Order Statistics Based Low-power Flash ADC with On-chip Comparator Selection

Abstract

High-speed flash ADCs are useful in high-speed applications such as communication receivers. Due to offset voltage variation in the sub-micron processes, the power consumption and the area increase significantly to suppress variation. As an alternative to suppressing the variation, we have developed a flash ADC architecture that selects the comparators based on offset voltage ranking for reference generation. Specifically, with the order statistics as a basis, our method selects the minimum number of comparators to obtain equally spaced reference values. Because the proposed ADC utilizes offset voltages as references, no resistor ladder is required. We also developed a time-domain sorting mechanism for the offset voltages to achieve on-chip comparator selection. We first perform a detailed analysis of the order statistics based selection method and then design a 4-bit ADC in a commercial 65-nm process and perform transistor-level simulation. When using 127 comparators, INLs of 20 virtual chips are in the range of -0.34 LSB/+0.29 LSB to -0.83 LSB/+0.74 LSB, and DNLs are in the range of -0.33 LSB/+0.24 LSB to -0.77 LSB/+1.18 LSB at 1-GS/s operation. Our ADC achieves the SNDR of 20.9 dB at Nyquist-frequency input and the power consumption of 0.84 mW.