Column-parallel sigma-delta analog-to-digital conversion for imagers

An analog-to-digital converter for an imager, said analog-to-digital converter comprising:

• a sensing circuit comprising:
• a first circuit for inputting a reset signal voltage and creating a reset current,
• a second circuit for inputting a pixel signal voltage and creating a pixel current, and
• an adjustment circuit for creating an adjustment current from a reference voltage by modulating a resistance in said adjustment circuit, said adjustment current being combined with the reset current when the reset current does not equal the pixel current;
• and
• a counter for counting a number of times the resistance is modulated and outputting a digital code corresponding to a difference between the pixel signal voltage and the reset signal voltage.

An imager comprising:

• a pixel array, said pixel array comprising a plurality of columns of pixels;
• a sample and hold circuit coupled to at least one column of said array;
• and
• an analog-to-digital converter coupled to input reset and pixel signal voltages from said sample and hold circuit, said analog to digital-converter comprising:
• a first circuit for inputting the reset voltage and creating a reset current,
• a second circuit for inputting the pixel signal voltage and creating a pixel current, and
• an adjustment circuit for creating an adjustment current from a reference voltage by modulating a resistance in said adjustment circuit, said adjustment current being combined with the reset current when the reset current does not equal the pixel current;
• and
• a counter for counting a number of times the resistance is modulated and outputting a digital code corresponding to a difference between the pixel signal voltage and the reset signal voltage.

A processor system comprising:

• a processor;
• and
• an imager coupled to said processor, said imager comprising a pixel array, said pixel array comprising a plurality of columns of pixels, a sample and hold circuit coupled to at least one column of said array, and an analog-to-digital converter coupled to input reset and pixel signal voltages from said sample and hold circuit, said analog to digital-converter comprising:
• a first circuit branch for inputting the reset voltage and creating a reset current,
• a second circuit branch for inputting the pixel signal voltage and creating a pixel current, and
• an adjustment branch for creating an adjustment current from a reference voltage by modulating a resistance in said adjustment branch, said adjustment current being combined with the reset current when the reset current is less than the pixel current;
• and
• a counter for counting a number of times the resistance is modulated.

A method of operating an imager, said method comprising the steps of:

• inputting a reset signal voltage and creating a reset current based on a first resistance;
• inputting a pixel signal voltage and creating a pixel current based on a second resistance;
• and
• creating an adjustment current by modulating a resistance and applying a reference voltage to the modulated resistance;
• combining the adjustment current with the reset current;
• and
• counting a number of times the resistance is modulated.