She understands my university life and is the closest to me in my thoughts. In bright conditions, photon shot noise is dominant in CIS compared to other noise sources. When measuring long distances, depth accuracy is reduced due to the lack of incoming photons, just like CIS in dark conditions.
Therefore, several approaches have been developed in the past to mitigate the trade-offs arising from multiple sampling.
Solid-state image sensor
Before we start comparing the two types of image sensors, we need to understand the main principle of image sensors. Therefore, detecting and calculating the intensity of light by counting the number of electrons produced by photons is the main function of an image sensor. Because of this critical drawback, CMOS image sensors using an APS pixel may have the potential to become widespread in the solid state image sensor market.
Both types of solid state image sensor (CCD, CMOS APS pixel) can be realized based on Metal Oxide Semiconductor (MOS) technologies.
Low light imaging
Research motivation
ADCs in CMOS image sensor
ADC architectures
The analog-to-digital converter (ADC) is not only used for CIS, but also for mixed-signal system-on-chip (SoC). Second, ramping architectures convert analog to digital by measuring the amount of time during which their reference signal and the sampled signal are the same. The flash ADC has the fastest conversion time than other types of ADCs because it can convert parallel analog value to digital value.
For this reason, the SF pixel is required for a large bias current to load the ADC input capacitance, and then it must be considered in the power consumption concept. As we know, the ADC structure must be smaller than other types of ADCs due to the sufficient pixel resolution in the ADC structure. Therefore, a time-to-digital converter (TDC) with timing is required to achieve this approach.
The latest CMOS image sensors are required for high resolution, which is above 14 bits to reduce temporal noise. However, to get conversion results, bitstream accumulation is needed to get conversion data because delta sigma ADC generates bitstream data serially. Successive approximation analog-to-digital converter circuit and operation 4-bit case The successive approximation ADC(SAR-ADC) is composed with a comparator, capacitor DAC, SAR logic digital block and n-bit register.
More discussion is needed about what order is most effective to obtain high image quality. In each column, it has a comparator and D flip-flop for converting analog value to digital value.
Chapter summary
Noise analysis in CMOS image sensor
Noise components in CMOS image sensor
Therefore, it can use pseudo-correlated double-sampling technique, which cannot completely eliminate reset noise in pixels. In my opinion, if we turn on reset transistor when we do multiple sampling reset values, thermal noise can be compensated because thermal noise does not freeze and is continuous. The photon shot noise can be described the statistical Poisson distributed nature of the arrival process of photons and the generation process of EHPs (electron-hole pairs) [2].
Similarly, multiple sampling, which samples the same value multiple times, is not effective in reducing photon shot noise. The flicker noise is generated by semiconductor traps, which originate from defects between oxide and substrate [14]. However, in CIS, the source follower in pixels is the dominant flicker noise source, but it cannot increase in size to reduce the flicker noise due to the shrinking size of pixels.
Therefore, reducing effects of flicker noise coming from source follower by circuit techniques should be considered to provide high quality images. Random telegraph noise (burst noise) is a type of electronic noise that occurs in ultrathin gate oxides and semiconductors. In other words, flicker noise in large devices is caused by the sum of many random telegraph noise in small area isolator [15].
Some noises, such as freeze reset noise, photon shot noise, and random telegraph noise, are irreversible. However, other noises, such as flicker noise and thermal noise from the readout circuit and pixel circuit, can be reduced by circuit techniques such as using a high gain preamplifier and multiple sampling techniques to increase SNR.
Circuit techniques for suppression noises in CMOS image
Previous multiple sampling techniques
However, due to the quantization noise coming from the increasing quantization step, the multisampling effect is limited compared to other common multisampling techniques. Therefore, under bright conditions, the effect of multiple sampling is limited due to the dominance of photon impact noise. As we know, the multisampling technique is not effective in bright conditions due to the dominance of photon impact noise.
However, a normal single slope operation must first be performed to check the location of the pixel values in order to perform multiple sampling. However, it has a disadvantage when the pixel output is on the boundary between the dark and light states. In short, many researchers in the past have been forced to use multiple sampling techniques to reduce readout noise and mitigate its effects.
Also, the effect of multiple sampling, signal dependence, and readout time are deeply related to the trade-offs that come from the multiple sampling technique. For example, signal dependence or increased quantization step must be sacrificed to preserve readout time.
Proposed signal dependency concept
As shown in the figure, the proposed idea will not increase the reading time compared to the conventional SS-ADC, which is the same as the Ramp1 signal, but it will achieve the signal-dependent multiple sampling technique only by adding some signals of the global ramp [12]. Therefore, the dark photo signal will be sampled at the maximum sampling number and the bright photo signal will be sampled at the same time.
Proposed ADC architecture
Ramp4 is the global signal for achieving a maximum number of samples and Ramp3 and Ramp2 are the global signal for an average number of samples. The Ramp_RST signal is intended to pull up each ramp generator to reset their initial state. Also, each ramp generator has its own Ramp_EN signal for indicating the appropriate range of each ramp signal.
If the output of the comparator is high in a certain region, the columns select the acquisition ramp generator for their pixel value. When RAMP4_RST goes high, all ramp signals except RAMP4 stop going low and RAMP4 goes to the initial value of the ramp generator. In the scheme of the proposed ADC, the "Ramp EN" signal defines the pixel value region for selecting the appropriate ramp signal.
In Figure 31, 'Ramp EN' signals are generated by shift register and AND gate trees to make simple digital logics. RAMP3EN RAMP3Region RAMP2Region RAMP2Region RAMP2Region RAMP2Region RAMP1Region RAMP1Region RAMP1Region RAMP1Region RAMP1Region RAMP1Region RAMP1Region RAMP1Region. Each column has their own ramp selector to select the appropriate ramp signal for multiple sampling signals independently.
The 'Ramp EN' signal and 'COMP OUT' switches the T-flip-flop when the 'Ramp EN' and 'COMP OUT' are high. Therefore, I choose to change the number of integration capacitors to compensate for mismatches of the slopes of the slope generators.
Measurement results
- Multiple sampling effect measurement results
- Linearity measurement
- Power break
- Depth measurement results
First, the blue line shows the theoretical multisampling effects, and the black line is the results of a conventional multi-frame single-slope ADC to indicate the effect of the conventional multisampling effects. Finally, the red line is the results of the proposed ADC by changing the sampling number (changing the input). However, if the standard deviation is less than 1LSB, the effect of multiple sampling is limited by the dominance of .
Therefore, we can conclude that the proposed ADC follows the theoretical effects of multiple sampling wells. Graph of the effects of the multiple sampling effect with theoretical curve (MF: multiple frame, MS: multiple sampling). Therefore, for using the proposed ADC architecture in CIS, the power consumption of ramp generators is not significant.
Moreover, in Figure 42 it indicates that the energy consumption estimation of conventional multiple sampling with single slope ADC for a 16 times the sampling number. Therefore, the digital power will increase more than 16 times compared to one of the conventional single-slope ADCs. Comparatively, the proposed ADC can reduce energy consumption by 87.8% with signal dependence for multiple sampling.
With proposed ADC, we can suppress depth error at multiple sampling effects to 38% compared to conventional single-slope ADC. We can conclude that proposed ADC can be used all applications of image sensors.
Conclusion and further works
Reset noise (kTC noise) suppression idea in depth imaging
In Chapter 3, we know that reset noise (kTC noise) is dominant for 3-T pixels due to mismatch between reset value and signal value. Therefore, in order to reduce the effects of reset noise, by turning on the reset transistor in pixels during multiple sampling of reset value, the reset value is not frozen and can be equalized, and it can reduce the noise power to 1/M. Therefore, we can reduce the standard deviation of the reset value. In my opinion, accurate analysis of the effect of multiple sampling in reset noise is needed.
Before analyzing it, I can do measurement with changing digital codes from FPGA in test PCB board. a) Long RST method (b) Short RST method.
Depth measurement results
Stoppa, et al., "Time of Flight Image Sensors in 0.18 uCMOS Technology: A Comparative Review of Different Approaches" Proceedings of the International Image Sensor Workshop, p. Lange, "3D Time-of-Flight Distance Measurement with Custom Solid State Image Sensors in CMOS/CCD Technology" Ph.D. Tang, et al., "Two-Step Single Slope/SAR ADC with Error Correction for CMOS Image Sensor", Sci.
Kim, et al., "An Area-Efficient and Low-Power 12-b SAR/Single-Slope ADC without Calibration Method for CMOS Image Sensors," in IEEE Transactions on Electron Devices, vol. Kawahito, et al., "Noise Reduction Effects of Column-Parallel Correlated Multiple sampling and source-follower driving current switching for CMOS image sensors,” in International Image Sensor Workshop (IISW), Jun. Lim, et al., “A 1.1 e- Temporal Noise 1/3.2-inch 8Mpixel CMOS Image Sensor Using Pseudo-Multiple Sampling,” in IEEE Int.
Shinozuka, et al., "A Single-Slope Based Low-Noise ADC with Input-Signal-Dependent Multiple Sampling Scheme for CMOS Image Sensors," in Proceeding of IEEE Int. Yeh, et al., "A 0.66erms− Temporal-Readout-Noise 3-D-Stacked CMOS Image Sensor With Conditional Correlated Multiple Sampling Technique, IEEE J. Lee, et al., "A Low Noise Single-Slope ADC with Signal -Afhængig Multiple Sampling Technique," i Proc.
13] S.-J Kim, et al., "A Three-Dimensional Time-of-Flight CMOS Image Sensor With Pinned- Photodiode Pixel Structure," IEEE Electron Device Lett., vol.
The energy equation of photoelectric effect
Conversion time equation of single-slope analog-to-digital converter
Reset noise equation in charge domain and voltage domain
Recombination-generation current equation
Minority carrier diffusion current equation
Photon shot noise equation
Flicker noise equation in voltage domain
High gain pre-amplifier technique noise suppression equation
Multiple sampling technique noise suppression equation
Conversion time of multiple sampling with single-slope ADC
Conversion time of multiple sampling with single-slope ADC
Conversion time of proposed signal-dependent multiple sampling ADC
Equation for cap modifying slope calibration
