Hello,
The 0.775V standard (1mW into 600 ohms) = 0dBm remains the standard for interconnections between professional audio boxes. Inside the boxes and at the circuit board level, impedances are rarely 600 ohms and engineers design each stage for maximum performance regardless of any standard level. Thus at each point within the equipment, "0 dB" may be defined as the maximum achievable voltage before clipping -- which depends on DC supply voltage and such. Thus chip manufacturers, including AD, specify their inputs and outputs by the maximum voltage they can handle. Any dB specifications will be relative to this level. For example, if the output can manage 0.9V max, and quiescent noise is 0.1 mV, the S/N would be rated as 20 log (0.9 / 0.0001) = 99 dB.
Inside the DSP there are no volts at all, only numbers. SigmaDSPs are fixed-point, with a number of bits from 5 to 8 before the binary point, and 23 or 24 bits afterward. Where a numbers' magnitude spills over from after the binary point to before it, is a logical place to spot the 0 dB level. This also corresponds to the clipping point for the chip's ADCs and DACs if included.
Best regards,
Bob