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

Yue, Xicai; Krapp, H. G.; Drakakis, E. M.

Abstract

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)V is 49 dB, 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 0.35 μm AMS CMOS technology, the comparator occupied an area of 130×140 μm while consuming 6.2 μW of power under a 1.5 V 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 100 KSPS. 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.

Citation

Yue, X., Krapp, H. G., & Drakakis, E. M. (2013). An output code offset-free comparator for SAR ADCs based on non-linear preamplifier and CMOS inverters. Microelectronics Journal, 44(5), 414-420. https://doi.org/10.1016/j.mejo.2013.02.011

• Journal Article Type: Article
• Acceptance Date: Feb 6, 2013
• Online Publication Date: Mar 13, 2013
• Publication Date: May 1, 2013
• Deposit Date: May 7, 2021
• Journal: Microelectronics Journal
• Print ISSN: 0026-2692
• Publisher: Elsevier
• Peer Reviewed: Peer Reviewed
• Volume: 44
• Issue: 5
• Pages: 414-420
• DOI: https://doi.org/10.1016/j.mejo.2013.02.011
• Public URL: https://uwe-repository.worktribe.com/output/5667465