Techniques for pixel-level analog-to-digital conversion

Two techniques for performing pixel level analog to digital conversion (ADC) are reviewed. The first is an over-sampling technique which uses a one bit first order (Sigma) (Delta) modulator for each 2 X 2 block of pixels to directly convert photocharge to bits. Each modulator is implemented using 17 transistors. The second technique is a Nyquist rate multi-channel-bit-serial (MCBS) ADC. The technique use successive comparisons to convert the pixel voltage to bits. Results obtained from implementations of these ADC techniques are presented. The techniques are compared based on size, charge handling capacity, FPN, noise sensitivity, data throughput, quantization, memory/processing, and power dissipation requirements for both visible an dIR imagers. From the comparison it appears that the (Sigma) (Delta) ADC is better suited to IR imagers, while the MCBS ADC is better suited to imagers in the visible range.

[1]  Abbas El Gamal,et al.  A Nyquist rate pixel level ADC for CMOS image sensors , 1998, Proceedings of the IEEE 1998 Custom Integrated Circuits Conference (Cat. No.98CH36143).

[2]  Gabor C. Temes,et al.  Oversampling Delta Sigma Data Converters , 1991 .

[3]  Christer Jansson,et al.  CMOS analog-to-digital conversion for uncooled bolometer infrared detector arrays , 1995, Defense, Security, and Sensing.

[4]  Bedabrata Pain,et al.  Low-power low-noise analog circuits for on-focal-plane signal processing of infrared sensors , 1993, Defense, Security, and Sensing.

[5]  M. E. Cox.  Handbook of Optics , 1980 .

[6]  Michael D. Godfrey.  CMOS device modeling for subthreshold circuits , 1992 .

[7]  Woodward Yang,et al.  A wide-dynamic-range, low-power photosensor array , 1994, Proceedings of IEEE International Solid-State Circuits Conference - ISSCC '94.

[8]  Eric R. Fossum,et al.  Active pixel sensors: are CCDs dinosaurs? , 1993, Electronic Imaging.

[9]  B. Fowler,et al.  A CMOS area image sensor with pixel-level A/D conversion , 1994, Proceedings of IEEE International Solid-State Circuits Conference - ISSCC '94.