(A) study on performance enhancement techniques for sub-ranging SAR ADC

Abstract

Successive approximation register (SAR) analog-to-digital converters (ADCs) have been developed with tremendous performance improvement during the last decade mainly due to their low-power, simple and digital-friendly architecture. However, the conversion speed that is inversely proportional to the resolution remains as one of the fundamental design bottlenecks, due to the increased number of decision cycles with a required digital-to-analog converter (DAC) settling accuracy in every decision cycle. To enhance the conversion speed of a SAR ADC, several architectural modifications could be tried. Time-interleaved (TI) architecture could be a good selection as a solution for increased number of decision cycles. In addition, as the conversion is more burdensome in the MSB part in terms of both settling speed and power consumption, subranging architecture could alleviate this issue with a fast and compact coarse ADC. So, in this paper, a SAR-assisted 4-way time interleaved SAR ADC that uses a double rate coarse ADC operation is presented. The coarse ADC operates with a higher rate operation to reduce the MSBs decision time. The mismatch problem between coarse and fine ADCs is solved by using series switching, loop-unrolled comparators, redundancy and background offset calibration. A simple metastability reduction technique for non-binary SAR ADC that does not require a lookup table is also proposed. The single channel ADC core occupies a $0.0128-mm^2$ area and consumes 1.9 mW under a 1-V supply. With an 80-MHz input, the ADC achieves an SNDR of 58.1 dB and an SFDR of 72.1 dB. The peak DNL and INL are 0.96 LSB and 1.6 LSB, respectively, and the figure of merit is 24.26 fJ/conversion-step. The 4-way interleaved ADC has TI timing initialization issues due to processing failure, the time-interleaving operation did not work properly, so the only temporary measurement was possible. The measured SNDR and SFDR were 48 dB and 57 dB, and the ADC occupies a $0.06-mm^2$.