Feedthrough-compensated image sensor

1. An integrated-circuit image sensor having pixels disposed in constituent rows and columns of a pixel array, the integrated-circuit image sensor comprising:

a first photodetector that accumulates photocharge in response to incident light;

a first and second capacitive nodes;

a first transfer gate disposed between the photodetector and the first capacitive node and having a control terminal coupled to a transfer-gate control line that extends in a row-wise direction across the pixel array;

a reset transistor coupled between a supply voltage node and the first capacitive node;

an output transistor having a control terminal coupled to the first capacitive node and an output terminal switchably coupled to a column output line that extends in a column-wise direction across the pixel array; and

a conversion-gain transistor coupled between the first and second capacitive nodes and having a control terminal coupled to a conversion-gain-select line that extends in the column-wise direction across the pixel array;

a comparator having a first input capacitively coupled to the column output line, a second input coupled to receive a threshold voltage, and an output to the conversion-gain-select line;

an auto-zeroing transistor coupled between the first input of the comparator and the output of the comparator; and

control circuitry to switchably couple the output terminal of the output transistor to the column output line throughout a readout interval and, during the readout interval, to:

concurrently assert (i) a reset pulse at a gate of the reset transistor to reset the first capacitive node to a reset voltage level and to generate, on the column output line, a first analog output signal corresponding to the reset voltage level, and (ii) an auto-zero pulse at a gate of the auto-zeroing transistor to establish, by switchably coupling the output of the comparator to the input of the comparator via the auto-zeroing transistor, a zero-amplitude at the input of the comparator corresponding to an amplitude of the first analog output signal;

assert a first transfer-enable pulse on the transfer-gate control line to enable transfer of accumulated photocharge from the photodetector to the first capacitive node via the first transfer gate and produce, on the column output line, a second analog output signal having an amplitude corresponding to the accumulated photocharge; and

enable the comparator to assert a conversion-gain-adjust pulse on the conversion-gain-select line according to whether the amplitude of the second analog output signal exceeds a sum of the zero-amplitude and the threshold voltage.

2. The integrated-circuit image sensor of claim 1 wherein the conversion-gain control transistor switchably couples the first and second capacitive nodes in parallel in response to assertion of conversion-gain-select signal on the conversion-gain-select line.

3. The integrated-circuit image sensor of claim 1 further comprising a sample-and-hold circuit coupled to the column output line.

4. The integrated-circuit image sensor of claim 3 wherein the control circuitry comprises readout circuitry to:

assert a first sample-and-hold pulse concurrently with assertion of the reset pulse to store, within a first capacitive storage element of the sample-and-hold circuit, a first readout voltage level corresponding to the amplitude of the first analog signal; and

assert a second sample-and-hold pulse during or after assertion of the transfer-enable pulse to store, within a second capacitive storage element of the sample-and-hold circuit, a second readout voltage level corresponding to the amplitude of the second analog signal.

5. The integrated-circuit image sensor of claim 1 , wherein the control circuitry is further to assert a second transfer-enable pulse on the transfer-gate control line, concurrently with assertion of the conversion-gain-adjust pulse by the comparator circuitry, if the amplitude of the analog output signal exceeds the sum of the zero-amplitude and the threshold voltage.

6. The integrated-circuit image sensor of claim 5 wherein transfer of accumulated photocharge from the first capacitive node via the first transfer gate is characterized by a first conversion gain and wherein assertion of the second transfer-enable pulse on the transfer-gate control line enables transfer of a residual portion of the accumulated photocharge to at least the second capacitive node, the transfer of the residual portion of the accumulated photocharge being characterized by a second conversion gain that is lower than the first conversion gain.

7. The integrated-circuit image sensor of claim 1 wherein the first photodetector, first and second capacitive nodes, first transfer gate, output transistor, and conversion-gain transistor are constituent elements of a first pixel within the pixel array, and wherein each other pixel of the pixel array is constituted by a respective other photodetector, other first and second capacitive nodes, other transfer gate, other output transistor and other conversion-gain transistor.

8. The integrated-circuit image sensor of claim 1 further comprising a second photodetector and a second transfer gate coupled between the second photodetector and the first capacitive node.

9. The integrated-circuit image sensor of claim 8 wherein the first photodetector and first transfer gate constitute elements of a first pixel within the pixel array, the second photodetector and the second transfer gate constitute elements of a second pixel within the pixel array and the first and second capacitive nodes, at least, are shared by the first and second pixels.

10. The integrated-circuit image sensor of claim 1 wherein the second capacitive node has a higher capacitance than the first capacitive node.