CMOS Active Pixel Sensors: An Introduction

1. CMOS Active Pixel Sensors: An Introduction Zeynep Dilli, Neil Goldsman, Martin Peckerar, Nibir Dhar

2. Active Pixel Sensors (adapted from El Gamal et al. and Yadid-Pecht et. al.)

3. Active Pixel Sensor Circuit • Simple version: 3 transistors (pixel reset, source follower, access), one photodiode • Not shown: Row read circuitry, a timed buffer connected to v_out and activated a set time after pixel access

4. Active Pixel Sensor Circuit • Operation: • Reset transistor gate pulsed, photodiode junction cap. charged up, source follower output follows • Photocurrent starts discharging, SF output follows • Access transistor gate pulsed, output cap.charged up • Bias current discharges output voltage linearly until the set read time for v_out

5. Operation: • Reset gate pulsed, photodiode charged up, SF output follows • i_pf discharges diode cap., SF output follows • Access gate pulsed, output cap. charged up • i_bias discharges output until the set read time for v_out

7. Active Pixel Sensor Circuit • Design Features: • Small circuit, just three transistors per pixel: Large fill factor possible • Photodiode operation isolated from readout circuit • Flexibility in design with respect to timing design vs. power consumption vs. readout speed vs. resolution • Converts current information from photodiode to voltage information, e.g. to be read by an A/D

8. Active Pixel Sensor Circuit • Design issues: • Transistor sizes should be as small as possible for the maximum photodiode/pixel ratio (“fill factor”) • Transistor sizes should be chosen carefully for enough current, SF gain, and isolation of SF output from the pixel output • Higher bias current: readout needs to be a smaller time after access is pulsed; timing gets tighter, reset period can be set smaller • Larger reset period: Better resolution, tighter readout timing required

9. Active Pixel Sensor Circuit • Can model photodiode with junction cap. and ideal current source in parallel