Analog-to-digital conversion in pixel arrays
An analog-to-digital converter (ADC) generates an output digital value equivalent to the difference between two analog signal values. The ADC receives a first analog signal level, a second analog signal level and a ramp signal. A counter is operable to count in a single direction. A control stage is arranged to enable the counter based on a comparison of the ramp signal with the first analog signal and the second analog signal. A digital value accumulated by the counter during a period when it is enabled forms the output. The ADC can perform the conversion during a single cycle of the ramp signal. The counter can be loaded with a starting digital value representing an exposure level accumulated during a previous exposure period. Techniques are described for reducing the conversion time.
1. An analog-to-digital converter for generating an output digital value equivalent to the difference between two analog signal values comprising:
at least one input for receiving a first analog signal level and a second analog signal level;
an input for receiving a ramp signal;
a counter which is operable to count in a single direction;
a control stage which is arranged to enable the counter based on a comparison of the ramp signal with the first analog signal and the second analog signal;
an output for outputting a value accumulated by the counter during a period when it is enabled;
wherein the control stage is arranged to accumulate digital signal values for elements of an image over multiple exposure periods and is further arranged to begin the counter at a starting digital value which represents a previously accumulated exposure level for an element of the image.
2. An analog-to-digital converter according to claim 1 further comprising a connection to a memory, and wherein the control stage is further arranged to:
read a digital value from the memory at the beginning of a processing operation for an exposure period, which digital value represents a previously accumulated exposure level of an element of the image;
output a digital value to the memory at the end of a processing operation for an exposure period, which digital value represents a new accumulated exposure level of the element of the image.
3. An analog-to-digital converter according to claim 1 wherein the control stage is arranged to determine whether to enable the counter based on the accumulated exposure level for an element of the image.
4. An analog-to-digital converter according to claim 3 wherein the control stage is arranged to make an independent determination of whether to enable the counter for each element of the image.
5. An analog-to-digital converter according to claim 1 wherein the control stage is arranged to use a dedicated counter to accumulate the digital signal value for a particular element of the image across the multiple exposure periods.
6. An analog-to-digital converter according to claim 1 wherein the pixel array is operable to perform time delay integration (TDI) and each exposure period is a TDI cycle.