The simplest I to V converter is the humble resistor. However, presenting a nonzero impedance to the source of input current is the disadvantage of the humble resistor. If the device providing the input current has very little compliance or does not produce a constant current as the output voltages changes, this can be fatal. A photovoltaic cell is a good example, a fancy name for a sun battery. Even the garden-variety signal diodes you use in circuits have a small photovoltaic effect (there are amusing stories of bizarre circuit behavior finally traced to this effect). The circuit below show us the good way to convert current to voltage while holding the input strictly at ground.

Ideal current to voltage converter circuit schematic

Since a photovoltaic diode can generate only a few tenths of a volt, the inverting input is a virtual ground (this is fortunate). The output of this particular circuit is 1 Volt per microamp of input current. The resistor from the noninverting input to ground could be omitted, but it is highly desirable The function of this resistor will be explained shortly in connection with op-amp shortcomings.