Selecting appropriate calibration points for an ultra low area 8-bit subrange ADC

Abstract

An ultra low area 8-bit subrange Analogue/Digital Converter that consists of a pair of Flash 4-bit converter stages is described in this paper emphasising on the appropriate method for its real time calibration. Its active area occupies only 0.04 mm2 and dissipates less than 22 mW. The sampling rate is higher than 500 MS/s and the achieved Signal to Noise and Distortion Ratio is higher than 40 dB. A voltage mode integer divider is used at the input of this Analogue/Digital Converter driving the two output Flash stages by its quotient and residue. The ultra low area and power is owed to the use of the integer divider at the input stage of the subrange converter and the employment of a “thermometer to binary encoder” instead of a full 4-bit Flash Analogue/Digital Converter for the generation of the 4 most significant bits. The calibration method employed for this Analogue/Digital Converter is based on the proper biasing of the resistor ladders used by the fine 4-bit Flash stage as well as on properly delaying and shifting of the divider signals that generate the residue.