Behavioral modeling of a Sigma-Delta Modulator for sensing photocurrent in a CMOS Image Sensor

We propose behavioral time-domain modeling for a Sigma-Delta Modulator (ΣΔM) to sense photocurrent in a CMOS Image Sensor (CIS). The model starts from transfer function for first-order single-bit ΣΔM in discrete-time, and then adds noise sources and nonlinearities by introducing noise floor and harmonic distortion, which determine system performance. The results were compared with the measurements of a CIS based ΣΔM circuit prototype fabricated with CMOS technology. The model showed good agreement with measurements.