Low noise active reset readout for image sensors

Abstract

Bandlimiting and capacitive feedback is used to reduce pixel reset noise in an image sensor without adding lag. An image sensor includes a reset circuit having a reset control loop for controlling pixel reset. The reset control loop includes, for example, a reset amplifier and a transistor. The reset amplifier has a first input coupled to a reset voltage and a second input coupled to a readout node (e.g., the source of a NMOS transistor). When the reset voltage exceeds the readout node voltage, the reset amplifier output voltage rises and turns on the transistor. The output voltage of the transistor then follows the reset voltage until the reset voltage stops rising and the readout node voltage overshoots the reset voltage. After the readout node voltage overshoots the reset voltage, the reset amplifier output voltage drops and turns the transistor off. With the transistor off, only the overlap capacitance of the transistor is used to control the reset control loop[. The overlap capacitance is combined with pixel capacitance to form a capacative voltage divider in the reset control loop for reducing pixel reset noise at the readout node. When the reset voltage is returned to ground or other suitable reference, the pixel reset is complete.