I have been trying to use my analog input pins to read a 0-10V signal and have found that whatever the signal is (coming from a calibrated power supply, the arduino spits out a lower voltage than it should. Way more of an error than the +-5% that the Opta manual says.
Example-- 10V from the power supply to the A3 pin, the arduino reads 9.11V. And the error is not consistent. at different voltages. 8V in gives 7.41out, 5V gives 4.65out and so on.
I have even attempted to use excel to see if the error fits any functions, and none of them are even accurate.