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    • A Low-Power, Medium-Resolution, High-Speed CMOS Pipelined ADC D. Meganathan Axel Jantsch Department of Electronics Engineering, Department of ICT, Madras Institute of Technology, Royal Institute of Technology (KTH), Anna University, Chennai, India-600044. Stockholm, Sweden-164 40. Each pipeline stage consists of a multiplying digital-to-Abstract— This paper presents the systematic design approach ofa low-power, medium-resolution, high-speed pipelined Analog-to- analog converter (MDAC) and an analog-to-digital subDigital Converter (ADC). The ADC is implemented in 180nm converter (ADSC). Each pipelined stage generates a coarsedigital CMOS technology. The converter achieves signal-to-noise ADSC output and a reconstructed residue signal for the laterdistortion ratio of 59.8 dB, spurious-free dynamic range of 89 dB stages. These digital codes are cascaded to obtain overalland effective number of bits of 9.64-bits at sampling speed of resolution of the ADC. Pipeline ADC operates using two-50MHz with an input signal frequency of 4MHz. The peak clock phases. During the phase Φ1 , CLK1 is high and thedifferential-nonlinearity of the converter is 0.28/-0.17LSB andintegral-nonlinearity of the converter is +0.42/-0.41LSB. The phase Φ 2 , CLK2 is high. During the Φ1 period, the first stageproposed 10-bit, 50MS/sec pipelined ADC consumes 24.5mW acquires the signal and in the next half cycle, it generates theamount of power from 1.8V supply. reconstructed residue for the second stage. The second stageKeywords- Analog-to-digital Sub Converter (ADSC), Signal-to- acquires the signal during the conversion state of the firstnoise distortion ratio (SNDR), Spurious Free Dynamic Range stage. In this paper, Section II discusses the design of THA,(SFDR), Dynamic Range (DR), Operational Transconductance MDAC and OTA. Section III discusses the low power circuitAmplifier (OTA). techniques to optimize the performance of the ADC. Section IV shows the performance of the ADC followed with the I. INTRODUCTION conclusion in Section V. High-speed and medium-resolution Analog-to-Digital II. DESIGN OF A THA, MDAC and OTAConverters (ADCs) are widely used in commercialapplications including data communication and image signal A. THA Designprocessing. In such applications, the reduction of powerconsumption associated with high speed sampling and high The capacitor flip-around THA is shown in Figure 2. Nolinearity is one key design issue in enhancing the portability charge-transfer occurs in this scheme and only two capacitorsand battery operation. Among many ADC architectures, are used. During the track phase (Φ1-phase), the differentialpipelined ADC is proved to be the most suitable for high- input signal is sampled by the input capacitors. During thespeed, medium-resolution and low-power consumption [1]. A hold phase (Φ2-phase), the input capacitors are flipped over bygeneric pipelined ADC is shown in Figure 1. First stage of the connecting their bottom plates to the output of the amplifier.pipelined ADC is front-end track-and-hold amplifier (THA). It By doing this, both the common-mode and differential-modeis followed by ‘m’ number of cascaded pipeline stages except charges are transferred [2].the final stage contains a simple flash ADC. Front End Q(Φ1 ) = Q(Φ 2 ) (1) THA Ф1-acquire Ф2- convert -CS (VIN-VCMI) = -CS (VOUT -VCMI) (2) Ф2-convert Ф1- acquire Hence VOUT = VIN (3) VIN THA Stage 1 Stage 2 Stage m When parasitic capacitance is ignored, the feedback factor CLK1 β of a flip-around THA is 1, whereas the feedback factor of a DFF CLK2 charge-transferring THA [3] is 0.5. As the feedback factor of DFF DFF the flip-around THA is twice as large compared to the charge- transferring THA, it requires only half of OTA gain- DFF DFF DFF bandwidth to produce the same closed-loop bandwidth. Thus, the same performance can be achieved with much less power Digital Error Correction Logic by using the flip-around architecture. The nonlinearity error Einput_ref produced by the pipelined ADC is given as D1 D2 Dm-1 Dm CLK1 Ф1 m-1 E i+1 CLK2 Ф2 Einput_ref = E1 + ∑ Gi i=1 (4) Figure 1. Generic Pipeline ADC 978-1-4244-8971-8/10$26.00 c 2010 IEEE
    • used to select the DASC output voltage according to the Φ2 ADSC input.VIN+ Φ1 CS CS + CF CS X VOUT- + VO U T = VIN - D VR E F (6) - CF CF VCMI Φ 2e VCMI Φ1e Φ1 VIN+ VCMO VIN- Φ1 Φ 2e Φ1e Φ1 CF + Φ1 X - VOUT+ VDASC+ - CS Φ2 VIN+ Φ1 CS Φ Φ2 VOUT Bootstrap + Switch VCMI Φ2 VCMI Φ1e - Φ1 V CMOS VIN- Φ1 Φ2 Φ1e CMO Switch + Φ - 1 CS Φ2 VOUT+ CMOS SWITCH VDASC- Φ2 Figure. 2 Switched Capacitor Flip-around THA CF Φ1 Note-Ф1 and Ф2 are non-overlapping clock phases used for track and hold operations. Ф1e and Ф2e are the early clock phases of Ф1 Note CS=CF VIN- and Ф2 respectively. Early clock phases are used to perform bottom VDASC+ plate sampling which minimizes the charge injection error [4] [5]. VIN+ B-bit Reference 1.5-bit Selector The thermal noise contribution of the pipelined ADC is VIN- ADSC VDASC- VDASC =DVREF 2 Vni 2 Vn i 2 stage2 2 VREF V B-bit Vn i = THA + Vni stage1 2 + 2 2 +........... Vni stagem m-1 (5) - 4 + 4 REF -VREF +VREF 1 G1 G1 G 2 ∏ j=1 Gj 2 Figure 3 1.5-bit per stage MDAC section Based on Equations (4) & (5), it has been observed that the C. OTA Design performance of the track-and-hold (T/H) circuit dominates the overall ADC dynamic characteristics and plays a major role in The OTA is implemented using telescopic architecture. determining the Spurious Free Dynamic Range (SFDR) and One of the tradeoffs of telescopic architecture is less output the Signal-to-Noise and Distortion Ratio (SNDR) of the voltage swing. Another tradeoff is that the input and output system [6]. Therefore improving the performance of the THA common-mode levels cannot be set independently of one can improve the SFDR and SNDR. The nonlinearity caused by another or equal to one another. The first tradeoff reduces the the input sampling switches at low supply voltage lowers dynamic range of OTA. Dynamic Range (DR) of OTA can be SFDR, SNDR and DR. Using an NMOS switch as a sampling maintained high by reducing thermal noise contribution of switch in the proposed T/H circuit has two limitations: input- OTA. Selecting the larger value of sampling capacitors dependent ON-resistance and input-dependent charge minimize thermal noise and capacitor mismatch nonlinearity injection. They will lead to nonlinear signal distortion. The errors. Thermal noise contribution of OTA can be further signal dependent nonlinearity error associated with switches reduced by selecting lesser gm value for cascode transistors can be reduced by using bootstrap switches. However, the and larger gm value for signal transistors. Larger value of gm design of bootstrap switch is complicated. Hence it is used in of signal transistors also pushes the Unity-Gain Frequency some critical places of the proposed ADC circuit. The other (UGF) to the higher value. The second tradeoff produces switches are CMOS switches which give better performance nonlinearity error. The nonlinearity error can be minimized by than NMOS switches. high loop gain of OTA. To achieve larger loop-gain, gain- boosting is used with the cascode transistors, as shown in B. Design of MDAC Section Figure 4. Normally gain-boosting amplifier has pole-zero doublet, which reduces settling speed of OTA. This can be The MDAC combines the functions of a track-and-hold, overcome by setting the bandwidth of the gain-boosting a DASC, a subtractor and a gain amplifier. Each MDAC amplifiers relative to that of the main amplifier. The unity- section is implemented using the switched capacitor circuit gain-bandwidth of the gain-boosted amplifier is designed to with the resolution of 1.5-bit-per-stage and an interstage gain satisfy the following criterian [8] [9]. of 2 as shown in Figure 3. [7]. During the track phase ( Φ1 =HIGH), the input signal VIN is applied to the set of βωMain < ωgain-boosted < ω2Pole-Main (7) capacitors CS and CF and simultaneously quantized per-stage Where ωMain and ω2Pole-Main are the unity-gain-frequency and resolution of Beff +1 bit through the ADSC function which has second pole of a main amplifier. ωgain-boosted is the unity-gain- V V frequency of gain boosted amplifier. The gain-boosting the threshold value of + REF and - REF . At the end of amplifiers, represented by amplifiers A1 and A2 in Figure 4, 4 4 are also implemented with the telescopic topology to provide the track phase, VIN is tracked across CS and CF and output of the largest possible gain by scaled replica of the Main the ADSC is latched. During the hold phase ( Φ 2 =HIGH), CF amplifier. Switched-capacitor common mode feedback closes negative feedback path around the OTA, the top plate controls stability of OTA. The unity-gain-frequency of gain- of CS is switched to the DASC output. The output of ADSC is boosted telescopic architecture achieves 400MHz which is
    • suitable for 50MS/sec sampling rate applications. The The charge-sharing type comparator with output latch differential output voltage swing of the OTA is 0.9V. The consumes the power of 55µW. OTA consumes the power of 3.6mW. The gain and phase angle versus frequency plots of OTA used in THA is shown in IV SIMULATION RESULTS Figure 5. VDD The layout of proposed low-power 10-bit, 50MS/sec VBIAS pipelined ADC is shown in Figure 7. The total power M7 M8 + - consumption of the ADC is 24.5mW. For measuring the + A1 INL/DNL performance of the ADC a slow moving ramp input - + M6 of 24 KHz is applied to the ADC input. The post-layout M5 simulated differential nonlinearity and integral nonlinearity for M3 M4 24 KHz full scale ramp signal at 50MS/s sampling rate are - + illustrated in Figures 8 and 9 respectively. The dynamic A2 VIN- linearity of the ADC is characterized by analyzing a fast VIN+ M1 + - M2 Fourier transform (FFT) of the output codes with a single-tone input. The simulated output FFT spectrum with 4 MHz and VCMFB Nyquist rate single-tone sinusoidal inputs are shown in Figure M9 10(a) and 10(b) respectively. The input is 0.9V at the Figure 4 Gain-boosted telescopic OTA sampling rate of 50MS/sec. Performance of the pipelined ADC is summarized in Table 1. Digital Error THA MDAC1 MDAC2 Correction Logic MDAC3 MDAC4 MDAC5 ADSCs Figure 5 Gain and phase angle Vs frequency MDAC6 MDAC7 MDAC8 CLK III. LOW POWER CIRCUIT TECHNIQUES Figure 7 Layout of 10-bit 50MS/s pipelined ADC A. Regenerative Latch Comparator Resistive divider type regenerative latch uses a positive feedback to implement comparator (ADSC) block [10] [11]. The architecture of proposed comparator is same as resistive divider comparator except the presence of active restore transistor, M5 connected in between the output nodes as shown in Figure 6. M5 transistor brings the VOUT+ and VOUT- in Figure 8 Differential nonlinearity code common-mode voltage level during reset phase. Hence it is named as charge-sharing comparator. The charge-sharing comparator takes the decision from common-mode voltage (Vcommon) to the output logic level which reduces the dynamic power dissipation and settling time of the comparator compared to the conventional regenerative latch comparators. In reset phase, normally the charge sharing comparator output will be in meta-stable state. But to drive the digital circuit, Figure 9 Integral nonlinearity code either a logic high or logic low is required. Hence the output latch is added to the comparator. VDD CLK M6 VOUT+ M4 M4’ VDD VOUT - VOUT+ Vcommon M5 ’ VOUT- M8 M8 CLK M3’ Reset M3 CLK Phase ComparisonVREF- VIN + VIN - VREF + Phase M2 M1 M1’ M2’ Figure 10 (a) Figure 10(b) CLK M7 Figure 10 (a) & (b) 1024 points FFT spectrum for 4MHz and 24 MHz Figure 6 Charge sharing comparator
    • Figure of merit is the unit used to evaluate the ADC by consuming less amount of power. Bootstrap andperformance of the ADC. More recently normalizing for the bottom-plate sampling techniques are incorporated tosupply voltage is added to the Figure of Merit (FOM), minimize nonlinearity errors. A modified dynamicespecially meaningful for the CMOS cores [12]. regenerative comparator and stage scaling further optimizes the overall power consumption. The FOM reveal that the Power VDD performance of the proposed ADC is better compared to the Figure of Merit (FOM) = (8) 2ENOB fS previous published works. The performance of this work is compared with that ofother pipelined ADCs referred to in publications based on REFERENCESEquation (8) is shown in Table 2. The FOM of the proposed [1]. Annema A.J., Nauta B., van Langevelde R. and Tuinhout H., “AnalogADC is lower than the reported ADCs. 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