Hi,
I've been watching this thread, but stayed out because, although I understand (some of) the theory, it's a very complex subject and I'm in danger of writing pages and pages.... Also, I suspect that it's fundamentally a "physical" problem, but don't have much of an idea what the cause might be. So just a few brief comments for now:
For photography, one unit of light level is the Lux, with full sunlight being approximately 100,000 Lux. But moonlight is less than 1 Lux (i.e. >100,000 times smaller) so if we apply that to the 300 mV full-scale from the SP-110 we would expect only a few microVolts in moonlight. Those voltage levels are very difficult to measure because of "noise" and thermoelectric effects. That just might be the cause of your problem - junctions between different metals at different temperatures can generate a (small) voltage difference.
The light (or energy) level that we're trying to measure is proportional to the current which is generated (and actually it's not too difficult to measure a current ratio of 1: 1 Million, or more). However, voltages are generally easier to "understand" and measure, and the conversion is easy - it's what a resistor does (V = I x R) ! Werk puts the resistor directly on the (PV) sensor (as presumably do Apogee) whilst Davis effectively amplify the current first and then convert to a voltage.
The voltage gain is indeed (almost) proportional to the feedback resistance (ratio). So for the very best stability (but not as convenient), the "Pot" could be replaced by a Metal Film (fixed) resistor (perhaps re-using the pot to balance the Input Offset) and the gain adjusted purely within the software.
It's worth considering the specification of the Op Amp: The "Input Offset Voltage" can be several hundred microVolts, which represents an "error" in the input voltage, that gets multiplied by the amplifier gain to produce an output that might represent several watt/m2. Also, although it is a "rail to rail" device, it cannot pull the output completely down to zero (milli-) Volts. The data sheet specifies about 6 mV at 20 microamps (pull-up), but I can't see why the current would be even as large as that. However, I do wonder if the designers of the SP-110 expected it to be used with a true "instrumentation grade" (i.e. very expensive) amplifier.
Cheers, Alan.
I've been watching this thread, but stayed out because, although I understand (some of) the theory, it's a very complex subject and I'm in danger of writing pages and pages.... Also, I suspect that it's fundamentally a "physical" problem, but don't have much of an idea what the cause might be. So just a few brief comments for now:
For photography, one unit of light level is the Lux, with full sunlight being approximately 100,000 Lux. But moonlight is less than 1 Lux (i.e. >100,000 times smaller) so if we apply that to the 300 mV full-scale from the SP-110 we would expect only a few microVolts in moonlight. Those voltage levels are very difficult to measure because of "noise" and thermoelectric effects. That just might be the cause of your problem - junctions between different metals at different temperatures can generate a (small) voltage difference.
The light (or energy) level that we're trying to measure is proportional to the current which is generated (and actually it's not too difficult to measure a current ratio of 1: 1 Million, or more). However, voltages are generally easier to "understand" and measure, and the conversion is easy - it's what a resistor does (V = I x R) ! Werk puts the resistor directly on the (PV) sensor (as presumably do Apogee) whilst Davis effectively amplify the current first and then convert to a voltage.
The voltage gain is indeed (almost) proportional to the feedback resistance (ratio). So for the very best stability (but not as convenient), the "Pot" could be replaced by a Metal Film (fixed) resistor (perhaps re-using the pot to balance the Input Offset) and the gain adjusted purely within the software.
It's worth considering the specification of the Op Amp: The "Input Offset Voltage" can be several hundred microVolts, which represents an "error" in the input voltage, that gets multiplied by the amplifier gain to produce an output that might represent several watt/m2. Also, although it is a "rail to rail" device, it cannot pull the output completely down to zero (milli-) Volts. The data sheet specifies about 6 mV at 20 microamps (pull-up), but I can't see why the current would be even as large as that. However, I do wonder if the designers of the SP-110 expected it to be used with a true "instrumentation grade" (i.e. very expensive) amplifier.
Cheers, Alan.