26-04-2017, 11:00
Hi,
The nominal output voltage from that sensor is rather low (but consistent with a single silicon photodiode) at only 14 mV / kW/m2, so the "input offset voltage" of the Op-Amp may be quite significant. That could cause the Op-Amp to either "ignore" or "invent" a small voltage near to the origin (zero), which would appear as a (rather severe) "non-linearity".
You may need to use an "instrumentation" grade (very low-offset) Op-Amp, or include a "bias offset" (balance) potentiometer. But my preference is to use a "Current to Voltage" or "Current to frequency" Op-Amp Configuration, the former as used by Davis (basically just a capacitor instead of a resistor in the Op-Amp feedback path).
With such an arrangement, I devised a simple microcontroller single-diode "Solar" sensor that was linear down to less than 10 mW/m2 (not far from moonlight
), even using the internal amplifier (having probably many mV of offset voltage). But on the KISS (Keep It SimpleS) principle, I now use a small "5 Volt" PV panel like Werk's.
Cheers, Alan.
Quote:I began to lose early morning and late evening readings altogether.
The nominal output voltage from that sensor is rather low (but consistent with a single silicon photodiode) at only 14 mV / kW/m2, so the "input offset voltage" of the Op-Amp may be quite significant. That could cause the Op-Amp to either "ignore" or "invent" a small voltage near to the origin (zero), which would appear as a (rather severe) "non-linearity".
You may need to use an "instrumentation" grade (very low-offset) Op-Amp, or include a "bias offset" (balance) potentiometer. But my preference is to use a "Current to Voltage" or "Current to frequency" Op-Amp Configuration, the former as used by Davis (basically just a capacitor instead of a resistor in the Op-Amp feedback path).
With such an arrangement, I devised a simple microcontroller single-diode "Solar" sensor that was linear down to less than 10 mW/m2 (not far from moonlight
), even using the internal amplifier (having probably many mV of offset voltage). But on the KISS (Keep It SimpleS) principle, I now use a small "5 Volt" PV panel like Werk's. Cheers, Alan.