Method and apparatus for backside illuminated image sensors using capacitively coupled readout integrated circuits

A back-side illuminated image sensor having two circuits capacitively coupled to each other at the pixel level, comprising:

• a capacitor having a first plate and a second plate;
• a first circuit comprising:
• a photon detector for receiving a plurality of photons and providing a charge signal corresponding to the received photons,
• an integrator, connected to the photon detector, for determining a cumulative signal based on the charge signal for a given exposure time and producing an integrator output signal based on the cumulative signal, and
• a comparator, connected to the integrator and the first plate of the capacitor, for producing a comparator output signal that is sent to the first plate of the capacitor when the integrator output signal has reached a threshold level;
• and a second circuit comprising:
• a receiver, connected to the second plate of the capacitor, for detecting a plurality of pulses from capacitively coupled signals received from the first circuit, and
• a counter, connected to the receiver, for counting the plurality of pulses detected by the receiver;
• wherein the first circuit and the second circuit are capacitively coupled to one another via the capacitor.