An 84dB-SNDR 1-0 Quasi-MASH NS SAR with LSB Repeating and 12-bit Bridge-Crossing Segmented CDAC

As the residue on the CDAC of a noise-shaping (NS) SAR ADC mainly contains quantization error (n_q) and comparator noise (ncomp), it is desirable to reduce them simultaneously to boost the SNR by NS. However, reducing ncomp by 2x would increase the comparator's power consumption by ∼ 4x. Therefore, many NS SAR ADCs adopt 8~10b CDAC and low-power comparators for comparable nq_q and ncomp. Then, the residue can be shaped by 1st-order NS with high OSR (25) [1] or 4th-order NS [2-3] to boost the SNR to over 85~dB, at the price of limited input BW or increased circuit complexity. Although pipelined NS SAR or MASH suppress the last-stage n comp effectively, amplifying the preceding stage's residue requires either power-hungry amplifier or dynamic amplifier with intense calibration [4]. Alternatively, LSB repeating can suppress n_comp power-efficiently [5]. The 78.4dB-SNDR NS SAR with 9bit CDAC in [6] suppress ncomp with 4-times LSB repeating and use the results to switch the CDAC after tri-level voting. However, the suppression effect of ncomp would be much limited for a precision CDAC (10 12bit) with more times LSB repeating, as it only translate the results into 3 different levels.