[ V_o = V_ref \cdot \frac\sum_i=0^N-1 b_i 2^i2^N \cdot \fracR_LR_L + R/3 \quad \text(if R_L not infinite) ]
from R mismatch ( \sigma_R ): [ HD_2 \approx \frac\sigma_RR \cdot \frac2^N-13 ]