Kwi-sung Yoo

A 2.1 M Pixels, 120 Frame/s CMOS Image Sensor With Column-Parallel

This paper presents a 2.1 M pixel, 120 frame/s CMOS image sensor with column-parallel delta-sigma (ΔΣ) ADC architecture. The use of a second-order ΔΣ ADC improves the conversion speed while reducing the random noise (RN) level as well. The ΔΣ ADC employing an inverter-based ΔΣ modulator and a compact decimation filter is accommodated within a fine pixel pitch of 2.25-μm and improves energy efficiency while providing a high frame-rate of 120 frame/s. A prototype image sensor has been fabricated with a 0.13-μm CMOS process. Measurement results show a RN of 2.4 e_rms^- and a dynamic range of 73 dB. The power consumption of the prototype image sensor is only 180 mW. This work achieves the energy efficiency of 1.7 e^-·nJ.

\Delta \Sigma

Over the last few years, the demands for high-density and high-speed imaging have increased drastically. Since CMOS image sensors have the advantages of low power consumption and easy system integration, they have become dominant over CCDs in the consumer market [1–4]. A column-parallel ADC architecture is the most widely used ADC in CMOS image sensors for high-speed and low-power operation [2–6]. The column-parallel architecture can be classified as: successive-approximation register (SAR) [2], cyclic [3], single-slope (SS) [4], and delta-sigma (ΔΣ) [5,6] ADCs. Although SAR ADCs have been utilized for high-speed imaging, such as UDTV, they require a DAC in a column, whose area is unacceptably large for consumer electronics with a fine pixel pitch. Cyclic ADCs have also been reported in high-speed imaging, but they have high power consumption and high noise levels. Since SS ADCs provide relatively high resolution with minimum area, they have been widely used in CMOS image sensors. However, SS ADCs require very fast clock signals leading to high power consumption in the case of high-speed imaging. Although ΔΣ ADCs have been investigated for low-noise imaging, they have only been applied for low-speed imaging with large pixel pitch because of the complexity of ΔΣ modulators and following decimation filters.

ADC Architecture

This paper proposes a low-power 240 frames/s 2.1 M-pixel CMOS image sensor with column-shared cyclic (CY) ADCs. Two-column shared CY-ADC architecture and two-level stacked ADC placement are employed for low-power and small pixel pitch design. The proposed CY-ADC uses only one OTA and four capacitors. Distributed clocking scheme using cascaded repeaters is proposed to reduce the required peak current. The prototype sensor was fabricated in a 0.13- μm 1P4M process with pixel pitch of 2.25 μm . The designed 10-bit ADC dissipates only 90 μW/channel with 1.5 V supply. The measured DNL and INL are +0.59/-0.83 LSB and +2.8/-3.6 LSB, respectively. The measured maximum pixel rate is 500 Mpixels/s with total power consumption of 300 mW.

A 2.1Mpixel 120frame/s CMOS image sensor with column-parallel ΔΣ ADC architecture

The demand for high-quality and high-speed imaging has increased. Column-parallel ≥14b A/D conversion is one of the major approaches to meet these requirements in CMOS image sensors (CIS). Oversampling ADCs such as incremental delta-sigma (I-ΔΣ) ADCs are the solution for a high-resolution ADC having tolerance of analog component errors. Oversampling reduces input temporal noise as well as the quantization error of the ADC itself [1]. However, an I-ΔΣ ADC is also classified as a slow ADC because it requires exponential conversion time to get higher bit resolution. To reduce conversion time, there are two alternative methods: 1) higher-order modulation, and 2) two-step conversion such as an extended-counting technique [2]. In this paper, the extended-counting (EC) method is used since a high-order structure requires more complex hardware and greater power consumption [1,2]. For a general two-step ADC, coarse conversion restricts the total ADC resolution since it determines accuracy of the residue. However, an EC ADC overcomes the accuracy limitation, since the I-ΔΣ can improve its precision through oversampling. Moreover, oversampling also suppresses the noise of the pixels source follower. Our 14b EC ADC is a blend of a 1st-order I-ΔΣ ADC and a cyclic ADC to simultaneously get high resolution and high speed.