Hi,
upto12bit a SAR ADC using capacitor DAC does not need calibration. For 12 bit and above we need calibration since that much of capacitor matching is not practical as we need to use bigger capacitors.
Also for high resolution you need to worry about the noise in the system as well. Normally the bulk of the noise comes from the comparator. So designing comparator preamplifier with the required noise performance is important.