Measuring a negative input on unipolar supply ADC
The incoming signal is adjusted so that its maximum peak to peak signal does not exceed 3.3V and then this signal is offset with a dc voltage of 1.65V.

1.65V becomes the new 0V.

EDIT

I've just re-read and I think you require a differential amp level shifter. Still easy to do and same sort of principle as my 1st answer BUT information about the input and the ADC is needed if you want better details - i.e. is the ADC also differential?