Ultra low readout noise architecture having a pixel sensor with dual active pixel sensor pixel and dual ramp line
A method to read out pixels includes reading a first pixel by resetting a first photodetector, integrating the first photodetector after resetting the first photodetector, resetting a first floating diffusion node coupled to the first photodetector and a second floating diffusion node coupled to a second photodetector, transferring charge from the first photodetector to the first floating diffusion node, comparing a first signal at the first floating diffusion node and a second signal at the second floating diffusion node and generating a first signal to latch a first counter value when the first signal is less than the second signal, incrementing the first signal and decrementing the second signal, and comparing the first signal and the second signal and generating a second signal to latch a second counter value when the first signal is greater than the second signal, wherein the difference between the second counter value and the first counter value indicates a first pixel level.
1. A pixel sensor, comprising:
a first active pixel sensor pixel, comprising:
a first floating diffusion node;
a first photodetector coupled to the first diffusion node; and
a first ramp line coupled to the first floating diffusion node;
a second active pixel sensor pixel, comprising:
a second floating diffusion node;
a second photodetector coupled to the second diffusion node; and
a second ramp line coupled to the second floating diffusion node;
and
a readout circuit, comprising:
a comparator having a first input connected to the first floating diffusion node and a second input connected to the second floating diffusion node; and
a latch coupled to the comparator.
2. The pixel sensor of claim 1 , wherein the readout circuit further comprises:
another comparator having a first input connected to a first output of the comparator, a second input connected to a second output of the comparator, and an output coupled to the latch.
3. The pixel sensor of claim 1 , further comprising a counter coupled to the latch.
4. The pixel sensor of claim 1 , further comprising a positive ramp generator and a negative ramp generator selectively coupled to the first and the second ramp lines.
5. The pixel sensor of claim 1 , wherein the first ramp line is a horizontal line that travels along a first row of active pixel sensor pixels including the first active pixel sensor pixel, and the second ramp line is a second horizontal line that travels along a second row of active pixel sensor pixels including the second active pixel sensor pixel.
6. The pixel sensor of claim 1 , wherein:
the first active pixel sensor pixel further comprises a first transfer gate coupling the first photodetector to the first floating diffusion node and a first reset gate coupling the first floating diffusing node to a first supply rail; and
the second active pixel sensor pixel further comprises a second transfer gate coupling the second photodetector to the second floating diffusion node and a second reset gate coupling the second floating diffusing node to a second supply rail.