A CMOS Image Sensor with Thermal Sensing Capability and Column Zoom ADCs

Abstract

This paper presents a CMOS image sensor (CIS) with a zoom ADC, to quantize in-pixel temperature sensors, as well as for faster readout speed of the image pixels while maintaining low quantization noise. The proposed 15 bit zoom ADC has a 4 bit Unit Capacitor Array (UCA) SAR and a 13 bit incremental 2nd-order delta-sigma ADC (DSADC), as its first and second stage, respectively. The proposed UCA with improved switching and decoding technique minimizes capacitor area and switching energy, by 50 % and 75 %, respectively, compared to a conventional binary weight array (BWA) counterpart. Measurement results on 4 chips show the proposed zoom ADC could operate at least twice as fast, when maintaining the same signal-to-noise ratio (SNR), or improve its SNR by 9 dB, when maintaining its sampling speed, compared to a DSADC only alternative. The proposed 15 bit ADC is measured a SNR of 80.1 dB and INL and DNL within ±1.5 LSB and ±1 LSB (full scale voltage is 1 Vp-p), when operating at 31 kHz. The incorporated imager-based temperature sensors are measured to have inaccuracies within ±0.6 °C on 4 chips, between-20 and 80 °C, when quantized by the same zoom ADC.