ANALOG-TO-DIGITAL CONVERSION IN IMAGE SENSORS
First Claim
1. A circuit, comprising:
- a correlated double sample circuit comprising a first input for receiving an analog signal from a pixel of a pixel array, the signal varying during a pixel readout period and having a first level during a first reset period and a second level during a second read period, a second input for receiving a time variant reference signal, and two sample capacitors; and
a comparator circuit that compares the time variant reference signal and the analog signal;
wherein the analog signal from the pixel and the time variant reference signal are constantly read onto one of the two sample capacitors respectively of the correlated double sample circuit during both the first reset period and the second read period; and
wherein the comparator circuit comprises a differential amplifier having one input receiving the combination of the analog pixel signal and the time variant reference signal, and a second input receiving a fixed reference signal.
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Abstract
An image sensor has a per-column ADC arrangement including first and second capacitors for correlated double sampling, and a comparator circuit. The capacitors are continuously connected to, respectively, the analog pixel signal and a ramp signal without use of a hold operation. The comparator circuit comprises a differential amplifier having one input connected to the junction of the two capacitors and another input connected to a reference signal. The reference signal is preferably sampled and held from a reference voltage. The use of a differential amplifier as first stage of the comparator addresses problems arising from ground voltage bounce when a large pixel array images a scene with low contrast.
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Citations
18 Claims
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1. A circuit, comprising:
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a correlated double sample circuit comprising a first input for receiving an analog signal from a pixel of a pixel array, the signal varying during a pixel readout period and having a first level during a first reset period and a second level during a second read period, a second input for receiving a time variant reference signal, and two sample capacitors; and a comparator circuit that compares the time variant reference signal and the analog signal; wherein the analog signal from the pixel and the time variant reference signal are constantly read onto one of the two sample capacitors respectively of the correlated double sample circuit during both the first reset period and the second read period; and wherein the comparator circuit comprises a differential amplifier having one input receiving the combination of the analog pixel signal and the time variant reference signal, and a second input receiving a fixed reference signal. - View Dependent Claims (2, 3, 4)
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5. A device comprising:
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an image sensor array including pixel element; a correlated double sample circuit comprising a first input for receiving an analog signal from the pixel element, the signal varying during a pixel readout period and having a first level during a first reset period and a second level during a second read period, a second input for receiving a time variant reference signal, and two sample capacitors; and a comparator circuit that compares the time variant reference signal and the analog signal; wherein the analog signal from the pixel and the time variant reference signal are constantly read onto one of the two sample capacitors respectively of the correlated double sample circuit during both the first reset period and the second read period; and wherein the comparator circuit comprises a differential amplifier having one input receiving the combination of the analog pixel signal and the time variant reference signal, and a second input receiving a fixed reference signal. - View Dependent Claims (6, 7, 8, 9, 10, 11)
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12. A method of converting an analog signal from an image sensor pixel, during a pixel readout period to a digital signal using an analog to digital converter circuit comprising a correlated double sample circuit having first and second sample capacitors, a time varying reference circuit enabled to provide a time varying reference signal, the first sample capacitor being continuously connected to the time varying reference circuit and the second sample capacitor being continuously connected to the pixel during the pixel readout period, the method comprising:
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(i) resetting the pixel, thereby sampling an analog signal equivalent to a pixel reset voltage onto the second capacitor; (ii) removing the pixel from reset, wherein the analog signal is corrected for pixel reset noise on the second capacitor; (iii) integrating light received on the pixel, wherein the analog signal is reduced according to the amount of light received; (iv) providing the time varying reference signal; (v) comparing the analog signal with the time varying reference signal; and (vi) outputting an output signal when the time variant reference signal corresponds to the analog signal; wherein;
said comparing is effectuated by applying the combination of the analog pixel signal and the time variant reference signal to one input of a differential amplifier, and applying a fixed reference signal to the other input of the differential amplifier. - View Dependent Claims (13, 14)
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15. A circuit, comprising:
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a pixel element outputting a voltage signal; a first capacitor connected to receive the voltage signal; a second capacitor connected to receive a ramping reference voltage signal; and a differential comparator having a first input connected to receive a summed output from the first and second capacitors and having a second input for receiving a fixed reference voltage. - View Dependent Claims (16, 17, 18)
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Specification