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 $(\mathrm{n}_{\mathrm{q}})$ and comparator noise ($\mathrm{n}_{\text{comp}}$), it is desirable to reduce them simultaneously to boost the SNR by NS. However, reducing n
comp
by 2x would increase the comparator’s power consumption by $\sim 4\mathrm{x}$. Therefore, many NS SAR ADCs adopt $8\sim 10\mathrm{~b}$ CDAC and low-power comparators for comparable $\mathrm{nq}_{\mathrm{q}}$ and n
comp
. Then, the residue can be shaped by $1^{\text{st}}$-order NS with high OSR (25) [1] or $4^{\text{th}}$-order NS [2–3] to boost the SNR to over $85\mathrm{~dB}$, 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 $n_{\text{comp}}$ power-efficiently [5]. The 78.4dB-SNDR NS SAR with 9bit CDAC in [6] suppress n
comp
with 4-times LSB repeating and use the results to switch the CDAC after tri-level voting. However, the suppression effect of n
comp
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.