Input Referred Comparator Noise in SAR ADCs

In a successive-approximation-register (SAR) ADC, comparator noise is often one of the dominant error sources. In this brief, the relation between the input referred noise of a comparator and its resulting noise in an SAR ADC is presented. A scaling factor less than unity is found when the comparator noise is referred back to the ADC input. When the comparator contains both thermal and flicker noise, the noise profile is maintained when referred back to the ADC input; however, due to aliasing caused by sampling, the flicker noise corner at the ADC output is pushed toward lower frequencies compared with the flicker noise corner frequency of the comparator. The results are analytically derived and proven by simulation and measurement.