Columnparallel adcs for cmos image sensors and their. Common architectures singleslope ramp today cyclic sar deltasigma. The time it takes for the integrator to trip the comparator is proportional to the unknown voltage t int v in. The conclusion of the study showed the efficiency of the dual slope adc as compared to other adcs, most especially the single slope adc keywords dual slope, adc, 8 bit, counter, cascaded 4 bit. However, multiramp signals are required for the proposed scheme and the first single slope adc must meet a full resolution noise specification. Single slope adc architecture the simplest form of an integrating adc uses a single slope architecture figures 1a and 1b. Each analog input channel is multiplexed to a single analog input source to be converted by means of a slope conversion method using a single precision comparator. Realtime calibration of a 14bit single slope adc with 290mhz onchip accelerated ramp generator for columnparallel image sensors jonathan bergey, sam bagwell, jackson law, wilson law. But, to achieve a highspeed operation without using a highspeed clock is almost impossible. A single slope adc, particularly suitable for use in a massiveparallel adc architecture in a readout circuit of a cmos imager. Singleslope analogtodigital conversion technique using msp430 mcus figure 2. For each adc channel, the subrange in which the voltage of the input signal falls is determined, and the corresponding ramp signal is selected for use in the a. A 12bit highspeed columnparallel twostep single slope ss analogtodigital converter adc for cmos image sensors is proposed. In order to achieve a high speed operation, using a very highspeed clock will increase the power consumption of the adc significantly.
The single slope adc suffers all the disadvantages of the digital ramp adc. Ee247 lecture 19 university of california, berkeley. Here, an unknown input voltage is integrated and the value compared against a known reference value. Deyan levski columnparallel adc readout architectures for cmos image sensors 235.
Integrating adc overview aka ramp and slope adc different flavors, e. Singleslope adc architecture the simplest form of an integrating adc uses a singleslope architecture figures 1a and 1b. The programmable current source feeds an external capacitor to generate the ramp voltage used in the conversion. This thesis presents the design of a 12bit column parallel twostep multi slope tsms analog to digital converter for low power cmos image sensors. This adc circuit behaves very much like the digital ramp adc, except that the comparator reference voltage is a smooth sawtooth waveform rather than a stairstep.
Improving single slope adc and an example implemented in. In its basic implementation, the dualslope converter, the unknown input voltage is applied to the input of the integrator and allowed to ramp for a fixed time period the runup period. Lets take a look at two of them, single slope adc and deltasigma adc. Twostep single slopesar adc with error correction for.
Microelectronics journal vol 81, pages 1208 november. The single slope adc suffers all the disadvantages. A columnparallel dualgain single slope adc comprises an input for receiving a signal vin, a sampleandhold stage which receives vin and outputs sampled signal vin,samp, a comparator, a counter, and a ramp generator which generates highgain hg and lowgain lg ramps, with the ratio of the lg ramp slope to the hg ramp slope being greater than 1. As a minimum, each device contains the integrator, zero crossing comparator and proc essor interface logic. Like the wellknown columnlevel singleslope adc, an mrss adc uses a very simple analog column circuit, which mainly consists of an analog comparator and some switches. The proposed adc employs a single ramp voltage and multiple reference voltages, and the conversion is divided into coarse phase and fine phase to. This paper presents an areaefficient and lowpower 12b successive approximation register single slope analogtodigital converter sarss adc for cmos image sensor cis applications. Singleslope analogtodigital ad conversion by stephen ledford csic product engineering austin, texas introduction the most common implementation for analogtodigital ad conversion among motorola microcontrollers is the successive approximation sar method. This differential single slope adc is composed of a differential comparator, a tdc timetodigital convert latch, an encoder, and two counters. Us7924207b2 single slope analogtodigital converter. Twostep single slope adc was proposed in the prior art 5, 6 in order to solve the operation speed issue of the traditional single slope adc. The single slope ss adc is a simplified architecture. Single slope adcs are very popular for a columnparallel adc in ciss because of its very simple circuit con.
These adcs are integrating type adc, meaning that they use the integrator for the conversion. Slope integrating adc slope integrating adc the single slope adc suffers all the disadvantages of the digital ramp adc, with the added drawback of calibration drift. Us10205463b1 dualgain singleslope adc with digital cds. Realtime calibration of a 14bit single slope adc with. An integrating adc is a type of analogtodigital converter that converts an unknown input voltage into a digital representation through the use of an integrator. Dual slope type adc in dual slope type adc, the integrator generates two different ramps, one with the known analog input voltage va and another with a known reference voltage vref. To convert analog signals to digital ones is needed to keep. The resulting readout structure, usually called a columnlevel adc is suitable for veryhigh resolution imagers. An alternative ad conversion technique uses the singleslope ad converter.
The tc500 is the base 16bit max device and requires both positive and negative power supplies. Pdf r 46 12bit columnparallel singleslope adcs with. Tsms adc architecture enables larger conversion speeds compared to widely implemented single slope architecture on its twostep single slope tsss mode. During charge phase, auxiliary clock aclk is the source clock, and therefore the sleep mode low. A plurality of ramp signals are generated which define nonoverlapping subranges of the full input range. The sensor used is a conventional 4t active pixel with a pinnedphotodiode as photon detector. There are a few ways of designing analogtodigital converters using an integrator. An example of sh circuit is given below the role of the capacitor is to be charged. Singleslope analogtodigital conversion technique using.
A critical building block in the singleslope columnparallel architecture is the ramp generator. The is the basic idea behind the socalled single slope, or integrating adc. Instead of using a dac with a ramped output, we use an opamp circuit called an integrator to generate a sawtooth waveform which is then compared against the analog input by a comparator. Adc architectures slope type converters successive approximation flash timeinterleaved parallel converter. Abstract single slope adc is a common building block in many asci or fpga based frontend systems due to its simplicity, small silicon footprint, low noise interference and low power consumption. A columnparallel architecture has become popular for analogtodigital converter adc integration in cis, in that it features a good tradeoff among frame rate, column number, noise, and power consumption. A dual slope adc ds adc integrates an unknown input voltage v. A highspeed cmos image sensor with columnparallel single. However, in a practical highspeed single slope adc, the overhead associated with the comparator turned out to be more significant than originally thought. A 12bit highspeed columnparallel twostep singleslope. A low noise global shutter cmos image sensor with multiple. The single capacitor cds circuit has only one capacitor so. At the end of that time it is attached to the reference voltage sw1 down. Spinbased logicinmemory analog to digital converter leveraging sheenabled domain wall motion devices soheil salehi, ronald f.
Columnparallel single slope adc with digital correlated. An alternative to a single adc is to move the task of analogto digital conversion into the readout column fig. Department of electrical engineering stanford university dps02 1. Sensor noise, fpn, linearity, and yield characteristics depend greatly on the ramp generator design and specifications. Each analog level in the adc range is stepped through and compared to the input signal figure 2. In this case, the known reference voltage must be stable and accurate to guarantee the accuracy of the measurement. Abstractthis paper presents a low noise cmos image sensor cis using 1012 bit configurable columnparallel single slope adcs ssadcs and digital correlated multiple sampling cms. The key operating principle of such structures is to translate an input voltage to a.
A singleslope 80mss adc using tw ostep timetodigital. Requirements on adcs for imaging ldr visible light. The time it takes for the integrator to trip the comparator is proportional to the unknown voltage tintvin. Simple dual slope ad converter dual slope ad converter output and timing dualslope adc consider this circuit. In this case, the known reference voltage must be stable and accurate. Single slope adc an advantage in using single slope adcs in a column parallel architecture is the simplicity of its design. A prototype imager using the mrss adc architecture was realized in a 0.
In operation the integrator is first zeroed close sw2, then attached to the input sw1 up for a fixed time m counts of the clock frequency 1t. This paper presents a high speed cmos image sensor cis with columnparallel single capacitor correlated double samplings cdss, programmable gain amplifiers pgas and single slope analogtodigital converters adcs. Analysis and improvement of ramp gain error in singleramp. Comparison of several ramp generator designs for column. First of all, because the ramp signal drives an adc in every column, it needs to be immune to kickback noise from. Single slope conversion or integrating conversion is a classical means of implementing an analogtodigital converter adc 1, and has the advantage of having a very simple implementation with minimal analog content. Columnparallel single slope adc with digital correlated multiple sampling for low noise cmos image sensors article pdf available in procedia engineering 25 december 2011 with 35 reads. Hence it is called a s dual slope a to d converter.
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