by side, comparing the results/readings.