Threshold Uniformity Improvement in 1b Quanta Image Sensor Readout Circuit

A new readout architecture for single-bit quanta image sensor (QIS) consisting of a capacitive transimpedance amplifier (CTIA) before a 1-bit quantizer to improve the threshold uniformity of the readout cluster is proposed in this paper. The 1-bit quantizer in the previous single-bit QIS had significant threshold non-uniformity likely caused by the fluctuation of the common-mode voltage of the jot output. To guarantee the stability of the common-mode voltage of input signals fed to the 1-bit quantizer, the CTIA is added before the 1-bit quantizer. A pipeline operation mode is also proposed so the CTIA and 1-bit ADC can work at the same time, thereby reducing the CTIA power consumption. A 2048 × 1024 high-speed test chip was implemented with 45 nm/65 nm stacked backside illuminated (BSI) CMOS image sensor (CIS) process and tested. According to the measured D-log-H results, a good threshold uniformity in the range of 0.3 to 0.8 e- for all readout clusters is demonstrated at 500 frame per second (fps) equivalent timing with 68 mW power consumption.