How to get -5 to 5V with AD5668
The AD5668 can only supply 0-5V at its outputs (or 2.5V if you use the -1 version with internal reference). The reference voltage has to be essentially within the supply rails. Applying -5V will destroy the chip. 

If you need -5 to +5 you will need to add an amplifier/level shifter at a given output- to multiply by 2 and add a suitable offset. That will require a negative supply of at least -5V (you won't quite get to +5 or -5 with a +/-5V supply). 

Here is how I would do it with this particular chip (-2 version with 0-5V out): 







simulate this circuit &ndash; Schematic created using CircuitLab

You only need one reference buffer (OA2) but you do need one amplifier per output. The output op-amps require power supplies of at least +/-5V. If you use closer to +/-5V you will need rail-to-rail output and input common mode range must include 3.33V. eg. OPA2188. You do need the reference buffer because the output impedance of the internal reference is 7.5K ohms. 

The stability will only be as good as the reference, so using the internal precision reference makes a lot of sense. If you were to use the power supply you would add a lot of noise and imprecision into the output. 

If you want high accuracy commensurate with the 16-bit DAC you can use a single precision resistor array for the two pairs of resistors such as a Vishay ACASA1002S2002P1AT.