REG ADC

[guest1]

Since i am new to PIC,
i have a small doubt,
is it possible to read a negative voltage in PIC
(ie., can we read values from -5v to 5v )
and wat does negative reference means
thanks

[Ttelmah]

No.
The ADC, has an input range, from Vref-, to Vref+, and these signals are allowed to be between Vss, and Vdd (they are allowed to go fractionally beyond these rails without damaging the chip - normally 0.3v beyond at each limit, but the ADC specifications are only met when they are inside the rails).
You can't have a 'negative reference', you have Vref-. Note that the 'minus' is after the Vref, rather than before. This distinguishes it from a voltage that is allowed to go '-', instead saying this is the 'negative end' of the circuit concerned (a little bit of 'standard shorthand'). This defines the bottom end of the ADC input range, but is not allowed to go below the 0v supply (with the caveat already made).
All you do, is add an external offset, with an op-amp.
Typically, add half the Vref, in an op-amp 'adder' circuit. At the same time, choose the gain of the circuit, to give the required input range.
If your source is very low impedance, aaccuracy is not terribly important, and you are using Vdd as the Vref+, then you can use a simple resistive divider, from Vdd to the input voltage.

Best Wishes