Designing Incremental Sigma-Delta ADCs for low Thermal Noise in Image Sensors

Incremental Sigma-Delta (ISD) ADCs are becoming popular in the imaging field mainly due to 2 characteristics: the intrinsic oversampling of the converter which decreases the thermal noise of the pixel source follower and the reduced accuracy required by the analog building blocks. In this paper we present some design guidelines for choosing the appropriate ISD architecture according to the imager design specifications in terms of conversion speed, power consumption and noise. Furthermore, we propose some novel solutions to common ISD design issues, such as modulator reference buffering, which hinder the implementation of this converter type in high speed, high resolution image sensors. Keywords—Incremental Sigma Delta, ADC, low noise, low power, shared opamp, reference buffering, calibration