Methods of calibrating knee-point and logarithmic slope in linear-logarithmic image sensors

A method of calibrating a linear-logarithmic image sensor pixel, comprising:

• at least partially depleting a photodiode within the pixel by driving a cathode of the photodiode with a pull-up current;
• supplying a floating diffusion region within the pixel with a first subthreshold current by passing first charge carriers from the at least partially depleted photodiode to the floating diffusion region;
• then
• reading a first voltage of the floating diffusion region containing the first charge carriers, as first calibration data;
• then, without adjusting a voltage of the floating diffusion region upon completion of said reading a first voltage, supplying the floating diffusion region with a second subthreshold current by passing second charge carriers from the at least partially depleted photodiode to the floating diffusion region;
• and then
• reading a second voltage of the floating diffusion region containing the first and second charge carriers therein, as second calibration data.

A method of calibrating a linear-logarithmic image sensor pixel, comprising:

• fully depleting a photodiode within the pixel;
• then
• discharging a floating diffusion region within the pixel;
• supplying the floating diffusion region with a first subthreshold current by passing first photo-generated charges of first conductivity type from the photodiode in an at least partially depleted state to the floating diffusion region;
• then
• reading a first voltage of the floating diffusion region as first calibration data;
• then
• without resetting the floating diffusion region by removing the first photo-generated charges therefrom, supplying the floating diffusion region with a second subthreshold current by passing second photo-generated charges from the at least partially depleted photodiode to the floating diffusion region;
• then
• reading a second voltage of the floating diffusion region containing the first and second photo-generated charge carriers therein, as second calibration data;
• and
• determining a knee-point voltage associated with a transition from a linear to logarithmic response of the pixel from the first and second calibration data.