An apparatus adapted to convert an input analog signal to an output digital signal includes means adapted to convert a first digital value of the output signal of the apparatus to a first analog value (IN1), wherein the first digital value is emitted by the apparatus at a first time instant, and also means for feeding back the first analog value to the input of the apparatus. The apparatus includes first means adapted to determine the difference between the first analog value and a second analog value (IN) of the input signal at a second time instant successive to the first time instant. The apparatus also includes second means adapted to convert the difference to a digital value and third means adapted to add or subtract the digital value of the difference to or from said first digital value by obtaining the output digital signal.