The DAC datasheet will include an input impedance specification for the reference. From this value, if it is fixed, it is possible to calculate if the voltage reference can supply enough current. However, this can become complicated with some DAC structures, such as R-2R, as they have an input impedance that varies substantially with the applied code. In such cases, the external voltage reference will need to be buffered. The buffer chosen for such voltage DACs should be a low noise, low offset error amplifier. This is because offset errors in the reference buffer will transfer to gain errors at the output of the DAC.