In general how do you compute the noise figure of a DAC?

Normally, you would look at the Noise Spectral Density (NSD) floor in your band of interest. That sets your transmitter's noise floor, so you could then calculate the distance from the NSD to thermal noise and that will give you an effective noise figure for the DAC. As long as the digital signal you generate has a much lower (>10 dB) noise floor than the DAC, this calculation will hold.

Normally, you would look at the Noise Spectral Density (NSD) floor in your band of interest. That sets your transmitter's noise floor, so you could then calculate the distance from the NSD to thermal noise and that will give you an effective noise figure for the DAC. As long as the digital signal you generate has a much lower (>10 dB) noise floor than the DAC, this calculation will hold.