To eliminate the DC component of an AC signal, run it through a capacitor.
how should I connect the capacitor???
Another option is a unity Summing Op-amp design with the signal on one leg and -1.40V on another. Though you do need to account for the inverting nature of the op-amp circuit.Though I do believe you can (at least on some of the atmel chips) use 2 ADCs to create a differential input and read below 0V. I could be wrong though.