Capacitor-Swapping Cyclic A/D Conversion Techniques With Reduced Mismatch Sensitivity

Abstract

This work proposes two capacitor-swapping techniques, random feedback-capacitor interchanging (RFCI) and averaging RFCI (ARFCI) techniques, for cyclic analog-to-digital converters (ADCs) to reduce the harmonic distortion caused by capacitor mismatch without trimming or calibration. The proposed RFCI and ARFCI techniques can be realized by simply rearranging the capacitor connections of the ADCs in different operation cycles. Hence, complicated circuits are not needed. The RFCI technique improves upon the spurious-free dynamic range (SFDR) of conventional ADCs without sacrificing the signal-to-noise-and-distortion ratio (SNDR). The ARFCI technique has better SNDR characteristics but less SFDR improvement than RFCI. With RFCI and ARFCI, the capacitor matching requirement is relaxed for high SFDR and the capacitance can then be reduced to meet the SNDR requirement, reducing the driving capability of the opamps, and thus reducing the total power and area of the ADCs. The prior commutated feedback-capacitor switching (CFCS) technique (see Yu , 1996) has less effect on the SFDR of cyclic ADCs but improves the signal-to-noise ratio (SNR). This work proposes a reconfigurable cyclic ADC architecture that can be easily reconfigured to operate with one of the RFCI, ARFCI, and CFCS techniques by a simple timing control circuit. This reconfigurable topology provides three conversion characteristics with one item of intellectual property (IP), rather than three separate IPs and thus greatly enhances the capabilities of cyclic ADCs.