Image sensor ADC and CDS per column
View Patent ↗A solid state imager includes an arrangement for converting analog pixel values to digital form on an arrayed per-column basis. An N-bit counter supplies an N-bit DAC to produce an analog ramp output providing a ramp signal with a level that varies corresponding to the contents of the counter. Latches or equivalent digital storage elements are each associated with a respective column. A counter bus connects the counter to latch inputs of said latches, and comparators associated the columns gate the latches when the analog ramp equals the pixel value for that column. The contents of the latch elements are transferred sequentially to a video output bus to produce the digital video signal. There can be additionally black-level readout latch elements, for storing a digital value that corresponds to the dark or black video level, and a subtraction element subtracts the black level value from the pixel value to reduce fixed pattern noise. An additional array of buffer latches can be employed.
1. Arrangement for converting analog pixel values from an array of pixels to a digital video signal, said array comprising a plurality of columns and at least one row, each column having at one pixel therein, and each column providing a respective pixel value; the arrangement comprising
an N-bit counter;
an N-bit DAC connected to said counter and having a ramp output providing a signal corresponding to a count existing on said counter;
a plurality of digital storage elements each associated with a respective one of said columns;
a counter bus connecting an output of said counter to inputs of said digital storage elements;
a plurality of comparators each associated with a respective one of said columns, and having one input connected to receive the respective column pixel value, another input connected to the ramp output of said N-bit counter, and a comparator output connected to a gating terminal of the respective digital storage element;
a video readout bus; and
means selectively transferring contents of said digital storage elements to said video output bus to produce said digital video signal.
2. Arrangement of claim 1 wherein each said digital storage element is a latch element.
3. Arrangement of claim 1 further comprising a plurality of black-level readout latch elements, each associated with a respective one of said columns; each black level readout latch element having a latch input connected to said counter bus and a gating terminal connected to the respective comparator output; a black-level readout bus; means for transferring the contents of the respective black-level latch elements to the black-level readout bus simultaneously with the transfer of the contents of the associated digital storage elements to said video readout bus; and a subtraction element connected to the video readout bus and the black-level readout bus.
4. Arrangement of claim 1 wherein said digital storage elements each include a first latch member having an input connected to said counter bus, and a buffer latch member, having an input connected to an output of the respective first latch member, a gate terminal, and an output connected to the video readout bus; and means providing a transfer signal to the gate terminals of said buffer latch members.
5. Arrangement of claim 1 , wherein said video readout bus comprises a sequence of DQ flip-flops.
6. Arrangement for converting analog pixel values from an array of pixels to a digital video signal, said array comprising a plurality of columns and at least one row, each column having at one pixel therein, and each column providing a respective pixel value; the arrangement comprising an N-bit counter; an N-bit DAC connected to said counter and having a ramp output providing a signal corresponding to a count existing on said counter; a plurality of digital storage elements each associated with a respective one of said columns; a counter bus connecting an output of said counter to inputs of said digital storage elements; a plurality of comparators each associated with a respective one of said columns; wherein each comparator is made up of a plurality of differential-input amplifiers, each of the differential amplifiers having a plurality of first input transistors, one of the first input transistors being located at each of the pixels within the periphery, and a second input transistor being located outside the periphery of the array and coupled to the first input transistors and to an output configuration circuit, and having one input connected to receive the respective column pixel value, another input connected to the ramp output of said N-bit DAC, and a comparator output connected to a gating terminal of the respective digital storage element; a video readout bus; and means selectively transferring contents of said digital storage elements to said video output bus to produce said digital video signal.
7. Arrangement of claim 6 wherein each said digital storage element is a latch element.
8. Arrangement of claim 6 further comprising a plurality of black-level readout latch elements, each associated with a respective one of said columns; each black level readout latch element having a latch input connected to said counter bus and a gating terminal connected to the respective comparator output; a black-level readout bus; means for transferring the contents of the respective black-level latch elements to the black-level readout bus simultaneously with the transfer of the contents of the associated digital storage elements to said video readout bus; and a subtraction element connected to the video readout bus and the black-level readout bus.
9. Arrangement of claim 6 wherein said digital storage elements each include a first latch member having an input connected to said counter bus, and a buffer latch member, having an input connected to an output of the respective first latch member, a gate terminal, and an output connected to the video readout bus; and means providing a transfer signal to the gate terminals of said buffer latch members.
10. Arrangement of claim 6 , wherein said video readout bus comprises a sequence of DQ flip-flops.