This is designed to make a variable voltage output from 0-24V with a LM317 or equal. A DAC delivers the input from 0-3.3V. The normal non-inverting amplifier controls the ADJ-Pin from the LM317. There is an offset of 1.25V between the ADJ-Pin and the OUT-Pin. To be able to get 0V at the output, you need -1.25V at the ADJ-Pin. This negative voltage is created by the virtual ground between the two diodes.
