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Dont put lpf because it will filtrate harmonic. I think the way to simulate SNR of DAC is:
1. Sample DAC output by fixed steps. The step should be equal to clk period.
2. Do fft on sampled data and calculate SNR
SNR is in the desired bandwidth. If you oversample, the quantization noise is spread over a wider frequency range and the noise in your desired signal bandwidth goes down. You have to LPF the DAC output because it makes signals at large multiples of the sampling frequency plus and minus the signal frequency. These will interact with the downstream circuitry possibly producing IMD into the desired signal frequency.
The SNR=6.02Nb+1.76 formula appears when you consider the quantization noise introduced by an IDEAL DAC (or ADC for that matter).
Therefore, in this calculation it is considered the output signal of the DAC directly (no LPF). Note also that an ideal DAC does not have generate distortion. The output spectrum of this ideal DAC is composed by the sinewave and the quatization noise.
In reality you have more noise (thermal, ...) and also distortion.
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