The modern DAC chip have a noise floor very low at the limit of the measurement instrumentation. It is very important to take care to avoid any unwanted noise coupling. With the USB port we connect the DAC at a computer; noise come from the ground connection and, if directly coupled at the ground plane of the DAC, will increase the noise floor at the analog stage.
The best strategy is to keep completely separated the ground plane of the USB port with the ground plane of the DAC chip, coupling the digital signals exclusively by optical or magnetic coupler. In our units we use an optical coupler from Texas Instruments the ISO7640, very fast (150Mbit/s) and with a very low transit time. So noise is stopped and signals are coupled.