Charles hit the nail on the head.

Code:
0-10v input  ----------+
                       |
                      _|_
                      | |
                      |R|
                      |_|
                       |
                       |
                       +--------- PIC ADC Input (0-5v)
                       |
                      _|_
                      | |
                      |R|
                      |_|
                       |
                       |
                       |
                     Ground