A multibit cascaded sigma-delta modulator with DAC error cancellation techniques

Noise reduction techniques are developed for a multibit cascaded sigma-delta (ÓÄ) modulator used in the analog interface of a digital signal processing system to improve its performance by reducing the errors introduced by digital-to-analog converters (DACs). The idea of the proposed architecture is to create extra feedback paths around the modulator to reduce the DAC errors further by properly designing the error cancellation logic. Transfer functions show that the DAC error at the final stage of the proposed architecture is totally cancelled, while DAC errors from other internal stages are shaped by an order higher than those in a conventional cascaded modulator.

The difficulty in circuit implementation of modulators with high resolution and bandwidth increases due to the imperfection of analog components in VLSI processes. Structural and circuit-level compensation techniques are generally used in developing such modulators. Major analog nonideal effects in a multibit cascaded ÓÄ modulator include coefficient mismatches, DAC nonlinearity errors, and integrator leakages. While providing solutions for each of these nonidealities, this dissertation focuses on the minimization of the DAC error since it causes the most performance deterioration.

A configurable fourth-order (2-1-1) ÓÄ modulator is implemented for architecture verification. This modulator can be configured as the proposed architecture as well as a conventional cascaded structure with various modulator orders. The design of the system's parameters and analog blocks are fully described in this dissertation. The system is fabricated by the AMI Semiconductor (AMIS) 0.5ìm double-poly triple-metal mixed- signal process through the MOSIS service. Measurement results show that with on-chip error of ±0.15 LSB for each DAC and an oversampling ratio (OSR) of 32, an improvement of 8dB of the proposed architecture over the conventional structure is observed.