Can we see a schematic?My first thought is op-amp offset voltage. The CA3140E is not a precision op-amp and its datasheet quotes an offset voltage as high as high as 15mV (5mV for the CA3140AE version). So 1.5mV is well within its specification.Look into using a "precision op-amp", such as the MAX9636AXT+T. 0.1pA of input bias current and 10uV of input offset voltage should work a lot better.--The Ruggeduino: compatible with Arduino UNO, 24V operation, all I/O's fused and protected
Offset voltage is +/-, so 5mV means +/-5mV.The LF351 offset voltage is also pretty bad -- you could just be getting a "lucky" part.In your application (before seeing the schematic) I think both offset voltage and input current are important, though op-amps tend to be better at minimizing their input currents (picoamps) than their offset voltages for a given cost.--The Flexible MIDI Shield: MIDI IN/OUT, stacking headers, your choice of I/O pins
But what about -1.5mV?