Optimization of Delta-Sigma ADC for Column-Level Data Conversion in CMOS Image Sensors

A delta-sigma analog-to-digital-converter (ADC) is designed, optimized and simulated for column-level data conversion in a CMOS image sensor. For a 0.18μm process, the design achieves 80dB of signal-to-noise ratio (SNR), including a 10dB margin for kTC noise not simulated, and consumes 210μW of power at a 50kHz sampling rate. Low power is realized mainly by using a first-order architecture and minimizing the capacitors. For the modulator, a boosted-folded-cascode operational transconductance amplifier (OTA) is optimized to achieve a gain of 90dB with a unity-gain bandwidth of 300MHz. The decimator is also optimized by placing part of the circuit at the chip level. Zero distortion is possible in the decimator due to the discrete-time nature of the input signal. The proposed ADC allows a reduction in the read-out nonlinearity of a CMOS image sensor, enabling a high SNR to be realized.