The VDAC in PSoC 3 or PSoC 5 can directly drive into high-input impedance circuit like opamp. When the VDAC is intended to drive an external resistive load, then the user must buffer the output of the VDAC using an opamp in voltage follower mode or a PGA in non-inverting mode. VDAC output can go directly to an external load if and only if the external load is capacitive. Following image shows the correct and incorrect configurations to use the VDAC in PSoC 3 or PSoC 5.
The PSoC 3 or PSoC 5 VDAC, by implementation, is an IDAC driving into a load resistance to make a voltage DAC. The two ranges of VDAC are determined by the resistance into which the IDAC drives. For 1 V and 4 V range, the IDAC drives a 4K and 16K resistor respectively. Because of this I to V conversion, the VDAC cannot drive an external resistive load directly. The external load will appear in parallel to the internal resistive load and will change the range of the DAC. Hence the VDAC cannot drive external resistive loads.