A Low-Power Column-Parallel Gain-Adaptive Single-Slope ADC for CMOS Image Sensors

: A low-power column-parallel gain-adaptive single-slope analog-to-digital converter (ADC) for CMOS image sensors is proposed. The gain-adaptive function is realized with the proposed switched-capacitor based gain control structure in which only minor changes from the traditional single-slope ADC are required. A switched-capacitor controlled dynamic bias comparator and a ﬂip-reduced up/down double-data-rate (DDR) counter are proposed to reduce the power consumption of the column circuits. A 12-bit current steering digital-to-analog converter (DAC) with a two-dimensional gradient error tolerant switching scheme is adopted in the ramp generator to improve the linearity of the ADC. The proposed techniques were experimentally veriﬁed in a prototype chip fabricated in the TSMC 180 nm CMOS process. A single-column ADC consumes a total power of 63.2 µ W and occupies an area of 4.48 µ m × 310 µ m. The measured differential nonlinearity (DNL) and integral nonlinearity (INL) of the ADC are − 0.43/+0.46 least signiﬁcant bit (LSB) and − 0.84/+1.95 LSB. A 13-bit linear output is acquired in nonlinearity within 0.08% of the full scale after calibration.


Introduction
CMOS image sensors (CISs) are widely used in mobile phones, surveillance security, autonomous driving, machine vision, and the Internet of Things. Figure 1 shows a CIS system including pixel array, control signal and clock generator, readout circuits, memory, and other modules. As illustrated in Figure 1, a typical 4T pixel includes a photodiode (PD), a transfer transistor (M TX ), a reset transistor (M R ), an amplifier transistor (M SF ), and a select transistor (M S ) [1].
Currently, the signal readout architecture with column-parallel analog-to-digital converters (ADCs) is often used in the CIS system. The column-parallel ADCs make it possible to read out image signals from the pixel array with less noise and higher bandwidth and to transfer the image data to outside at very high pixel rate with no degradation of signal quality [2]. The column-parallel ADC is an important part of the CIS readout circuits and a key element to affect the performance of CISs. As the resolution of CIS increases, the pitch of pixel decreases. The column-parallel ADC must be designed under very severe constraints because it must be embedded into the narrow column and arranged as an array of many elements [2].
The column-parallel single-slope ADC (SS-ADC) architecture is widely adopted for CIS because of its simplicity, low power consumption, and high linearity [3]. Figure 2a shows the single-slope ADCs in a CIS system consisting of a global ramp generator, comparators, and up/down counters. The ramp generator is One of the main drawbacks of the SS-ADC is the long conversion time. The clock cycles per conversion is an exponential function of the ADC bit depth. In SS-ADCs, it is generally thought to be difficult to get both high speed and high A/D resolution because the A/D resolution is restricted by the number of clock cycles [6]. On the one hand, with the increasing demands of high-quality imaging, high-resolution ADCs are required. On the other hand, the analog-to-digital conversion time per row is only a few microseconds in high resolution CISs. Thousands of SS-ADCs work simultaneously and the counting clock usually goes up to several GHz, which makes the power consumption quite high. In high resolution CISs, the power consumption of the SS-ADCs, which is over 40% of that of the entire CIS [7], has become a major problem. The trade-off between power consumption and conversion time has become a big challenge in the design of SS-ADC.
Recently, some methods to reduce the power consumption of the SS-ADC in CIS have been published. In Reference [8], a hold-and-go counter scheme with a global counter is proposed to realize CDS and reduce power consumption. However, the speed of the global counter is limited because of its large load. In Reference [9], a global 5-bit least significant bit counter is shared among every 248 columns and the power consumption of the counters is reduced. However, the refreshing of the memory cells in columns will lead to a large amount of power waste. In Reference [10], a passing window technique is used to save power in LSB memories and data bus driver, but the complexity and area of the column ADC will increase. A dynamic SS-ADC [11] based on a dynamic bias comparator and a two-step counter can achieve high energy efficiency but the fully dynamic circuits may cause more random noise and power supply variation.
In this paper, a low-power gain-adaptive SS-ADC is proposed to reduce the conversion time and power consumption of SS-ADC. The gain-adaptive function is realized with the proposed switched-capacitor based gain control structure. To reduce the power consumption, a switched-capacitor controlled dynamic bias comparator and a flip-reduced up/down double-data-rate (DDR) counter are proposed. In addition, a 12-bit current steering DAC with two-dimensional gradient error tolerant switching scheme is adopted in the ramp generator to improve the linearity of the ADC. With the 500 MHz counting clock and 2.2 µs for the signal level in per conversion, a conventional SS-ADC can only achieve 10-bit A/D conversion, while the proposed ADC acquires 13-bit linear digital outputs based on the gain-adaptive function and DDR counter.
This article is organized as follows. Section 2 illustrates the concept of the proposed gain-adaptive SS-ADC. Section 3 presents the circuit configuration of the SS-ADC including the comparator, the counter, and the ramp generator. Section 4 describes the nonlinearity of the gain-adaptive SS-ADC and the calibration method. Section 5 shows the experimental results. Finally, conclusions are presented in Section 6.

Concept of Gain-Adaptive SS-ADC
The pixel response to light intensity is plotted in Figure 3 along with main noise sources of CIS. Temporal noise in CISs can be divided into two main categories. Most of the noise sources are basically independent of light intensity, such as thermal noise, 1/ f noise, etc., which can be regarded as a "noise floor" and limit the dynamic range of the CIS [12]. In order to ensure that the quantization noise of the ADC does not deteriorate the performance of the system noise, the ADC is usually designed to make the quantization noise lower than the noise floor. The other type of noise is photon shot noise, which is dependent on the signal level and can be expressed as follows: (1) where N sig is the signal level and n shot N sig is corresponding photon shot noise. N sig and n shot N sig are both expressed in the number of electrons. With the increase of light intensity, photon shot noise becomes the dominant noise source of the system. In this situation, the quantization noise of the ADC is much lower than the photon shot noise, and the ADC actually has a better noise performance than is required. Therefore, in the situation of high light intensity, even if the quantization noise of the ADC is increased, it will not affect the overall noise performance of the system. The reason why the SS-ADC is slow is because its working principle is continuously integrating with the smallest quantization step in the entire input range. If the quantization step can be increased with the increase of light intensity, the clock cycle required can be greatly reduced, the speed of the SS-ADC can be effectively increased. Within a certain conversion time, the required counting clock frequency is reduced which will reduce the power consumption. Based on the above analysis, multiple-ramp [12], nonlinear-slope [9,13,14], and gain-adaptive [7,15,16] techniques have been proposed to realize high A/D resolution in a short conversion time, which is shown in Figure 4. The multiple-ramp signal path occupies more area and the calibration of offset errors among different ramps is difficult. The nonlinear-slope functions usually produce significant nonlinearity, especially at the knee point. Conventional gain-adaptive techniques employ a programmable gain amplifier (PGA) in each column to change the analog gain according to the pixel output. However, the power consumption and area occupied of the column-parallel PGAs are large. To solve the problems, we proposed a low-power gain-adaptive SS-ADC with a switched-capacitor based gain control structure. The block diagram of the proposed gain-adaptive SS-ADC is shown in Figure 5. The gain-adaptive function is realized by adjusting the slope of the ramp signal in each column according to the signal level of the pixel. A high-gain (HG) ramp with a small slope is realized in the dark condition and a low-gain (LG) ramp with a large slope is realized in the bright condition. The slope adjustment is realized by the switched-capacitor based gain control structure. This approach only requires minor changes from a conventional single-slope ADC with small overhead of area by adding several switches and registers..  Figure 6 illustrates the switched-capacitor based gain control structure. In a conventional SS-ADC, the ramp signal V RAMP and pixel output V PXD are connected to the input of the comparator through two capacitors C IP and C I N of the same value. The capacitors allow the comparator to achieve the auto-zero operation to eliminate the offset voltage and determine the appropriate operating point. In the gain control structure, the capacitor C I N between the pixel output node V PXD and the negative input terminal of the comparator V I N is set to 4C, where C is the unit capacitance value. The capacitor C IP between the ramp signal V RAMP and the positive input terminal of the comparator V IP is divided into three parts of 1:1:2.

Gain Control Structure
The upper plates of the three capacitors (C IP0 , C IP1 and C IP2 ) are connected to the V IP node, and the lower plate of the capacitor C IP0 is connected to V RAMP . The lower plates of C IP1 and C IP2 are connected to V RAMP or ground controlled by the switches. AG <1: 0> and AGB <1: 0> are two sets of opposite control voltage to control the corresponding switches. The switches (S0 and S1) connected to the lower plate of the capacitor C IP0 are fixed and used for circuit matching. V RAMP is an input ramp signal with a certain slope. By changing the connection of the capacitors between V RAMP and V IP nodes, the voltage relationship between V RAMP and V IP can be changed to adjust the slope of the ramp signal in V IP node. The smaller the slope of the ramp, the higher the analog gain of the pixel.
The voltage relationship between V IP and V RAMP with different control voltages is shown in Figure 7. In Figure 7a, the control voltages are AG<1: 0> = 00 and AGB<1: 0> = 11, so the lower plates of the capacitors C IP1 and C IP2 are connected to V RAMP . V IP is consistent with V RAMP , and the analog gain is 1, which is the low-gain (LG) scheme in this design. In Figure 7c, the control voltages are AG<1: 0> = 10 and AGB<1: 0> = 01, so the lower plate of capacitor C IP2 is connected to ground, and the lower plate of capacitor C IP1 is connected to V RAMP . The relationship between V IP and V RAMP is Here, and the analog gain is 2, which is the medium-gain (MG) scheme in this design. Similarly, the voltage relationship in Figure 7d can be obtained as follows: The analog gain is 4, which is the high-gain (HG) scheme in this design. The gain schemes and corresponding control voltages are summarized in Table 1.    Figure 8 shows the timing diagram of the proposed gain-adaptive SS-ADC in a horizontal scanning period. In (1), the pixel reset level is sampled for analog CDS with the same method in [17]. The comparator is reset and the comparator offset voltage is sampled during (2). The reset level is down-counted in the counter respectively in (3) with the high-gain scheme. After signal readout in (4), the pixel output voltage is compared with two threshold voltages V OM1 and V OM2 to estimate the voltage level. The control voltages are obtained by the comparison results of the pixel output voltage to the threshold voltages and stored in the flipflops. If V PXD is darker than V OM1 , AG<0> is equal to 1. If V PXD is brighter than V OM1 , AG<0> is equal to 0. If V PXD is darker than V OM2 , AG<1> is equal to 1. If V PXD is brighter than V OM2 , AG<1> is equal to 0. According to the control voltages obtained in (5), a gain-adaptive signal readout scheme is implemented in process (6). In dark condition, the high-gain (HG) scheme is employed as the blue line in Figure 8. The low-gain (LG) scheme is employed in the bright condition as the red line in Figure 8. The black line in Figure 8 represents the medium-gain (MG) scheme.

Comparator
In the conventional SS-ADC, as the input ramp signal changes continuously, a static comparator is usually adopted and always kept on, which leads to significant static power waste. If a lower current is supplied to the static comparator, the response of the comparator will slow down, causing a risk of the degradation of the A/D conversion performance [5]. In fact, a high supply current only needs to be maintained during the comparison period to ensure the speed of the comparator, while the current can be lower during the non-comparison period, which only changes the common static operating point of the comparator without affecting the comparison. For the above reasons, a switched capacitor controlled dynamic bias comparator is proposed to reduce the power consumption. As shown in Figure 9, the proposed comparator consists of first amplification stage (Amp1), second amplification stage (Amp2), and output stage. The gate of the tail current transistor of Amp1 is biased by a switched capacitor.  Figure 10 shows the timing diagram of the control signal of the comparator during an entire A/D conversion period, which includes an autozero period, comparison periods, and hold periods. For simplicity, the gain-adaptive function is not shown in the diagram. The working process of the comparator is explained as follows: • During the autozero period, switches S1 and S6 are closed and S5 is open. The lower plate of the capacitor C B1 is connected to the ground and the gate voltage of the transistor M6 (V G ) is equal to V B .

•
During the comparison period, switches S1 and S6 are open and S5 is closed. The lower plate of the capacitor C B1 is connected to the boost voltage V BST and V G = V B + V BST .

•
During the hold period, the switches S1 and S5 are open and S6 is closed, and V G is equal to V B .
During the entire A/D conversion, the average current of Amp1 is where I 1 is the current of Amp1 during the autozero and hold periods, I 2 is the current of Amp1 during the comparison period, T AD is the time of an A/D conversion period, T AZ is the time of an autozero period, T H is the total time of hold periods, and T CMP is the total time of comparison periods. While I 1 is lower than I 2 , the average current I ave is lower than the static current in the conventional structure, which is equal to I 2 . As shown in Figure 10, switch S1 is only closed for a short time during the autozero period, which might prevent noise coupling between different columns during comparison periods. Switch S3 is closed once before each comparison to set the output of Amp1 (V 1 ) high to ensure the same initial state for comparison. The transistor M8 is used to clamp the output voltage of Amp2 (V 2 ) to prevent the transistor M10 operating in the triode region.

Counter
A double-data-rate counter [18], also called double-edge-counting (DEC) counter, is capable of performing counting at both edges of the clock. In a traditional up/down DDR counter, a latch is used so that the lowest bit circuit can operate at the same frequency as the input clock. However, a latch and an inversion selector in the LSB circuit perform high-speed operation in synchronization with a high frequency input clock and consume a large amount of current [19].
To reduce the power consumption, the lowest bit circuit is redesigned in the proposed counter as it consumes the most power. Figure 11 shows the block diagram of the proposed flip-reduced up/down DDR counter [20]. In the proposed structure, the latch in the LSB circuit is replaced by two D flip-flops. The outputs of the two D flip-flops (QX and QN) change only when the output of comparator (CMP) changes. The lowest bit circuit can count at both edges of the input clock, but only records the parity information of the lowest bit. The proposed counter counts with twice the frequency as an input clock, but the LSB circuit (DDR Cell) flips much fewer times than conventional counters in both down and up counting phases.   The DDR counter needs to be modified to realize CDS operation with the gain-adaptive function. The counting process of the signal voltage is shown in Figure 13. Since the reference voltage is converted in HG scheme, if the signal voltage is also converted in HG mode, the counter will work directly without adjustment. If the signal voltage is converted in MG scheme in which the quantization step is 2 LSB, the DDR Cell is directly input to Counter Cell [2], and Counter Cell [1] is bypassed. In order to ensure correct counting, the reference counting result of Counter Cell [1] should be written into the DDR Cell before the conversion of the signal voltage. Similarly, the quantization step in LG scheme is 4 LSB and the DDR Cell is directly input to Counter Cell [3]. Counter Cell [1] and Counter Cell [2] are bypassed. The reference counting result of Counter Cell [2] should be written into the DDR Cell before the conversion of the signal voltage. The modified DDR Cell is shown in Figure 14. A XOR gate is used to decide whether to change the value of D[0] for signal level conversion. A latch is used to hold the value of D[0] of the reference level conversion.

Ramp Generator
A 12-bit current steering DAC with 6+6 architecture and two-dimensional gradient error tolerant switching scheme [21] is adopted as the ramp generator to improve the linearity of the ADC. The block diagram of the ramp generator is shown in Figure 15. When the most significant bit (MSB) unary source is divided into 16 elements located in 16 separated regions of the array, the linear gradient errors and quadratic errors can be averaged and eliminated effectively. Figure 16 shows the scheme for 6-bit unary decoding of the MSB current source. The 6-bit unary decoding array is generated from a basic 4-bit decoding array [22]. Each unary source has exactly one element in each 8×8 region.  A common noise canceling section [23] is adopted to reduce the effect of power supply noise on the ramp generator. The differential output of DAC is input to the common noise canceling section. The power supply noise is regarded as common noise which can be canceled through a differential operation between the differential ramp signals. The output of the ramp generator is where V DAC is the single output of the DAC.

Group Delay Scheme
The column ADCs are divided into groups as shown in Figure 17. The clock and control signals are delayed for every four column ADCs to prevent gates in different groups, especially the counters, from flipping at the same time, which would cause transient large current from source. The clock signals for adjacent groups are in the opposite phase to further suppress power source variations. Figure 18 shows the diagram of supply current of counters during an A/D conversion period with/without group delay scheme. The simulation result of the maximum value of the transient current of 1000 counters is also shown in the diagram. Two horizontal bit lines are grouped by parity for every bit so that the frequency of the readout clock can be reduced to half and the power consumption in the readout phase can also be lower.

Nonlinearity and Calibration
The gain-adaptive function realized by adjusting the slope of the ramp to change the analog gain is relatively simple, but the quantization result usually has nonlinearity and needs calibration. One of the main reasons of the nonlinearity is that the comparator is not an ideal model. When the ramp signal reaches the pixel output voltage, the comparator output does not immediately toggle, and there will be a delay time. Figure 19 is a timing diagram of the comparator comparison process, in which the total counting time of the counter t c is defined as Here, t p is the time from the beginning of the ramp to when the ramp voltage is the same as the pixel voltage. For a certain pixel voltage, t p is inversely proportional to the slope of the ramp. t d is the time from when the ramp voltage is same as the pixel voltage to when the comparator output toggles, defined as the toggle delay of the comparator. Since t d is not inversely proportional to the slope of the ramp s r , for a certain pixel voltage, changing the slope of the ramp by a certain ratio will not completely inversely change the total counting time by the same ratio. Figure 20 shows the simulation result of the total counting time of the SS-ADC with different slopes of the ramp signal under a certain input voltage. The relationship between t c and s r is not completely inversely proportional, reflecting a kind of nonlinearity. In addition, due to the non-idealities in the circuit, the slope of the ramp will change, resulting in gain errors. In the switched-capacitor structure, gain errors may come from the effects of charge injection and clock feedthrough of the switches, or the influence of the parasitic capacitance of each node on the charge distribution relationship. The gain error brought by the slope error of ramp signals is also an important source of the nonlinearity of the quantization result.
The quantization result with nonlinearities produced by the above two reasons is shown in Figure 21. The nonlinearity between the counting time and the slope of the ramp will produce a jump near the gain switching point, which can be regarded as an offset voltage. The slope mismatch will produce a gain error, which is reflected as the nonuniformity of slope between quantization lines of different gain schemes.
The gain ratio of α is defined as The gain ratio of β is defined as The calibration of gain errors is achieved by multiplying the MG and LG quantization results by the ratio α and β. The offset between the MG and HG lines is The offset between the LG and HG lines is For a MG quantization result D MG , the calibration result iŝ For a LG quantization result D LG , the calibration result iŝ

Experimental Results
The single-slope ADCs prototype is fabricated in a 180 nm standard CMOS process, and the microphotograph of the chip is shown in Figure 22. In order to focus on the characterization of the ADC design, only 100 columns of single-slope ADCs are implemented in this prototype. The control signals are generated on chip and the counting 500 MHz clock is generated externally and buffered on chip. The SS-ADC is implemented within a column pitch of 4.48 µm and a vertical length of 120 and 190 µm for the comparator and the counter, respectively.   Figure 24 shows the power breakdown of the SS-ADC in one column with a total power of 63.2 µW. The single-column power of the global ramp generator is calculated by dividing its power by the number of columns. Figure 25 illustrates the relationship between the power consumption of the proposed comparator and the boost voltage V BST . With the proposed dynamic bias technique, 32% of the power consumption of the comparator can be saved when V BST is chosen as 0.6V. As shown in Figure 26, 54% of the power consumption of the counter is saved with the proposed DDR structure compared with the conventional ripple counter and 20% of the power consumption is saved compared with a previous DDR counter [19]. According to the post layout simulation, the ramp generator can drive more columns, and therefore the power consumption of one-column SS-ADC can be lower in a higher resolution design with the counting clock of same frequency. Figure 27 shows the linearity of the gain-adaptive SS-ADC. With the 500 MHz counting clock and 2.2 µs for the signal level in per conversion, a conventional SS-ADC can only achieve 10-bit A/D conversion, while the proposed ADC achieves 13-bit linear digital outputs based on the gain-adaptive function and DDR counter. The nonlinearity of the proposed ADC is only 0.08% of the full scale after calibration.   Table 2 summarizes the performance of the implemented ADC compared with prior works. This work achieves low power consumption and high A/D resolution. The power efficiencies were compared using figure of merits (FoMs) calculated as follows [3]:

Conclusions
A 63.2 µW per-column gain-adaptive single-slope ADC for CMOS image sensors was designed and fabricated in a 180 nm CMOS process. With the proposed switched-capacitor controlled dynamic bias comparator and flip-reduced up/down DDR counter, total power consumption is reduced. With the proposed gain-adaptive function, the ADC achieves total 13-bit linear A/D resolution in nonlinearity within 0.08% of the full scale after calibration. The occupied area of one column ADC is 4.48 µm × 310 µm. The prototype ADC has a competitive FoMs performance with the proposed gain-adaptive structure and low power techniques. The proposed techniques are suitable for low-power and high-resolution CMOS image sensors.
A limitation of this work is that the performance of the proposed ADC was independently tested without pixels. However, the proposed gain-adaptive SS-ADC has already been implemented in a 5-megapixel CIS chip, which has been taped out and is being manufactured in a 90 nm CIS process. The performance of the proposed techniques will be tested and verified in the CIS system.