An 18b 12.5MHz ADC with 93dB SNR

Abstract

This ADC is aimed at medical imaging such as digital x-ray where multiple channels from a photodiode array are multiplexed to a number of ADCs. These ADCs need to have a dynamic range in excess of 90dB to cater for the distance and composition of tissues that the x-rays have passed through. INL matching between converters of about 1 LSB at the 16b level is also required to avoid visible artifacts across pixel boundaries. These INL and dynamic range specifications are met by a number of commercially available 16b successive approximation (SA) ADCs but none at conversion rates more than 4 MHz. Conversion rate can be increased by determining 2 or more bits per bit trial by using a low resolution flash within a SA ADC. In [1] the residue amplifier (RA) that drives the inputs of the flash converter is within the SA loop. Sufficient time must be allowed for this RA to settle to the required accuracy each bit trial. [2] has no gain between the SA DACs and the flash and so avoids any RA settling delays. However to avoid upsetting the SA algorithm, the flash comparator errors must remain smaller than the size of each trial space. As a result this latter approach has so far been limited to low to medium resolution ADCs.