9.4 A 4th-Order Cascaded-Noise-Shaping SAR ADC with 88dB SNDR Over 100kHz Bandwidth

High-resolution, sub-MHz-bandwidth data converters are essential for audio and sensor applications and are conventionally implemented as sigma-delta (SD) converters. The dependence of SD ADCs on op-amps inherently results in difficulties for process scaling, power efficiency improvement, and area reduction. A promising alternative to SD ADCs is the emerging noise-shaping (NS) SAR ADC, which provides high resolution with high energy efficiency and compactness. For medium-high SNR applications, some NS-SAR ADCs already exhibit better performance than SD ADCs [1, 2]. However, NS-SARs for applications that require SNR >85 dB and hundreds of kHz of bandwidth are currently lacking. The SNR is limited because most recent NS-SAR ADCs are restricted to a low-order NTF. Additionally, noise from the loop filter limits performance. In this work, we propose a cascaded-noise-shaping (CaNS) SAR architecture that cascades two 2nd-order NS stages to provide a 4th-order NTF for high resolution, while consuming similar power and area to a 2nd-order NS-SAR. Our prototype achieves 88dB SNDR over a 100kHz bandwidth while consuming only 120µW and occupies 0.02mm2, resulting in a Schreier FoM of 177dB. The proposed architecture enables the NS-SAR to be used in low-power, area-critical audio applications in place of SD ADCs.