Well, that's precisely my point. I can't really include an op-amp circuit before the Arduino input because this circuit is to be included in a prosthesis where space is very limited. I would like to anchor the reading to another value ranging from 0 to 255, e.g. an input value of a 200mV coming from the EMG signal detector would give me a value of about 51 on the scale of 0 to 255; what I want to do is tell the Arduino that the 200mV voltage is actually a 0 and when the voltage goes up to 400mV, I would like to assign that voltage a value of, say, 221, on the range of 0 to 255. What I can't get right is the C++ syntax to get this effect as I would like it to be executed.
That's also the reason why I convert the voltage value on a range from 0 to 255, because I was thinking instead of getting those small mV readings I could get whole number readings ranging from 0 to 255 so that the interpretation could be friendlier and perhaps play with these values establishing ranges for example when the input value would vary between 0 - 52, I would like a PWM output of about 153, so I don't know if you understand where I'm getting at, it's more about the association of a value to another and, then again, this sounds simple and might be incredibly easy to program but I would like some help due to the fact that I'm new to the C++ language. 