Practical modeling of comparator metastability for conventional and LSB-first SAR ADCs

A practical model for characterizing comparator metastability errors in SAR ADCs is presented. Previous work used uniformly-distributed ADC input signals to calculate signal-to-metastability error ratio (SMR), in the interest of making the derivation easy. The contributions of this work are three-fold: first, it is shown that for characterizing metastability it is more reasonable to use input signals with normal or Laplace distributions. Second, it is shown that the simplifying assumptions in previous work overestimated SMR by as much as 18dB compared to this more accurate analysis. Finally, it is shown that LSB-first SAR ADCs achieve SMR performance equal to or better than conventional SARs, depending on bandwidth.