DAC8775EVM: How the enable the external power(+-15V) apply to the EVB

Alex,

This is described on Page 6 of the EVM User's Guide. Pasting that below.

When using the DAC8775 internal buck-boost converters to supply VPOS_X and VNEG_X, install JP1 through JP8 (or the two jumpers that correspond to the channel of interest) in the 1-2 position, or "inside" position, as indicated by Table 3. When using external equipment to supply VPOS_X and VNEG_X, install JP1 through JP8 (or the two jumpers that correspond to the channel of interest) in the 2-3 position, or "outside" position, as indicated by Table 3. In this configuration, the VPOS_X supplies are connected to the PVDD/AVDD net and the VNEG_X supplies are connected to the AVSS net. If bipolar supplies are used, connect an appropriate negative supply voltage to AVSS through terminal block J3, as shown in Figure 3. If unipolar supplies are used, connect AVSS to ground through terminal block J3.