This paper presents a 14-bit column-parallel two-step compact hybrid ADC for low-power digital IRFPA application, with the area and speed performances compromise. The proposed two-step ADC works in two phases: in the first phase, A 7-bit SAR ADC performs the coarse quantization; in the second phase, a 7-bit SS ADC further performs fine quantization to complete the residue voltage conversion. In this work, the number of unit capacitors is reduced to 1/128th of that of a conventional 14-bit SAR ADC, and the main clock of fine SS ADC can be reduced to 50MHz at the sampling rate of 238 kS/s. More importantly, by sharing analog circuits between the SAR ADC and the SS ADC, the power consumption and layout area are reduced consequently. The proposed two-step ADC is designed in 0.18 μm standard CMOS process. The simulation results show that the proposed two-step ADC has a differential nonlinearity of −0.87/+0.43 LSB and an integral nonlinearity of −0.86/+0.71 LSB. The layout of the proposed ADC can be implemented in the pixel pitch less than 10 μm. The conversion time is 4.2 μs, and the power consumption of each column ADC is only 65 μW with a 3.3V power supply. The simulation results indicate the proposed two-step ADC is suitable for low-power digital readout circuits in the small pixel pitch IRFPA.
With the development of science and technology, image sensors are more and more widely used, such as digital cameras and surveillance cameras. However, due to the physical characteristics of the photodetector, which performance is sensitive to the variation of the operating temperature. Therefore, a digital temperature sensor integrated on the chip is required to measure the operating temperature and assist in correction and compensation. Traditional scheme integrates one temperature sensor on the whole image sensor chip, which can’t reflect the temperature distribution for each pixel. It’s desirable to implement temperature measurement in pixel level for accurate correction, but existing temperature sensor occupying area of hundred μm2, which can’t be input to the pixel of image sensor. In additional, the power consumption of each temperature sensor is μW-level, which will dissipate considerable power for million temperature sensors. In this paper, a pixel-level integrated temperature sensor is proposed. The circuit is composed of only a capacitor and a conventional diode. The readout circuit is similar to that of the active pixel of image sensor, thus the ADC (Analog-to-Digital Converter) and other readout circuits and be multiplexed. The temperature sensor integrated in pixel is designed, which area is only 0.21 μm2. The simulation results show the increased power consumption for 50Hz working pixels don’t exceed 4%. It’s confirmed that the proposed pixel-level integrated temperature sensor can measure the temperature of each pixel and assisting in the accurate correction of image sensor in pixel level.
This paper presents a logarithmic response burst mode IRFPA ROIC with pixel level integration of BDI structure and memory cells. BDI structure provides stable bias for the detector, and converts detector current into logarithmic voltage fast. On-chip high speed video record is achieved by high speed sampling the logarithmic response voltage and storing it into the memory cells in order. A column level SAR ADC is used to convert the outputs of memory cells into digital code. The prototype chip with 64×64 pixels was designed and fabricated. Ultra-high speed video capturing at 1Mfps with 100 consecutive frames is successfully demonstrated.
This paper presents a two-step ADC architecture for dual band infra-red focal plane array (IRFPA). It has advantages of small pixel area and low power over conventional pixel level ADC while maintaining the characteristic of enhancing well capacity of ROIC. The proposed two-step ADC with 16 bits is designed for middle / long wave (MW/LW) dual band IRFPA: 10-bit pulse frequency modulation (PFM) based pixel level ADC with 6-bit column paralleled Successive Approximation Register (SAR) ADC is employed for LW detector and 6-bit PFM based pixel level ADC with 10-bit column paralleled SAR ADC is employed for MW detector. Layout area of the pixel is 30 μm × 30 μm. The simulation result shows the DNL of the proposed two-step ADC for LW detector and MW detector are 0.6 LSB and 0.9 LSB respectively.
Digital infrared (IR) focal plan array (IRFPA) is one of the most significant characteristic of advanced IR imaging systems, it is implemented by integrating ADCs into the readout integrated circuit (ROIC). Successive Approximation Register (SAR) ADC is popular for column-level ADC architecture since it has low power and high resolution. In this paper a 14-bit RC hybrid SAR ADC with unary capacitor array is proposed, it has better linearity performance area compared with conventional SAR ADC. The proposed SAR ADC introduces resistor ladder for the last 6-bit conversion and it is shared by the whole SAR ADC array, so single SAR ADC’s layout’s length can be reduced, The proposed SAR ADC is designed in 130 nm standard CMOS process, the size of the SAR ADC core is 30 μm × 560 μm. the post simulation result show that its power consumption is 102 μW when the sampling rate is 100 kHz, and the worst DNL is 0.4 LSB when mismatching of 0.2% of capacitor array is introduced. The proposed SAR ADC suits for digital IRFPA applications.
Digital readout integrated circuit (ROIC) has become the development tendency of ROIC for infrared focal plane array (IRFPA) due to its advantages such as improved ability of resisting interference, high readout rate and low readout noise. Compared with traditional analog ROIC, column-parallel analog-to-digital converters (ADCs) are integrated on digital ROIC. Because of the non-ideality of CMOS process, the column nonuniformity of digital ROIC is more obvious than that of analog ROIC due to device mismatch, which will lead to obvious column fixed pattern noise (FPN). In order to reduce column nonuniformity, the main sources of column nonuniformity in digital ROIC are analyzed firstly, including the mismatch between column analog readout channels, as well as ADCs. Then, analytical models have been developed to reveal the relationship between the column FPN and design parameters of digital ROIC. And the numerical computation with CMOS process parameters is implemented. The results show that the column FPN caused by digital ROIC can be reduced to less than 0.1% when the transistor area of tail current source in column analog readout channel is larger than 10μm2 and input transistor area of the first operational amplifier in ADC is larger than 20μm2 . By the help of the proposed mismatch model, column FPN of digital ROIC can be reduced over ten times with optimized design parameters, which provides beneficial guidance for digital ROIC design.
KEYWORDS: Synthetic aperture radar, Quantization, High dynamic range imaging, Signal to noise ratio, Readout integrated circuits, Amplifiers, Image sensors, Imaging systems
This paper presents a two-step ADC architecture for high dynamic range, high sensitivity image sensor. The proposed two-step ADC architecture works in two phases: the coarse quantization phase in each pixel, digital integration technique is applied to increase the well capacity as well as system’s dynamic range, and a capacitive transimpedence amplifier (CTIA) scheme is employed to achieve high sensitivity; The fine quantization phase in the column which reduces the bit width of the pixel-level ADC, pixel-level ADC’s noise and layout area are reduced consequently. The proposed two-step ADC with 18 bits is designed in 0.18 μm standard CMOS process. The optimized assignment for the bit width of pixel-level ADC and column paralleled ADC is applied. The simulation shows the signal to noise ratio (SNR) is 93.5 dB. The dynamic range is 108 dB. The least sensible electrons are 781 e-. The simulation results indicate the proposed twostep ADC is suitable for high dynamic range, high sensitivity image sensor.
In order to achieve high sensitivity for low-light-level CMOS image sensors (CIS), a capacitive transimpedance amplifier (CTIA) pixel circuit with a small integration capacitor is used. As the pixel and the column area are highly constrained, it is difficult to achieve analog correlated double sampling (CDS) to remove the noise for low-light-level CIS. So a digital CDS is adopted, which realizes the subtraction algorithm between the reset signal and pixel signal off-chip. The pixel reset noise and part of the column fixed-pattern noise (FPN) can be greatly reduced. A 256×256 CIS with CTIA array and digital CDS is implemented in the 0.35μm CMOS technology. The chip size is 7.7mm×6.75mm, and the pixel size is 15μm×15μm with a fill factor of 20.6%. The measured pixel noise is 24LSB with digital CDS in RMS value at dark condition, which shows 7.8× reduction compared to the image sensor without digital CDS. Running at 7fps, this low-light-level CIS can capture recognizable images with the illumination down to 0.1lux.
An incremental sigma-delta ADC is designed for column-parallel ADC array in CMOS image sensor. Sigma-delta modulator with single-loop single-bit structure is chosen for power consumption and performance reasons. Second-order modulator is used to reduce conversion time, without stability problem and large area accompanied by higher order sigma-delta modulator. The asymmetric current mirror amplifier used in integrator reduces more than 30% power dissipation. The digital filter and decimator are implemented by counters and adders with significantly reduced chip area and power consumption. A Clock generator is shared by 8 ADCs for trade-off among power, area and clock loading. The ADC array is implemented in a 0.18-μm CMOS technology and clocked at 10 MHz, and the simulated resolution achieves 15-bit with 255 clock cycles. The average power consumption per ADC is 118 μW including clock generator, and the area is only 0.0053 μm2.
A novel data transmission circuit for digital image sensors is presented. Large amounts of data are divided into m groups of n bits each. Each group of data is stored in an n-bit shift register. Under the control of a column scanner, at a time, only one group of data is selected to output serially to a common output line. Thus, as the scanner scans all the groups of data, the large amounts of data are serially output through the common line. A current-mode circuit transforms the output data into a low voltage swing signal which propagates over the common output line fast. A sense amplifier receives the low voltage swing signal and then recovers the full swing signal. Finally, a low-voltage differential signaling ( LVDS ) transmitter, which is fed with the full swing signal, transmits the data out chip. Because there is always only one of the m shift registers operating, the power consumption is greatly reduced. The simulation results show that the proposed circuit works correctly at a date rate of 400Mb/s. For n=14, and m=8, 32, 128, and 256, the power consumption of the prototype is as low as 1/4, 1/15, 1/50, and 1/80 that of the traditional serial link respectively.
The readout integrated circuit (ROIC) is a bridge between the infrared focal plane array (IRFPA) and image processing circuit in an infrared imaging system. The ROIC is the first part of signal processing circuit and connected to detectors directly, so its performance will greatly affect the detector or even the whole imaging system performance. With the development of CMOS technologies, it’s possible to digitalize the signal inside the ROIC and develop the digital ROIC. Digital ROIC can reduce complexity of the whole system and improve the system reliability. More importantly, it can accommodate variety of digital signal processing techniques which the traditional analog ROIC cannot achieve. The analog to digital converter (ADC) is the most important building block in the digital ROIC. The requirements for ADCs inside the ROIC are low power, high dynamic range and small area. In this paper we propose an RC hybrid Successive Approximation Register (SAR) ADC as the column ADC for digital ROIC. In our proposed ADC structure, a resistor ladder is used to generate several voltages. The proposed RC hybrid structure not only reduces the area of capacitor array but also releases requirement for capacitor array matching. Theory analysis and simulation show RC hybrid SAR ADC is suitable for ADC array applications
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