LK1 is used to select the power source for the AD7747. LK1 in the 5.0 V position selects the 5 V supply from the USB connector, J1. LK1 in the 3.3 V position (default) selects the 3.3 V regulated output from the on-board ADP3303 voltage regulator. LK1 in the V_EXT position selects an external power supply, supplied via J2.