Oversampling noise-shaping successive approximation adc

Abstract

A successive approximation Analog to Digital Converter (ADC), comprising: a sample and hold device arranged to sample and hold an input signal at the beginning of a conversion cycle; a successive approximation register that sequentially builds up a digital output from its most significant bit to its least significant bit; a digital to analog converter that outputs a signal based on the output of the successive approximation register; a comparator that compares the output of the digital to analog converter with an output of the sample and hold device and supplies its output to the successive approximation register; and a residual signal storage device arranged to store the residual signal at the end of a conversion cycle; and wherein the successive approximation ADC is arranged to add the stored residual signal from the residual signal storage device to the input signal stored on the sample and hold device at the start of each conversion cycle. After each ADC full conversion by the SAR, the analog conversion of the digital output is as close to the original input signal as the resolution will allow. However there remains the residual part of the input signal that is smaller than what can be represented by the least significant bit of the digital output of the SAR. In normal operation, successive outputs of a SAR for the same input will result in the same digital value output and the same residual. By storing the residual at the end of each conversion and adding the residual onto the input signal of the next conversion the residuals are accumulated over time so that they may affect the output digital value. After a number of conversions, the accumulated residuals add up to more than the value represented by the LSB of the register and the digital value will be one higher than if a conversion had been performed on the input signal alone. In this way, the residual signal affects the output value in time and thus can be taken into account by processing the digital output in the time domain.