Automatic generation of common-centroid capacitor arrays with arbitrary capacitor ratio

The key performance of many analog circuits is directly related to accurate capacitor ratios. It is well known that capacitor ratio precision is greatly enhanced by paralleling identical size unit capacitors in a common-centroid geometry. In this paper, a general algorithm for fitting arbitrary capacitor ratios in a common-centroid unit-capacitor array is presented The algorithm gives special care to both non-integer and identical ratios in order to minimize mismatch. A method for capacitance mismatch estimation based upon an oxide gradient model is also introduced It enables the comparison of different unit-capacitor array assignments. Layout issues are discussed with emphasis on a generic routing model. Both the algorithm and the mismatch estimation method are implemented in an automatic capacitor array generation tool.