An Automatic On-Chip Calibration Technique for Static and Dynamic DAC Error Correction in High-Speed Continuous-Time Delta-Sigma Modulators

Abstract

Calibration is preferred over dynamic element matching (DEM) techniques to correct nonlinearity in digital-to-analog converters (DACs) in high-speed multi-bit continuous-time (CT) delta-sigma modulators (DSMs) to avoid introducing extra excess loop delay. The state-of-the-art calibration techniques, however, involve substantial external control, making it difficult for a complete on-chip realization. In this paper a highly automated on-chip technique for calibrating differential nonlinearity (DNL) and inter-symbolinterference (ISI) errors of a multi-bit DAC in a CTDSM is presented. The proposed technique implements a Calibration Control System (CCS), equipped with a Finite State Machine (FSM) logic, that automates the entire calibration process with minimal intervention. The calibration loop utilizes the modulator itself to produce the digital estimates of the DNL and the average ISI errors of each unit element of the DAC. These digital estimates are then used to configure auxiliary DACs for correction of the errors in the main DAC. For every unit element in the main DAC, an 8-bit dynamic auxiliary DAC injects a calibrated compensation current in the loop at every up-transition of the input data to cancel the average ISI error, and a 5-bit constant current source array corrects its DNL error. Design considerations and post-layout simulation results for a 50-MHz bandwidth, 1.8GS/s 4 th -order CTDSM are presented. The modulator has a 4.8 dB and a 10.8 dB improvement in SNDR and SFDR respectively with calibration, leading to a dynamic range of 71 dB with a total power consumption of 37.7 mW from 1.3 V and 1 V supplies.