The analog output of a DAC may be either voltage or current. In both cases, it is important to know the output impedance. Current-output R-2R DACs have code-dependent output impedance; therefore, its output must drive the virtual ground of an op amp in order to maintain linearity. The AD5545 16-bit Current Output DAC is an excellent example of this architecture. A suitable interface circuit is shown here. A 2.5V external voltage reference and the AD8628  auto-zero op amp are used as an output current to voltage converter.