Bulk Voltage Trimming Offset Calibration for High-Speed Flash ADCs

A bulk voltage trimming offset calibration technique is presented for flash analog-to-digital converters (ADCs). Offset calibration is achieved by digitally adjusting the bulk voltages of the preamplifier input devices. Without introducing additional capacitive loading in the analog path, this technique improves the accuracy of flash ADCs while not impairing their high-speed performance. A 4-bit ADC in 90-nm CMOS with the proposed technique achieves 3.71 effective number of bits (ENOB) at 5-GS/s sampling rate with 2.5-GHz effective resolution bandwidth (ERBW). The calibration generally improves ENOB by approximately 0.5 bit after calibration. The ADC consumes 86 mW at 5 GS/s with a 2.5-GHz input achieving a 1.32-pJ/convstep figure of merit. The ADC occupies 0.135-mm² chip area.