You can tie an R to Vcc to input and one from that input to signal,
a third to ground, and offset the voltage seen by A/D. That will allow
handling a negative signal. Attached a calculator to help selecting
them. Be careful of input Z structure of A/D when you choose values.
This is one simple method. It has errors due to non precision of Vcc
(use a reference) and R tolerance (use precision Rs).
Regards, Dana.