Low-power/small-area pixel readout with reciprocal response
An imaging device includes a pixel-circuit and a readout circuit. The pixel-circuit includes a photodetector configured to accumulate electrical charge in response to incident light and to transfer the electrical charge to a storage node. The readout circuit is configured to (i) charge a capacitance with a charge current that depends on the electrical charge in the storage node while comparing a voltage on the capacitance to a threshold, and (ii) generate an output signal indicative of a time duration needed to charge the capacitance until reaching the threshold, the time duration being inversely-related to the electrical charge accumulated by the photodetector.
1 . An imaging device, comprising:
a pixel-circuit, comprising a photodetector configured to accumulate electrical charge in response to incident light and to transfer the electrical charge to a storage node; and
a readout circuit, configured to:
charge a capacitance with a charge current that depends on the electrical charge in the storage node while comparing a voltage on the capacitance to a threshold; and
generate an output signal indicative of a time duration needed to charge the capacitance until reaching the threshold, the time duration being inversely-related to the electrical charge accumulated by the photodetector.
2 . The imaging device according to claim 1 , wherein the readout circuit is configured to represent the output signal by a digital value.
3 . The imaging device according to claim 2 , wherein a resolution of the digital value is set according to a maximum signal-to-noise ratio (SNR) that accounts for (i) the SNR of the output signal and (ii) quantization noise.
4 . The imaging device according to claim 1 , wherein the time duration needed to charge the capacitance is inversely-related to a difference between the electrical charge accumulated by the photodetector and a defined offset.
5 . The imaging device according to claim 4 , wherein the pixel circuit is further configured to set the defined offset responsively to a reset charge.
6 . The imaging device according to claim 1 , further comprising a column line that connects a group of pixel circuits to the readout circuit.
7 . The imaging device according to claim 1 , wherein the pixel circuit comprises an amplifier that is configured to drive a column line with a column-line voltage indicative of the electrical charge on the storage node.
8 . The imaging device according to claim 1 , wherein the readout circuit is further configured to sink a column-line current from a column line, and to set the charge current that charges the capacitance depending on the sunk current.
9 . A method for imaging, comprising:
accumulating electrical charge by a photodetector in response to incident light, and transferring the electrical charge to a storage node;
charging a capacitance with a charge current that depends on the electrical charge in the storage node while comparing a voltage on the capacitance to a threshold; and
generating an output signal indicative of a time duration needed to charge the capacitance until reaching the threshold, the time duration being inversely-related to the electrical charge accumulated by the photodetector.
10 . The method according to claim 9 , further comprising representing the output signal by a digital value.
11 . The method according to claim 10 , wherein a resolution of the digital value is set according to a maximum signal-to-noise ratio (SNR) that accounts for (i) the SNR of the output signal and (ii) quantization noise.
12 . The method according to claim 9 , wherein the time duration needed to charge the capacitance is inversely-related to a difference between the electrical charge accumulated by the photodetector and a defined offset.
13 . The method according to claim 12 , further comprising setting the defined offset responsively to a reset charge.
14 . The method according to claim 9 , further comprising, using an amplifier, driving a column line with a column-line voltage indicative of the electrical charge on the storage node.
15 . The method according to claim 9 , further comprising sinking a column-line current from a column line, and setting the charge current that charges the capacitance depending on the sunk current.