Mini 2440 , How to set reference voltage for ADC input

Hi , I want to read negative voltage and positive voltage together. Adc
reads only positive voltage . How to set an interval for -3,5 to +3,5

Reduce the -3.5 to +3.5 V swing down to something smaller, ie by a factor
of 1/2 so that you have a swing of -1.75 to +1.75v using a resistor divider
network.  Then apply an offset of, say 2.5 volts to an op-amp with gain of
1.  The output will now be something like 0.75V to 4.25V to stay within the
linear output swing on a single supply 5V op-amp.  

Then the rest is maths.

I don't understand , I use GPIO of mini2440 , Do you mean make a circuit 
or  make it by software ...?

I thought one would use an ADC input and "condition" the signal as I
described. This is a hardware solution.

Or buy an external ADC that can handle -negative voltages (I am not aware
if they are even available) that outputs a SPI or I2C data format. Or, to
make it even more difficult one that outputs the digital representation of
your analogue signal in 8 parallel lines and then you could use a 8 bit
GPIO to gather this information.

Maybe, some more details of what you are trying to do might be appropriate.

hai 

i dont have such kind of problem but how can i able to set the reference
voltage my ref voltage is 5.214645v

please help me 

Thanks in advance 

Regards
Jack

As far as i know is the input range of the FriendlyArms ADC between 0V and
3.3V.  So you must design the peripheral circuits to fit this range. That's
all. The rest is maths, as mentioned above.

please tell me that peripheral circuit i am new to mini2440

Thank in advance

Regards
Nicky

This is not a mini2440 related problem but electronics. Usually it is done
by voltage dividers and/or op-amps to put the input in this range. What you
want to measure. I can not believe that you have a signal with a precision
of 6 steps behind the comma ;-)