Exploiting Combinatorial Redundancy for Offset Calibration in Flash ADCs

Process variations in advanced CMOS nodes limit the benefits of scaling for analog designs. In the presence of increasing random intra-die variations, mismatch becomes a significant design challenge for circuits such as comparators. In this paper we describe and demonstrate the details of a statistical element selection (SES) methodology that relies on the combinatorial growth of subsets of selectable circuit elements (e.g., input transistors in a comparator) to provide redundancy for post-manufacturing calibration of specifications (e.g., offset). A test chip consisting of an array of digitally calibrated comparators with built-in combinatorial redundancy was manufactured in 65 nm bulk CMOS. Over 99.5% of the comparators satisfy the given offset specification compared to 15% for Pelgrom-type sizing. A second test chip in the same process consists of an 8-bit, 1.5 GS/s flash ADC and achieves 37 db SNDR at low frequencies. The total power is 35 mW, 20 mW in the S&H and 15 mW in the ADC core. The figure of merit is 0.42 pJ/conv.