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 oversampling 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 uses 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 and IR 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.