Analysis of Switched-Capacitor Common-Mode Feedback Circuit

A detailed analysis of the dc behavior of switched-capacitor common-mode feedback circuit (SC-CMFB) is presented. A mathematical model, useful for analysis, is developed and the expressions for the output common-mode (CM) voltage, with and without considering the charge injection of switches and leakage currents, are derived. Further, the expression for dc CM settling time, is presented. The effect of parasitic capacitances, dc CM gain, charge injection error, and leakage currents, on the steady–state value of the dc CM voltage is analyzed and design guidelines to minimize these errors are presented. Finally, an improved version of the SC-CMFB circuit is analyzed. This circuit has very low errors due to charge injection and leakage currents and settles much faster than the traditional SC-CMFB circuit.