fraser wrote:This is expected behaviour, but you will only see these voltages with a shunt alone, or when floating. The floating voltage is 2V (there is a 2V reference connected to the positive input terminal through 1MOhm). When voltage is applied to the terminals this will be negated and the measurement will be correct.
With no shunt and attenuated laser power I am getting wrong voltage from the photodetector with the VCP1000. When the attenuation is strong, the VCP1000 shows 2 V as if no voltage source is connected. I can understand that because Si photodiodes have huge internal impedance, may be Ghoms.
That opens the following question: which impedance of the voltage source should be OK for the VCP1000? Or maybe, since you say "when voltage is applied to the terminals this will be negated", which minimum voltage that would be? 2 V? Then the measurement range of the VCP1000 is limited by 2 V from below, correct? It is 2 v to 40 V then?