Global shutter image sensor

Abstract

In one example, a method comprises: within an integration period: enabling a photodiode of a pixel cell to accumulate charge responsive to incident light, and transferring the charge from the photodiode to a charge storage device of the pixel cell. The method further comprises: performing, using a sampling capacitor, a sample-and-hold operation to convert the charge stored in the charge storage device into a voltage; and generating a digital output based on the voltage to represent an intensity of the incident light received by the photodiode.

What is claimed is: 
     
         1 . A method comprising:
 within an integration period:
 enabling a photodiode of a pixel cell to accumulate charge responsive to incident light, and 
 transferring the charge from the photodiode to a charge storage device of the pixel cell; 
   performing, using a sampling capacitor, a sample-and-hold operation to convert the charge stored in the charge storage device into a voltage; and   generating a digital output based on the voltage to represent an intensity of the incident light received by the photodiode.   
     
     
         2 . The method of  claim 1 , wherein the sampling-and-hold operation comprises:
 enabling a sampling switch to cause the sampling capacitor to sample the charge stored in the charge storage device to develop the voltage; and   disabling the sampling switch to cause the sampling capacitor to hold the voltage.   
     
     
         3 . The method of  claim 2 , wherein the enabling of the sampling switch starts at a first time within the integration period and ends at a second time after the integration period ends. 
     
     
         4 . The method of  claim 3 , wherein the transferring of the charge from the photodiode to the charge storage device starts after the first time and ends before the second time. 
     
     
         5 . The method of  claim 1 , wherein:
 the pixel cell further includes a shutter switch coupled with the photodiode;   the integration period is started based on disabling the shutter switch; and   the integration period is ended based on enabling the shutter switch.   
     
     
         6 . The method of  claim 1 , wherein the pixel cell further includes:
 a transfer switch coupled between the photodiode and the charge storage device; and   a reset switch coupled with the charge storage device;   wherein the integration period is started based on disabling the reset switch and the transfer switch; and   wherein the integration period is ended based on enabling the reset switch and the transfer switch.   
     
     
         7 . The method of  claim 6 , wherein generating the digital output based on the voltage comprises:
 generating, using a comparator, a decision based on the voltage and a threshold;   controlling a memory to store a count value from a counter based on the decision; and   providing the count value as the digital output.   
     
     
         8 . The method of  claim 7 , wherein the threshold increases or decreases with time; and
 wherein the count value measures a time when the threshold intersects with the voltage.   
     
     
         9 . The method of  claim 7 , further comprising: resetting the comparator prior to transferring the charge. 
     
     
         10 . The method of  claim 9 , wherein the charge storage device is coupled with the comparator via an AC capacitor;
 wherein the voltage is a first voltage;   wherein the method further comprises:
 obtaining, using the AC capacitor, a first sample of a reset voltage of the charge storage device caused by a prior reset operation of the charge storage device; 
 obtaining, using the AC capacitor, a second sample of an offset of the comparator when the comparator is in the reset state; 
 storing a second voltage across the AC capacitor based on the first sample of the reset voltage and the second sample of the offset; and 
 outputting, based on the first voltage and the second voltage, a third voltage to the comparator; and 
   wherein the digital output is generated based on the third voltage.   
     
     
         11 . The method of  claim 7 , wherein an output of the comparator is coupled with the memory via a selection switch; and
 wherein the method further comprises:
 enabling the selection switch to transmit the decision to the memory when the pixel cell is selected to store the digital output in the memory; and 
 disabling the selection switch to block the decision from the memory when the pixel cell is not selected to store the digital output in the memory. 
   
     
     
         12 . The method of  claim 7 , wherein the comparator is coupled with a power supply via a power switch; and
 wherein the method further comprises:
 enabling the power switch to enable the comparator to generate the decision when the pixel cell is selected to store the digital output in the memory; and 
 disabling the power switch to disable the comparator when the pixel cell is not selected to store the digital output in the memory. 
   
     
     
         13 . The method of  claim 7 , wherein the comparator is part of the pixel cell. 
     
     
         14 . The method of  claim 1 , wherein the charge storage device comprises at least one of: a floating drain node, or a pinned storage node. 
     
     
         15 . A method comprising:
 within an integration period:
 enabling a first photodiode to accumulate a first charge responsive to incident light; 
 transferring the first charge from the first photodiode to a first charge storage device; 
 enabling a second photodiode to accumulate a second charge responsive to the incident light; and 
 transferring the second charge from the second photodiode to a second charge storage device; 
   performing, using a first sampling capacitor, a first sample-and-hold operation to convert the first charge stored in the first charge storage device into a first voltage;   performing, using a second sampling capacitor, a second sample-and-hold operation to convert the second charge stored in the second charge storage device into a second voltage;   generating a first digital output based on the first voltage to represent an intensity of the incident light received by the first photodiode;   storing the first digital output at a memory;   reading the first digital output from the memory;   generating a second digital output based on the second voltage to represent an intensity of the incident light received by the second photodiode; and   after the first digital output is read from the memory, storing the second digital output at the memory.   
     
     
         16 . The method of  claim 15 , further comprising:
 within the integration period:
 enabling a third photodiode to accumulate a third charge responsive to incident light; and 
 transferring the third charge from the third photodiode to a third charge storage device; 
   performing, using a third sampling capacitor, a third sample-and-hold operation to convert the third charge stored in the third charge storage device into a third voltage;   generating a third digital output based on the third voltage to represent an intensity of the incident light received by the third photodiode; and   storing the first digital output and the third digital output at the memory prior to reading of the first digital output from the memory.   
     
     
         17 . The method of  claim 16 , wherein the first photodiode is part of a first pixel cell;
 wherein the second photodiode is part of a second pixel cell; and   wherein the third photodiode is part of a third pixel cell.   
     
     
         18 . The method of  claim 16 , wherein the first pixel cell and the third pixel cell are in a first row of pixel cells of an image sensor; and
 wherein the second pixel cell is in a second row of the pixel cells of the image sensor.   
     
     
         19 . The method of  claim 16 , wherein each of the first, second, and third digital outputs is generated using, respectively, a first comparator, a second comparator, and a third comparator; and
 wherein the first, second, and third comparators are reset during, respectively, the first, second, and third sample-and-hold operations.   
     
     
         20 . The method of  claim 19 , wherein:
 the first comparator is part of the first pixel cell;   the second comparator is part of the second pixel cell; and   the third comparator is part of the third pixel cell.