Differential CVD waveform in PIC18F45K50
First off, I assume; 


You are referring to using the inbuilt ADC without any external hardware as per Microchip's application note AN1478 (http://ww1.microchip.com/downloads/en/AppNotes/01478A.pdf)
Your scope is set to AC coupling (otherwise that -1.5V peak would have fried your PIC)


Just setting your oscilloscope to DC coupling will most likely get you a waveform that will resemble much more the image which you supplied (also in the application note I linked). I would also recommend increasing your time spent in the pre-charge phase. It is difficult to be sure in AC coupling but it looks to me like it does not have time to reach the full scale voltage before you transition to the acquisition phase.