Variable gain column amplifier adapted for use in imaging arrays
An imaging sensor using a novel bit line processing circuit, that circuit, and the method of processing the pixel outputs from an image sensor using that processing circuit are disclosed. The image sensor includes an array of pixel sensors, a signal digitizing circuit, and a digitizing controller. Each pixel sensor generates a voltage signal that is a function of a charge on the photodetector in that pixel sensor, and couples that voltage signal to a bit line in response to a first signal. The signal digitizing circuit is connected to the bit line, the digitizing circuit converting the voltage signal to a plurality of output digital values, the output digital values having selectable levels of digitization noise. The digitizing controller generates the level of noise based on the voltage signal. The signal digitizing circuit includes a variable gain amplifier and an ADC having a fixed number of bits.
1. An apparatus comprising:
an array of pixel sensors, each pixel sensor comprising a photodetector, a charge conversion circuit, and a gate transistor, said charge conversion circuit generating a voltage signal that is a function of a charge on said photodetector, and said gate transistor coupling said voltage signal to a bit line in response to a first signal;
a signal digitizing circuit connected to said bit line, said digitizing circuit converting said voltage signal to a plurality of output digital values, said output digital values having a plurality of selectable levels of quantization noise, one of said selectable levels of quantization noise being selected in response to an output control signal; and
a digitizing controller that generates said output control signal based on said voltage signal, wherein said digitizing controller comprises a variable gain amplifier having a plurality of gain values that amplifies said voltage signal to generate an amplified signal that is digitized by an analog-to-digital converter (ADC) having a fixed number of bits.
2. The apparatus of claim 1 further comprising a double sampling circuit that stores a first voltage that represents a measurement of a reset voltage measured at a first gain value and a second voltage that measures said voltage signal, said double sampling circuit outputting a measurement of said amplified signal minus said first voltage divided by a factor that depends on which of said plurality of gain values was used to generate said amplified signal.
3. The apparatus of claim 1 wherein said variable gain amplifier comprises a capacitive transimpedance amplifier having a feedback capacitance that is determined by said output control signal.
4. The apparatus of claim 3 wherein said digitizing controller comprises a latching comparator that compares an output of said capacitive transimpedance amplifier to a reference value.
5. The apparatus of claim 4 wherein said output digital values comprise a value indicative of said output control signal.
6. An apparatus comprising:
a signal digitizing circuit connected to an input line, said signal digitizing circuit converting a voltage signal on said input line to a plurality of output digital values, each output digital value having a different level of digitization noise, one of said output digital values being selected for output in response to an output control signal; and
a digitizing controller that generates said output control signal based on said voltage signal, wherein said signal digitizing controller comprises an amplifier having a plurality of gain values that amplifies said voltage signal to generate an amplified signal that is digitized by an analog-to-digital converter (ADC) having a fixed number of bits.
7. The apparatus of claim 6 wherein said variable gain amplifier comprises a capacitive transimpedance amplifier having a feedback capacitance that is determined by said output control signal.
8. The apparatus of claim 7 wherein said output selection signal comprises a latching comparator that compares an output of said capacitive transimpedance amplifier to a reference value.
9. The apparatus of claim 8 wherein said output digital values comprise a value indicative of said output control signal.
10. A method for operating an imaging array comprising an array of pixel sensors, each pixel sensor comprising a photodetector, a charge conversion circuit, and a gate transistor, said charge conversion circuit generating a voltage signal that is a function of a charge on said photodetector, and said gate transistor coupling said voltage signal to a bit line in response to a first signal, said method comprising:
coupling one of said pixel sensors to said bit line;
amplifying said voltage signal corresponding to said one of said pixel sensors with a variable gain capacitive transimpedance amplifier to generate an output analog signal, said variable gain capacitive transimpedance amplifier having a variable gain determined by said voltage signal; and
digitizing said output analog signal with an analog-to-digital converter (ADC) having a fixed number of bits.
11. The method of claim 10 further comprising storing a first voltage that represents a reset voltage measured at a first gain value and a second voltage that measures said output analog signal; and
correcting said output analog signal by an amount that depends on said first voltage divided by a factor that depends on said variable gain.
12. The method of claim 10 wherein said variable gain capacitive transimpedance amplifier has a capacitive feedback circuit having a plurality of different capacitance values and said capacitive feedback circuit has a capacitance that is set by selecting one of said different capacitance values in response to said voltage signal being coupled to said bit line.