Abstract: A fourth-order continuous-time sigma delta modulator with 20-MHz bandwidth, implemented in 130-nm CMOS technology is presented. The modulator is comprised of an active-RC operational-amplifier based loop filter, a 4-bit internal quantizer and three current steering feedback DACs. A three-stage amplifier with low power is designed to satisfy the requirement of high dc gain and high gain-bandwidth product of the loop filter. Non-return-to-zero DAC pulse shaping is utilized to reduce clock jitter sensitivity. A special layout technique guarantees that the main feedback DAC reaches 12-bit match accuracy, avoiding the use of a dynamic element matching algorithm to induce excess loop delay. The experimental results demonstrate a 64.6-dB peak signal-to-noise ratio, and 66-dB dynamic range over a 20-MHz signal bandwidth when clocked at 480 MHz with 18-mW power consumption from a 1.2-V supply.

Key words: continuous-time