Hi @Moto, your unit is working as expected. In the link below, see the discussion on whether the DAC is being set to optimize for THD or optimize for THD+N. They are different settings and these aren’t adjustable by the user, although they might be at some point down the road. In short, to optimize for THDN loopback, you’d want to have the software adjust the relays and run the DAC very close to full-scale output… And if you optimize for THD, you want to have the software adjust the relays and run the DAC about 15-20 dB away from full scale.
So, if you specify 0 dBV full scale output, and you wanted to optimize for THD, you’d set the DAC attenuators to 0 dB (18 dBV full scale output) and then send a -18 dBFS output stream to the DAC. If you wanted to opimtize for THDN, you’d set the full scale output to 8 dBV, and send a -8 dBFS output stream to the DAC. This assumes you have full manual control, which isn’t an actual option.