Abstract:
An active pixel sensor includes mixed analog and digital signal integration on the same substrate. The analog part of the array forms the active pixel sensor, and the digital part of the array does digital integration of the signal.

Description:
BACKGROUND 
   Active pixel sensors are well known in the art. A basic description of the active pixel sensors found in U.S. Pat. No. 5,471,515, the disclosure of which is incorporated by reference to the extent necessary for proper understanding. 
   An active pixel sensor, and many other image sensors, have inherent trade-offs. Typically, the trade-off is made between sensitivity, versus motion resolution, versus space resolution. 
   For example, we obtain sensitivity by increasing the integration time. However, with a higher integration time, motion becomes more choppy, and hence motion sensitivity is decreased. Sensitivity can also be increased by increasing the pixel size. However, space resolution then decreases, again supporting the trade-off. 
   Integrated circuit designers continually attempt to put more circuitry on a chip. Lines on the chip are becoming smaller: for example, current technology may use a 0.11 micron process for digital circuitry. However, the image sensor, which is effectively analog, may be subject to a physical minimum size. A pixels that has too small a size and/or high gain, would have insufficient capacitance to allow the sensor to obtain the signal to noise ratio required for quality image acquisition. 
   SUMMARY 
   The inventor recognized that memory size can form an effective tradeoff against pixel size. The present specification describes receiving information in an analog photosensor array, and integrating that information in on-chip digital memory. According to this system, an analog array is placed on the same substrate with a digital memory. The information from the analog array is sampled periodically, and the integration is carried out in the digital memory. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a basic block diagram; and 
       FIGS. 2 and 3  respectively show more detailed block diagrams of the circuitry. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The basic system is shown in  FIG. 1 . An analog image detector  100 , preferably a CMOS image sensor, reads out the image at some time period, e.g., between 1 microsecond and 1 millisecond. Each pixel is coupled to a digital memory  110 . Digital memory integrates the instantaneous information received from the pixels. 
   Current frame times are preferably either approximately 33 milliseconds for a 30-frames per second system, or approximately 16 milliseconds for high motion resolution of 60 frames per second. 
   In addition to the other advantages noted above, this architecture allows pixel capacitance to be reduced and pixel gain to be increased, since the pixel need provide only instantaneous values, and does not need to integrate the incoming charge. 
   The signal integration process is divided into two parts: an analog part in the active pixel sensor  100  and a digital part in digital random access memory  110 . 
   A first embodiment is shown in  FIG. 2 .  FIG. 2  shows the active pixel sensor array  100 , coupled with an analog signal processor  202 , column A/D converters  204 , a digital processor  206 , and a digital memory array  110   a  of the digital memory  110 . The analog signal processor  202  includes column analog double sampling circuitry for sampling both signals and references to decrease the pixel fixed pattern noise. Preamplifiers with adjustable gains, can also be used to increase the sensitivity and provide an automatic exposure control, as is known in the art. 
   The system as described herein uses column parallel A/D conversion, where one separate A/D converter is provided for each column of the active pixel sensor array. In this system, digital integration may be used for oversampling the A/D converter. Digital sampling can reduce the quantization noise density, and hence increase the effective resolution of the system proportionally to arise of the frame bit. Preferably the system operates with an AC input noise of about half of the least significant bit. 
   The digital signal processor (DSP)  206  provides arithmetic operations such as addition, subtraction, division, and multiplication, and also includes a buffer memory to maintain intermediate results. DSP  206  can also act to digitally correct column digital fixed pattern noise.  FIG. 3  shows a system similar to that in  FIG. 2  but with twice as many digital memory arrays  110   a  and processing circuits  206 . 
   In operation, the sensor is preferably a CMOS image sensor that is of a sufficiently small size that it cannot integrate for a desired frame period. The information from the sensor is sampled by the column A/D converts  204  at an oversampled rate. Each sample is stored in the digital memory array  110   a , and the values are integrated in that memory  110   a . A digitally integrated value can be subsequently read from the digital memory array  110   a.    
   Although only a few embodiments have been disclosed in detail above, other modifications are possible in the preferred embodiment.