Reset noise suppression in CMOS image sensor using charge control technique
In a photodiode based CMOS image sensor, reset noise plays a significant role in limiting the detection of the sensor, especially in low light conditions. In a standard active pixel sensor (APS) the reset noise (i.e. the hard reset noise) is of the order of kTC where k is the Boltzmann constant, T is the temperature, and C is the photodiode capacitance of the CMOS image sensor.
Several methods for reducing the reset noise have been published in the past. In this thesis, a new circuit of the CMOS image sensor with significant reduction in the reset noise using the charge control mechanism is presented. In this technique (charge control mechanism), the reset noise is reduced by precisely determining when to stop charging the capacitive sensor. The voltage across the photodiode is monitored by employing a column level comparator which is part of a feedback circuit connected to the reset transistor.
The simulation results obtained from a 4 transistor per pixel 0.5microm CMOS technology show that the reset noise can be reduced to less than kT23C . This noise reduction is achieved with high fill factor and without adding any image lag.