How to measure the negative values of current and voltage

Analog to Digital Converter by its name digitalize analog signal to its equivalent digital output. An analog-to-digital converter (abbreviated ADC, A/D or A to D) is an electronic integrated circuit, which converts continuous signals to discrete digital numbers. The reverse operation is performed by a digital-to-analog converter (DAC). The digital output may be used in different coding schemes, such as binary, Gray code or two's complement binary.

PIC has inbuilt analog to digital convertor which can be configured as 8 bit or 10 bit. For our project we have configured the ADC to be 8 bit thus the resolution of ADC will be 2⁸=256 i.e it can have 256 values. The ADC has 8 input channel. Besides input channels , the ADC has to be referenced with maximum and minimum reference for ADC conversion. But the default value will be 0 to +5 voltage for many ADC.  
Thus, in order to measure the negative values, negative references voltage should be given to micro controller and it should be translated into its original value through simple calculation. 

Figure: Reference voltage generation through 5.12 Volt battery

By changing the value of resistors the reference voltages can be controlled.