An Activity-Triggered 95.3 dB DR $-$75.6 dB THD CMOS Imaging Sensor With Digital Calibration

Abstract

Imaging sensors are being used as data acquisition systems in new biomedical applications. These applications require wide dynamic range (WDR), high linearity and high signal-to-noise ratio (SNR), which cannot be met simultaneously by existing CMOS imaging sensors. This paper introduces a new activity-triggered WDR CMOS imaging sensor with very low distortion. The new WDR pixel includes self-resetting circuits to partially quantize the photocurrent in the pixel. The pixel residual analog voltage is further quantized by a low-resolution column-wise ADC. The ADC code and the partially quantized pixel codes are processed by column-wise digital circuits to form WDR images. Calibration circuits are included in the pixel to improve the pixel linearity by a digital calibration method, which requires low calibration overhead. Current-mode difference circuits are included in the pixel to detect activities within the scene so that the imaging sensor captures high quality images only for scenes with intense activity. A proof-of-concept 32 times 32 imaging sensor is fabricated in a 0.35 mum CMOS process. The fill factor of the new pixel is 27%. Silicon measurements show that the new imaging sensor can achieve 95.3 dB dynamic range with low distortion of -75.6 dB after calibration. The maximum SNR of the sensor is 74.5 dB. The imaging sensor runs at frame rate up to 15 Hz.