A method of measuring signals related to a photodiode based sensor and calculating a corrected data value thereof is disclosed. A nominal reset voltage value of the photodiode may be measured. A knee point voltage may be applied to the photodiode and resets a voltage on the photodiode to the knee point voltage when the voltage on the photodiode falls below the knee point voltage. Applying the knee point voltage may extend the dynamic range of the sensor. An output voltage of the photodiode at end of an integration time of the photodiode may be measured. The knee point voltage may be applied again after the end of the integration time. A voltage value of the photodiode of the knee point voltage may be measured. The nominal reset voltage value, the output voltage of a sensor and the knee point voltage may be reported to calculate the corrected data value.
1. A method for adjusting a pixel voltage, comprising:
applying a first voltage to a photodiode and to a floating diffusion capacitance after an integration period, the first voltage sufficient to substantially fill the photodiode with electrons;
applying a second voltage to the photodiode after applying the first voltage and after the integration period; and
reading a voltage of a photodiode after applying the second voltage by:
resetting the floating diffusion capacitance;
transferring the voltage of the photodiode to the floating diffusion capacitance; and
transferring the voltage of the floating diffusion capacitance to an output node, wherein the voltage of the floating diffusion capacitance serves as a knee point voltage.
2. The method of claim 1 , further including:
starting an integration time of the photodiode before applying the first voltage to the photodiode and before applying the second voltage to the photodiode;
applying a third voltage to the photodiode in response to being in the integration period;
transferring charge from the photodiode to the floating diffusion capacitance to generate a pixel voltage; and
reading the pixel voltage on the floating diffusion capacitance.
3. The method of claim 1 , further including:
resetting the floating diffusion capacitance to a first reset voltage level; and
reading a first reset voltage level wherein the first reset voltage level serves as a nominal reset voltage.
4. The method of claim 3 , further including:
generating a calibration voltage to be a difference between the nominal reset voltage and the pixel voltage if the difference between the nominal reset voltage and the pixel voltage divided by a gain factor is greater than a difference between the nominal reset voltage and the knee voltage or generating the calibration voltage to be a difference between the nominal reset voltage and the knee voltage added to the product of the gain factor and a difference between the knee voltage and the pixel voltage if a value of the difference between the nominal reset voltage and the pixel voltage divided by the gain factor is less than the difference between the nominal reset voltage and the knee voltage, wherein the gain factor is a ratio of the integration time to a point in time that the photodiode applying the third voltage to the photodiode.
5. A method for adjusting a pixel voltage of a pixel, wherein the pixel comprises a first transistor and a photodiode, the first transistor having a control terminal and first and second current carrying terminals, the first current carrying terminal coupled for receiving a source of potential and the second current carrying terminal coupled to the photodiode, the method comprising:
setting the photodiode to a first voltage, wherein the first voltage substantially fills the photodiode with electrons;
applying a second voltage to the control terminal of the first transistor which places a third voltage on the photodiode, the second voltage less than the voltage at the first current carrying electrode of the first transistor and greater than a saturation voltage of the pixel; and:
reading the third voltage by resetting a floating diffusion capacitance, then transferring the third voltage to the diffusion capacitance, and transferring the third voltage on the floating diffusion capacitance to an output node.
6. The method of claim 5 , further including:
starting an integration time of the photodiode before applying the first voltage to the photodiode;
applying the second voltage to the control terminal of the first transistor during the integration period to charge the photodiode to the photodiode;
transferring the charge from the photodiode to the floating diffusion capacitance to generate a pixel voltage; and
reading the pixel voltage on the collection capacitance.
7. The method of claim 6 , further including:
resetting the floating diffusion capacitance to a first reset voltage level; and
reading a first reset voltage level wherein the first reset voltage level serves as a nominal reset voltage.
8. The method of claim 6 , wherein resetting the diffusion capacitance includes resetting the diffusion capacitance either before or after setting the photodiode to the first voltage level.
9. A method for extending a dynamic range of a pixel that includes a photodiode and a transistor, the transistor having a control electrode and first and second current carrying electrodes, the first current carrying electrode coupled for receiving a source of potential and the second current carrying electrode coupled to the photodiode, the method comprising setting the photodiode to a first voltage level by applying a voltage to the control terminal of the transistor that is less than a voltage of the first current carrying electrode and setting the photodiode to a second voltage level during an integration time.
10. The method of claim 9 , wherein setting the photodiode to the second voltage level increases the pixel voltage to a voltage level that is greater than a saturation voltage of the pixel.
11. The method of claim 9 , further including setting the photodiode to the second voltage level a plurality of times during the integration period.
12. The method of claim 9 , further including:
determining a nominal reset voltage of the photodiode;
determining the second voltage level;
determining a pixel voltage of the photodiode; and
generating a pixel correction factor using the nominal reset voltage, the first voltage level, and the pixel voltage.
13. The method of claim 12 , wherein determining the nominal reset voltage of the photodiode comprises:
resetting a floating diffusion capacitance of the pixel; and
determining the charge on the floating diffusion capacitance after it has been reset, wherein the charge on the floating diffusion capacitance serves as a measure of the nominal reset voltage.
14. The method of claim 13 , wherein determining the pixel voltage of the photodiode comprises:
transferring the charge of the photodiode to the floating diffusion capacitance at the end of the integration time; and
determining the charge collected from the photodiode, wherein the collected charge serves as a measure of the pixel voltage.
15. The method of claim 14 , wherein determining the second voltage level comprises:
initializing the floating diffusion capacitance to a first initialization voltage after the integration time;
initializing the photodiode to a second initialization voltage after the integration time;
setting the photodiode to the first voltage level by applying a voltage to the control electrode of the transistor that is less than a voltage of the first current carrying electrode;
transferring charge from the photodiode to the floating diffusion capacitance after setting the photodiode to the first voltage level; and
determining the charge collected on the floating diffusion capacitance, wherein the charge collected by the floating diffusion capacitance serves as a measure of the second voltage level.
16. The method of claim 15 , wherein generating the pixel correction factor comprises subtracting the pixel voltage from the nominal reset voltage to generate a first difference value that serves as the pixel correction factor if the first difference voltage divided by a gain factor is greater than a second difference that is generated by subtracting the second voltage level from the nominal reset voltage, wherein the gain factor is a ratio of the integration time to a point in time that the photodiode was set to the second voltage level during the integration time.
17. The method of claim 15 , wherein generating the pixel correction factor further comprises subtracting the second voltage level from the nominal reset voltage to generate a first difference value, subtracting the pixel voltage from the second voltage level to generate a second difference value, generating a gain product by taking the product of a gain factor and the second difference value to generate the gain product and adding the first difference value to the gain product to generate a value that serves as the pixel correction factor if the difference between the nominal reset voltage and the pixel voltage divided by the gain factor is less than the first difference value, wherein the gain factor is a ratio of the integration time and a point in time that the photodiode was set to the second voltage level during the integration time.