Unless you have a USB input on your DAC/AMP you don't without some sort of digital to digital converter like a Mutec MC3+USB or Sonore Ultra Digital.
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Your NAD C328 amp has 4 digital audio inputs (2 optical/2 coaxial). It does not have a USB input connection. If your Chromebook only supports USB output, you are going to need a digital to digital convertor was suggested above.
I think the above will work but please confirm this. Search on USB to optical converter for additional options.