Detection circuit for photo sensor with stacked substrates

A pixel in a photo sensor, comprising:

• a portion of a first substrate including a photodiode, a floating diffusion point, and a first transistor between the photodiode and the floating diffusion point to transfer charge from the photodiode to the floating diffusion point responsive to turning on the first transistor;
• a portion of a second substrate including a current source, a conductive line, a first switch selectively coupling the conductive line to the floating diffusion point, and a first capacitor configured to store a signal voltage responsive to turning on the first switch, the second substrate overlapping the first substrate, the signal voltage representing an amount of charge transferred from the photodiode to the floating diffusion point;
• and
• a pixel level interconnect between the floating diffusion point and the conductive line to carry the signal voltage.