This paper presents a passive NS-SAR ADC structure with a passive gain of 4, and a low size-ratioed two-input pair comparator. It achieves an ??? = (1 − 0.8?^(−1))^2. A 6-bit NS-SAR prototype is designed and simulated. Power spectral densities show that the proposed second-order NS suppresses the in-band noise by 9dB more than the first-order state-of-the-art NS.