An analog-to-digital conversion (ADC) system is operated with a duty cycle. During the ON period, the ADC circuits perform analog-to-digital conversions of an analog input signal. During the Standby period, the ADC system is in either a standby state or a foreground calibration state. The ADC system operates in a reduced-power mode in the standby state. In the foreground calibration state, the ADC system performs a portion of a foreground calibration cycle during a calibration time slot. The foreground calibration cycle is performed over multiple calibration time slots. The foreground calibration cycle and the calibration time slots are configurable by changing the values of control registers that represent calibration parameters.