Hi,
I think I understood
parts of what you're trying to do. Off the top of my head, if your -ve voltage is reasonably low impedance, you could do a voltage divider between the +2.4 and -2.4 to get 0 V, and then compare the signal with "GROUND". I'm not sure how well that would work. It would most likely be slow. There may also be a switched-capacitor way of doing this. For example, charge cap A to +2.4, and cap B to -2.4, then add the two caps together and compare with "GROUND".
Its also possible to generate a negative voltage with a negative charge-pump, but its not that accurate. Actually I have a sneaking suspicion that you already did that, and you're trying to make it more accurate
You are going to have to explain what you mean by "connect the source opposite". What is the source? Also, as RobG mentioned, you should probably describe your application so people know what you're talking about...
cheers,
Aaron