This is the circuit that I found on the internet. Its purpose is to ensure the input voltage to the ADC falls between 0 to 5V. 8)
From the figure below, the first op-amp is designed to change the input voltage so that it does not go over zero volts, for example changes -2.5 to 2.5V, to -5 to 0V. The second op-amp is configured in negative amplifier mode with a gain of unity changing -5 to 0V to 0 to 5V. The third part with the two diodes is for protection, e.g. say a -1 to 6V signal makes it through the op-amps the diodes will cut off the peaks of the waveform making sure no damage is done to the A2D.