I don't understand what you're trying to get out... You can't get a 1.5V inverted square wave out, but you could get a 5V inverted square wave (if that's what you want)...
[u]PWM[/u] is NOT analog. There is no digital-to-analog converter so you cannot get 1.5V out of the Arduino (without additional hardware).
You can write a value that averages 1.5V but I don't think that's what you want.
A square wave isn't really "analog" but you can use an analog input to read it.
If you want to invert the square wave and boost it to 5V you can simply write a high whenever the input is less than 0.75V and write a low whenever the input is greater than 0.75V. (3/4ths of a volt should read about 767 and there's no need to convert to voltage.)