A Low-Power Pilot-DAC Based Column Parallel 8b SAR ADC With Forward Error Correction for CMOS Image Sensors

Successive-Approximation-Register (SAR) Analog- to-Digital Converters (ADC) have been shown to be suitable for low-power applications at aggressively scaled CMOS technology nodes. This is desirable for many mobile and portable applications. Unfortunately, SAR ADCs tend to incur significant area cost and reference loading due to the large capacitor array used in its Digital-to-Analog Converter (DAC). This has traditionally made it difficult to implement large numbers of SAR ADC in parallel. This paper describes a compact 8b SAR ADC measuring only 348 μm×7 μm. It uses a new pilot-DAC (pDAC) technique to reduce the power consumption in its capacitor array; moreover, the accuracy of the pDAC scheme is protected by a novel mixed-signal Forward Error Correction (FEC) algorithm with minimal circuit overhead. Any DAC error made during pDAC operation can be recovered later by an additional switching phase. Prototype measurements in 0.18 μm technology shows that the DAC's figure-of-merit (FoM) is reduced from 61.3 fJ/step to 39.8 fJ/step by adopting pDAC switching with no apparent deterioration in Fixed-Pattern Noise (FPN) and thermal noise.