IMAGING SYSTEMS HAVING IMPROVED ANALOG-TO-DIGITAL CONVERTER CIRCUITRY
An image sensor may include an array of image pixels coupled to analog-to-digital conversion circuitry. To reduce space on the integrated circuit on which the image sensor is formed, the analog-to-digital conversion circuitry may have at least some portions that are shared between columns of the array of image pixels. In particular, individual delta-sigma modulators may be coupled to each column of the pixels to convert the charge generated by the pixels into digital bits, and shared accumulator circuitry may convert the digital bits into a digital signal. The shared accumulator circuitry may use pre-determined weights to reduce the number of required accumulation steps, and sequentially process the bits corresponding to two or more columns of the pixels. The digital signal may be stored in a shift register to further reduce the area required for the analog-to-digital conversion circuitry.
1 . An image sensor comprising:
an array of pixels arranged in rows and columns, wherein the pixels are configured to generate charge in response to incident light; and
analog-to-digital conversion circuitry coupled to the array of pixels, wherein the analog-to-digital conversion circuitry comprises:
a plurality of delta-sigma modulators configured to convert the generated charge into digital signal bits, wherein each of the delta-sigma modulators is coupled to one of the columns of pixels, and
shared accumulator circuitry configured to produce a digital signal from the digital signal bits, wherein at least two of delta-sigma modulators are coupled to the shared accumulator circuitry.
2 . The image sensor defined in claim 1 further comprising:
shared storage circuitry coupled to the shared accumulator circuitry, wherein the shared storage circuitry is configured to store the digital signal corresponding to each of the pixel columns that are associated with the shared accumulator circuitry.
3 . The image sensor defined in claim 2 wherein the shared storage circuitry comprises shift registers and wherein one of the shift registers is configured to sequentially store the digital signals associated with all of the pixel columns.
4 . The image sensor defined in claim 3 further comprising:
a plurality of additional shift registers, wherein each of the additional shift registers is coupled to a given one of the delta-sigma modulators.
5 . The image sensor defined in claim 4 wherein the shared accumulator circuitry is configured to use pre-determined weights in combination with the digital signal bits to produce the digital signal.
6 . The image sensor defined in claim 5 wherein the shared accumulator circuitry comprises a hybrid weight adder that uses the pre-determined weights and the digital signal bits to produce the digital signal.
7 . The image sensor defined in claim 6 wherein the hybrid weight adder comprises first and second multiplexers coupled to an adder, and wherein the first and second multiplexers each select a given one of the pre-determined weights to send to the adder to be combined with the digital bits.
8 . The image sensor defined in claim 7 wherein the pre-determined weights are the same for each column of pixels across the array of pixels.
9 . The image sensor defined in claim 7 wherein the pre-determined weights vary for the columns of pixels across the array of pixels.
10 . The image sensor defined in claim 5 wherein the additional shift registers are configured to store different numbers of digital bits based on the column of pixels with which the additional shift registers are associated.
11 . A method of operating an image sensor and processing circuitry, the method comprising:
generating charge in response to incident light using image pixels that are arranged in rows and columns in an array of image pixels;
generating digital signal bits from the generated charge using delta-sigma modulators that are each coupled to a given one of the columns of image pixels in the array;
accumulating the digital signal bits associated with a first column of the pixels;
performing weighted addition using the accumulated digital signal bits associated with the first column of the pixels and pre-determined weights to produce a first digital signal;
accumulating the digital signal bits associated with a second column of the pixels; and
performing weighted addition using the accumulated digital signal bits associated with the second column of the pixels and pre-determined weight to produce a second digital signal.
12 . The method defined in claim 11 wherein performing the weighted addition comprises performing hybrid weighted addition using the accumulated digital signal bits and the pre-determined weights.
13 . The method defined in claim 12 further comprising:
after producing the first digital signal, storing the first digital signal in a first shift register; and
after producing the second digital signal, shifting the first digital signal to a second shift register and storing the second digital signal in the first shift register.
14 . The method defined in claim 11 wherein accumulating the digital signal bits associated with the first column of the pixels comprises storing the digital signal bits in a first shift register and wherein accumulating the digital signal bits associated with the second column of the pixels comprises storing the digital signal bits in a second shift register.
15 . The method defined in claim 14 wherein storing the digital signal bits in the first register comprises storing a first number of digital signal bits in the first register and wherein storing the digital signal bits in the second register comprises storing a second number of digital signal bits in the second register, and wherein the second number of digital signal bits is greater than the first number of digital signal bits.
16 . An image sensor comprising:
an array of pixels arranged in rows and columns;
a plurality of delta-sigma modulators, wherein each column of pixels is coupled to a given one of the delta-sigma modulators; and
shared accumulation circuitry coupled to at least two of the delta-sigma modulators.
17 . The image sensor defined in claim 16 further comprising:
shared storage circuitry coupled to the shared accumulation circuitry.
18 . The image sensor defined in claim 17 wherein the shared storage circuitry comprises a plurality of shift registers and wherein the shared accumulation circuitry comprises hybrid weight addition circuitry.
19 . The image sensor defined in claim 18 further comprising:
an additional plurality of shift registers coupled between the at least two delta-sigma modulators and the shared accumulation circuitry.
20 . The image sensor defined in claim 19 wherein the delta-sigma modulators are configured to generate digital signal bits that correspond to charge generated by the columns of the pixels and wherein the hybrid weight addition circuitry is configured to generate digital signals based on the digital signal bits and pre-determined weights.