A Novel Continuous-Time Common-Mode Feedback for Low-Voltage Switched-OPAMP

A novel rail-to-rail and fast continuous-time common-mode feedback (CMFB) strategy is presented proper for low-voltage Switched-OPAMP (SO) circuits. The threshold voltage change due to bulk signal is used to measure the output voltage. To satisfy speed requirements, averaging common-mode (CM) level and amplifying error signal are realized in a single block. Finally, the measured CM level is controlled by applying an error-voltage dependent current to the output nodes. As a design example, a modified low-voltage switched-OPAMP in a cascade 2-1 delta-sigma modulator adopting the proposed technique is presented. SPICE simulation using a 0.18µm technology and VDD=0.9V is provided which confirms the expected accuracy and speed of the CMFB circuitry.