Fully Differential ADC with Rail-to-Rail Common-Mode Range and Nonlinear Capacitor Compensation

Abstract

- One of the sources of nonlinearity in charge redistribution analog-to-digital converters (ADC' ‘s) is capacitor voltage dependence. While it is possible to address this problem through capacitor fabrication technology improvements, situations arise where it is more desirable to use circuit techniques. The conventional fully differential charge redistribution converter topology eliminates errors proportional to the capacitor linear voltage coefficient, but its comparator is subjected to the common-mode input signal. When converting unbalanced differential signals, linearity is achieved only with large comparator common-mode rejection. An alternative differential converter topology is presented that isolates the comparator from the input common-mode signal, resulting in a common-mode rejection ratio of -73 dB. In addition, a circuit that cancels the error caused by the quadratic capacitor voltage coefficient is described, and measurements show that it is capable of increasing the converter linearity by an order of magnitude. © 1990 IEEE