# Introduction any and the communication systems today digital signal processing (DSP) to resolve the transmitted information. Therefore between the received analog signal and DSP system an analog-todigital interface is necessary. This interface achieves the digitization of received waveform subject to a sampling rate requirement of the system. Being a part of communication system, the low power constraint mentioned above the A/D interface also needs to address to the low power constraint. There are many ADC architectures; pipelined ADCs are advantageous and widely used in applications with signal bandwidths that are too high for oversampling delta-sigma ADCs and resolution requirements that are too high for flash ADCs. Nevertheless they are sensitive to distortion introduced by the residue amplifiers in their first few stages, and residue amplifier distortion tends to be inversely related to both power supply voltage and power consumption. Therefore, the residue amplifiers are usually the dominant consumers of power in highresolution pipelined ADCs, particularly in low supply voltage designs [1]- [6]. Among the key building blocks in pipelined ADCs are the residue amplifiers that interface successive converter stages. Especially in the converter front-end, these gain elements have to meet very stringent speed, noise, and linearity requirements and tend to dominate overall power dissipation. To address this issue, a variety of techniques have been developed to minimize amplifier power in pipelined ADCs. Among them, stage scaling [3], [4], optimization of the per-stage resolution [5]- [7], and amplifier sharing techniques [8], [9] are commonly used. In addition to their dominance in power consumption, it has also been recognized that residue amplifiers are most susceptible to complications that arise from continuing integrated circuit technology scaling [10], [11]. For implementations in future deep submicron processes, it is often predicted that limited supply headroom and low intrinsic device gain may lead to a relative power increase in such noise-limited precision analog circuit blocks [12], [13]. In this paper, a pipelined ADC is designed that achieves superior SNR, and operates at 100Msps, with power dissipation less than 70mW. Section II discusses pipelined ADC architecture, section III discusses design of software model for proposed pipelined ADC. Section IV discusses the schematic design of pipelined ADC and layout design. Section V presents conclusion. a) Architecture of Pipelined ADC The purpose of analog to digital converts is to sample and digitize an analog signal. The more precisely the analog signal is converted to digital, the more information can be obtained from it. It is also desirable to convert high bandwidth or high frequency analog signals; thus, ADCs must be capable of a fast sampling rate as well being accurate. The pipeline ADC is the architecture of choice for applications that require both speed and accuracy and where latency is not ear 2012 Y concern. The basic idea behind the pipeline ADC is that each stage will first sample and hold the input then it is compared with VREF/2. If the input is greater than VREF/2, output a 1 for that stage and pass the input voltage directly to the next stage. If the input is less than VREF/2, output a 0 for that stage and multiply the input voltage by 2 before passing it to the next stage. Figure 1 shows the block diagram for this basic operation. Fig. 1 : Pipeline ADC Block Diagram [1] There are a few challenges with the basic pipeline ADC architecture that this project will attempt to address. Before looking at the sources of error, it is worth noting that an error in the early stages of the pipeline will propagate through the pipeline affectively being amplified by 2 by each successive stage. Errors can be created by the comparators not switching at the correct point. This means that the comparator may have some offset which will result in it making the wrong decision. The sample and hold may also have some offset causing the wrong voltage to be passed to the comparator which will result in the same problem of the comparator making a wrong decision. The other source of error is the multiply by 2 function, because it is difficult to multiply by a gain of exactly 2. These limitations with real op-amps and comparators will result in integral nonlinearity (INL) and differential nonlinearity (DNL) errors. This design requires 12-bit resolution. This means that there will be 2 12 or 4096 possible output bit combinations. Assuming a VREF of 1V, 1LSB or the level of analog resolution is given by To correct the errors caused by the offsets in the comparators and the sample and hold op-amps, a technique called 1.5 bits/stage will be used. The name 1.5 bits/stage is based on the fact that each stage has an output with three possible cases consisting of a and b signals, where ab can be 00, 01, or 11. The ideal transfer curve for the 1.5 bits/stage of vin versus vout is shown in Figure 2. And, the relationship between vin and vout can be expressed as: The functional block diagram of the pipelined ADC is simulated using Simulink in MATLAB. The 1-bit single stage converter behavioural block has been designed and simulated. The behaviour of the block has been developed using the equations given below. ? If Vin > Vmid Vresidue=2(Vin-Vref) ? If Vin < Vmid Vresidue=2(Vin) Using a reference value this single stage block would act as a comparator and gives a bit as output. Again that bit will be converted in to analog value that value again compared with the actual input signal and the difference signal will be produced. The difference signal will be in the range of half of the actual input range. Since the total pipelined architecture has been developed using these similar types of 1-bit single stage blocks should give the same input rage to all stages. To get that voltage range we should multiply the error signal with 2 then we can give output of one stage to input of next stage. Every stage has its own sample and hold circuit operating at 100 MS/s. the output of the sample and hold stage will be consider as the input to the 1-bit converter. Figure 3 shows the software reference model of 1-bit conversion. The 3dB Bandwidth of the op-amp is selected as 100 MHz to meet the application of the architecture. The schematic has simulated by the cadence specter and layout has drawn by the virtuoso. The simulated transient and ac analysis waveforms are shown in the Figure 8. The digital outputs for a given input analog sample are not generated at the same time. MSB comes first and LSB last. The time delay between adjacent bits is one half clock cycle. All bits need to be synchronized. The 1-bit digital output from the first stage is delayed by 12 half cycles and the output from the second stage is delayed by 11 half cycles and so on. The output from the last stage is delayed by a half cycle. The delay block is made of D flip-flops (DFF) implemented with transmission gate and static NAND gates. Since sampling rate is only 100 MS/s and the word length is 12 bits the carry ripple is not an issue under 0.13µm process. g) 1-Bit Single stage of Pipelined Architecture This stage is consist of sample and hold circuit followed by 1-bit ADC, 1-bit DAC, subtracted and multiplier. The analog signal will be sampled and fed to the comparator acts as the 1-bit ADC that would give the 1-bit digital output. Before giving to the comparator the sample signal should lift to the 0.9V of the DC voltage, so that the comparator can compare the value to the threshold voltage and give the output. # Conclusion The goals of this paper is First different ADC architectures were analyzed to determine the optimal topology for the given performance specifications with minimum power consumption. Second the exact implementation of the chosen architecture was investigated in an effort to use the minimum amount of power. This Paper involved designing an integrated CMOS Analog-to-Digital converter for communication and video applications. The performance specifications were 12-bits, power dissipation less than 70 mW, area should be less than 6.5mm 2 and static performance parameters such as INL and DNL should be less than 1 LSB and 0.5 LSB in order to make a monotonic ADC. This pipelined ADC has been met the performance requirements. The ADC was designed in 0.13um technology at an operating voltage of ±1.8 V. Sample and hold circuit is designed by the switched-capacitor implementation. We can use the rail to rail op amp to increase the input voltage range of the ADC but by increasing the input voltage swing the resolution will be affected. Common-mode drift issue Since there is no common-mode feedback inside the loop the commonmode drift caused by the mismatch of capacitors, offset of op amp and charge injection will accumulate stage by stage. Careful design and layout are supposed to minimize the total drift within 100 mV such that the residue output signal will not be out of saturation. But this problem is likely to cause trouble if the commonmode signal is not controlled well as expected. This page is intentionally left blank ![Global Journal of Researches in EngineeringVolume XII Issue vv v v XI Version I Journals Inc. (US)](image-2.png "M") 2![Fig.2 :1.5 Bits/Stage Transfer Curve (Single-ended)[1] ](image-3.png "Fig. 2 :") 3![Fig. 3 : Simulink model for the 1-bit conversion stage The input and output waveforms of this single stage are shown in Figure 4. The 1 MHz input sine wave has been sampled by the 100 MHz clock signal. Theoretically we can estimate the signal to noise ratio (SNR). SNR (worst case) = 6.02 n+4.24 dB Hence we have established the boundary conditions for the choice of the resolution of the converter based upon a desired level of SNR. Based on this calculation the 12-bit pipelined ADC having (SNR) 68 dB.](image-4.png "Fig. 3 :") 467![Fig 4 : Digital and Residue output waveforms of the single stage pipelined ADC](image-5.png "Fig 4 :Fig. 6 :Fig. 7 :") 89![Fig. 8 : Wideband opamp schematic diagram and AC analysis d) 1-Bit ADC design using CMOS InverterIn this paper however a new approach the Threshold Inverter Quantizer (TIQ) based on systematic transistor sizing of a CMOS inverter in a full-flash scheme eliminates the resistor array implementation of conventional Comparator array flash designs. Therefore no static power consumption is required for quantizing the analog input signal making the idea very attractive for battery-powered applications. We can estimate safe analog input rage as follows:Analog range = Vdd -(V TN + | V TP |), where V TN and V TP are the threshold voltages for large NMOS and PMOS devices, namely the V THO value from the model parameter data set used during the entire design process.](image-6.png "Fig. 8 :Fig. 9 :") 10![Fig. 10 : Schematic Diagram of 1-Bit DAC f) Buffer Designing This buffer stage is consist of number of Dffs, these Dff are designed using Tx-gates and NAND gates. The Schematics and Layout are shown in Figure 11. Through the practical calculations, I got the average rise and fall time is 0.18ns, and Setup Time is 154ps.](image-7.png "Fig. 10 :") 11![Fig. 11 : Schematic Diagram of 12-Bit Buffer](image-8.png "Fig. 11 :") 1213![Fig. 12 : Schematic Diagram of 1-Bit Conversion StageThe digital output again converted to the analog value through the 1-bit DAC, Uses two reference voltage levels. This converted value will be subtracted from actual sampled signal to produce an error signal using a difference circuit this signal often called as residue signal. This residue signal again multiplied by two with an open loop amplifier. The difference and the multiplier are working at 100 MS/s and input rage of +/-250mV.](image-9.png "Fig. 12 :Fig. 13 :") 14![Fig. 14 : Top-Level Schematic of pipelined ADCFigure15shows the frequency domain analysis of pipelined ADC. The output of ADC is captured and is processed using FFT processor to convert the time domain discrete samples to frequency domain samples. The frequency domain samples obtained are plotted and SNR is measured. In order to estimate the performance of ADC, a known input with single frequency is used as test vector. The frequency spectrum obtained is analyzed for the number of harmonics. The first five harmonics are considered to compute SNR.](image-10.png "Fig. 14 :") 15![Fig. 15 : Frequency domain response of pipelined ADC.](image-11.png "Fig. 15 :") ![Design and Implementation of Low Power 12-Bit 100-MS/s Pipelined ADC Using Open-Loop Residue Amplification](image-12.png "") ![Design and Implementation of Low Power 12-Bit 100-MS/s Pipelined ADC Using Open-Loop Residue Amplification](image-13.png "") 1PreviousPresent WorkworkTechnologyCMOSCMOS0.13um0.35umPower Supply 3V± 1.8VInputSignal200kHzup to 10 MHzFreqInputvoltage---+/-250mVrangePower290uW70mWdissipationArea7.9 mm26.5 mm2Sampling freq 75MHz100 MHz 1 © 2012 Global Journals Inc. (US) * SHLewis PRGray A pipelined 5 * Msample/s 9-bit analog-todigital converter IEEE J. Solid-State Circuits Dec. 1987 SC-22 * A 10 b 150 MS/s 123 mW 0.18 _m CMOS Pipelined ADC S.-MYoo IEEE Int. Solid-State Circuits Conf. Dig. Tech. Papers Feb. 2003 * TCho PRGray A10 b 20 * MSample/s, 35mWpipeline A/D converter IEEE J. Solid-State Circuits 30 Mar. 1995 * DWCline PRGray 13 * Msamples/s pipelined analog to-digital converter in 1.2-_m CMOS IEEE J. Solid-State Circuits 31 Mar. 1996 * Optimizing the stage resolution in pipelined, multistage, analog-to-digital converters for video-rate applications SHLewis IEEE Trans.Circuits Syst. II 39 Aug. 1992 * Systematic design for optimization of high-speed self-calibrated pipelined A/D converters JGoes JCVital JEFranca IEEE Trans.Circuits Syst. II 45 Dec. 1998 * A 14-bit 10-MHz calibration-free CMOS pipelined A/D converter LASinger TLBrooks Symp. VLSI Circuits Dig. Tech. Papers June 1996 * A 2.5-V, 12-b, 5-Msample/s pipeline CMOS ADC PCYu H.-SLee IEEE J. Solid-State Circuits 31 Dec. 1996 * A 69 mW 10 b 80 MS/s pipelined CMOS ADC B.-MMin PKim DBoisvert IEEE Int. Solid-State Circuits Conf. Dig. Tech. Papers Feb. 2003 * An 8 b 80 MSample/s pipelined ADC with background calibration JMing SHLewis IEEE Int. Solid-State Circuits Conf. Dig.Tech. Papers Feb. 2000 * An 8-bit 13-MSamples/s digital-backgroundcalibrated algorithmic ADC EBBlecker OEErdogan PJHurst SHLewis Proc. Eur. Solid-State Circuits Conf Eur. Solid-State Circuits ConfStockholm, Sweden Sept. 2000 * Analog circuit performance and process scaling A.-JAnnema IEEE Trans. Circuits Syst. II 46 June 1999 * A 12-Bit 75-MS/s Pipelined ADC Using Open-loop Residue Amplification BorisMurmann BernhardEBoser IEEE Journal of Solid-State Circuits 38 12 Dec. 2003 * A 600 MS/s 5-bit pipelined analog-to-digital converter using Digital reference Calibration AVarzaghani CK KYang IEEE Journal of Solid-State Circuits 41 2 Feb. 2006 * Low-Power Pipeline ADC for Wireless LANs JArias VBoccuzzi LQuintanilla LEnríquez DBisbal MBanu JBarbolla IEEE Journal of Solid-State Circuits 39 8 Aug. 2004 * An 8 b 80 MSample/s pipelined ADC with background calibration JMing SHLewis IEEE Int. Solid-State Circuits Conference Feb. 2000 * AliTangel ChoiKyusun The CMOS Inverter as a comparator in ADC designs University Park, PA16802 USA The Pennsylvania State University * A Classic Wide Band Operational Amplifier Design MaysamGhovanloo Ann Arbor, MI University of Michigan * A 12 bit 80 Msps Pipelined ADC Core with 190mw consumption from 3v in 0.18um Digital CMOS ArashLoloee AlfioZanchi ShereefHuawen Jin EduardoShehata Bartolome 2002 Texas Instruments, Inc.-Wireless Infrastructure Data Converters 12500 TI Boulevard Dallas, TX(U.S * Digital Integrated Circuits-A design Perspective JMRabaey 1996 Prentice Hall India * Analog Integrated Circuit Design DJohns KMartin 1997 John Wiley New York * CMOS Mixed Signal Circuit Design PhillipEAllen & Holberg ;R JacobBaker CMOS Analog Circuit Design Oxford University Press 2002. 2002 NJ