An output code offset-free comparator for SAR ADCs based on non-linear preamplifier and CMOS inverters

• Published in: Microelectronics 2013-05, Vol.44 (5), p.414-420
• Main Authors: Yue, Xicai, Krapp, H.G., Drakakis, E.M.
• Format: Article
• Language: English
• Subjects: Cascading; Chip formation; CMOS; CMOS invertor; Comparator; Comparators; Inverters; Non-linear amplifier; Nonlinearity; Operating transconductance amplifier (OTA); Preamplifiers; Successive approximation registers (SAR) ADC; Synthetic aperture radar
• ISSN: 1879-2391; 0026-2692; 1879-2391
• DOI: 10.1016/j.mejo.2013.02.011

The Tanh transfer function of the differential pair operating in weak inversion is employed to implement the non-linear pre-amplification for the comparator of the SAR ADC. This non-linear preamplifier achieves a low gain for large input signals therefore the comparator can operate at a full speed (overdrive-free), while the high gain it achieved for small input signals improves the accuracy of the comparator. By combining the non-linear preamplifier and CMOS inverters, the issue of the comparison point shift in CMOS inverter due to fabrication process can be neglected, providing an output code offset-free solution. Simulations for the most critical zero-crossing overdrive cases for a 12-bit SAR ADC demonstrated that by cascading four stage differential pairs, the gain difference between the input signals of (1/2)LSB and (1/4)Vref is 49dB, while the output of the preamplifier for the input of (1/2)LSB is big enough to tackle the comparison point shift in the following comparison stage. Fabricated using the 0.35μm AMS CMOS technology, the comparator occupied an area of 130×140μm2 while consuming 6.2μW of power under a 1.5V single power supply which is suitable for low-power applications. Chip tests demonstrated that the fabricated chips achieved 12-bit resolution without offset when working at 100KSPS. This pure analog comparator could lead to higher resolution or higher speed by cascading more stages of the non-linear pair or by applying more current in the last stage of the non-linear pair.