Delay and Energy Efficient Low-Voltage Dual Tail Comparator

Delay and Energy Efficient Low-Voltage Dual Tail Comparator

1 .Tirumalareddy Anji Reddy Electronics and Communication Engineering

Annamacharya College of Engineering & Technology Andhrapradesh.

2 .A.Sivaiah

Electronics and Communication Engineering Annamacharya College of Engineering & Technology Andhrapradesh.

AbstractThe necessity for ultra low-power, area efficient, and high speed analog-to-digital converters is forcing toward the use of dynamic regenerative comparators to boost up speed and power efficiency. In this paper, an analysis on the delay of the dynamic comparators will be presented and analytical ex- pressions calculated. From the analytical expressions, design- ers can achieve a perception about the main contributors to the comparator delay and completely given the tradeoffs in dy- namic comparator design. Based on the analysis and by adopt- ing sleep transistor approaches a new dynamic comparator is proposed, where the circuit of a conventional dual tail compar- ator is modified for low-power and fast operation even in small supply voltages. Without complicating the design and by add- ing few transistors, the positive feedback during the regenera- tion is strengthened, which results in remarkably reduced delay time. The simulation results in 45nm technology shows that in the proposed dual tail dynamic comparator both the power consumption and delay time are significantly reduced. The maximum clock frequency of the proposed comparator can be increased to 2.5 and 1.1 GHz at supply voltages of 1.2 and 0.6 V, while consuming 1.4 mW and 153 W, respectively. The standard deviation of the input-referred offset is 5 mV at 0.8 V supply.

Index TermsDynamic clocked comparator, Dual-tail compara- tor, sleep approach, dynamic ,controlled dual tail,low power ,high speed ADCs design.

1 INTRODUCTION

Comparator circuit compares two voltage or current signals and determines which one is greater. The result of this com- parison is indicated by the output voltage. These are mostly used in ADCs. Dynamic latched comparators are useful for many applications such as high speed analog to digital con- verters (ADCs) due to high speed, low power consumption, high input impedance and full swing output. They use posi- tive feedback mechanism with one pair of back to back cross coupled inverters (latch) in order to convert a small input voltage difference to a full scale digital level in a small time interval. The desire of many ADCs, are high-speed, low power comparators with small chip area. In high speed ultra deep sub micrometer (UDSM) CMOS comparators, the technology suffers from lower supply voltages. This is in- tense especially when considering the fact that threshold voltages of the devices have not been scaled at the same speed as the supply voltages of the modern CMOS pro- cesses.This low voltage leads to limited common-mode in-

put vary, that is vital in several high-speed ADC architec- tures, like flash ADCs. Supply boosting technique (SBT) is suitable for sub-micron CMOS processes containing MOSFET transistors with threshold voltages comparable to the supply voltage, but these introduce some reliability is- sues.Techniques using body-driven transistors, current- mode design and dual-oxide processes, which may handle higher supply voltages, are developed to satisfy the low volt- age design challenges The most well known technique is sleep approach, in this an additional sleep transistors such as an extra sleep PMOS or sleep NMOS transistor is placed be- tween VDD and pull up network is placed to reduce the amount of power dissipation. In this paper the analytical ex- pressions about the delay of dynamic comparator has been presented for different architectures Furthermore, depend on the double-tail structure a new controlled double tail comparator is presented, which does not require boosted voltage or connecting of too many transistors. Just by add- ing a few small size transistors to the conventional double tail dynamic comparator, maximize the latch speed and a new sleep approach, thus providing a new choice to low leakage power VLSI designers.

This paper is organized as follows. Section II describes the operation of the regenerative dynamic comparators and the problems and conclusions of each structure is discussed the analytical expressions for the delay of the comparators are presented the proposed comparator and its sleep transistor approach is presented in Section III. Simulation results are represented in Section IV and conclusions in Section V.

1. REGENERATIVE DYMANIC COMPARATORS

   1. Conventional dynamic comparator

      Conventional dynamic comparator widely used in ADCs, with high input impedance, full swing output and no static power consumption. The disadvantage is the fact that due to several stacked transistors, a sufficiently high supply voltage is needed for a proper delay time and this structure has only one current path, via tail transistor Mtail, which defines the current for both the differential amplifier and the latch (the cross-coupled inverters). While one would like a small tail current to keep the differential pair in weak inversion and obtain a long integration interval and a better Gm/I ratio, a large tail current would be desirable to enable fast regenera- tion in the latch and the tail transistor operates in triode re- gion which is not acceptable for regeneration.

   2. Conventional dual tail Dynamic Comparator

      A conventional dual tail comparator is shown in Fig 1. This topology has less stacking and therefore can operate at lower supply voltages compared to the conventional dynamic com- parator. The double tail enables both a large current in the latching stage and wider mtail2, for fast latching independ- ent of the input common-mode voltage (Vcm), and a small current in the input stage (small Mtail1), for low offset. The operation of this comparator is as follows (see Fig (c)). Dur- ing reset phase (CLK = 0, Ntail1, and Ntail2 are off), tran- sistors P2-P1 pre-charge f2 and f1 nodes to VDD, which in turn causes transistors NR1 and NR2 to discharge the output nodes to ground. During decision-making phase (CLK = VDD, Mtail1 and Mtail2 turn on), P2-P1 turn off and volt- ages at nodes f2 and f1 start to drop with the rate defined by IMtail1/Cf2(1) and on top of this, an input-dependent differ- ential voltage Vf2(1) will build up. The intermediate stage formed by NR1 and NR2 passes Vf2(1) to the cross coupled inverters and also provides a good shielding between input and output, resulting in reduced value of kickback noise. Similar to the conventional dynamic comparator, the delay of this comparator comprises two main parts, t0 and tlatch. The delay t0 represents the capacitive charging of the load capacitance Clout is given as

      Fig 2. Transient simulations of the dynamic dual tail comparator for input voltage difference of Vin = 5 mV, Vcm = 0.7 V, and VDD = 0.8V

      This equation shows that V0 depends strongly on the trans- conductance of input and intermediate- stage transistors, input voltage difference (Vin), latch tail current, and the capacitive ratio of CLout to CL,f2(1). Substituting V0 in latch regeneration time, the total delay of this comparator is achieved as follows

      = +

      = 2 + (2)

      2

      ,

      (C V ) 2 (C V )

      (C V ) 2 (C V )

      t Lout Thn Lout Thn

      (1)

      = 2 +

      o IB1

      Itail2

      2

      ,

      After the first n-channel transistor of the latch turns on (for instance, N4), the corresponding output (e.g.,Outn) will be

      ( 22 ,2(1) 821.21.2

      ) (3)

      discharged to the ground, leading front p-channel transistor (e.g., P3) to turn on, charging another output (Out1) to the supply voltage (VDD) , the regeneration time (tlatch) is ob- tained

      (2)

      Fig 1. Schematic diagram of dual tail comparator

      From the equations two important conclusions are devel-

      oped on the dual tail dynamic comparator.

      1. The voltage difference at the first stage outputs (Vf2/f1) at time t0 is increased then the amount of delay would pro- foundly reduce.

      2. In this comparator during the reset phase both the transis- tors charged from ground to vdd due to this the amount of power dissipation increases.

   3. Controlled DualTail Dynamic Comparator

   Fig 3. Schematic diagram of the controlled dual tail comparator

   The operation of the controlled dual tail comparator

   = ()

   is same as dual tail comparator. During reset phase (CLK = 0, mtail1 and mtail2 are off, avoiding static power), P1 and P2

   ,+1,2

   pulls both f1 and f2 nodes to VDD, hence transistor Pc1 and

   = (/2) (5)

   Pc2 are cut off. Intermediate stage transistors, NR1 and NR2,

   ,+1,2

   reset both latch outputs to ground. During decision-making phase (CLK = VDD, mtail1, andNtail2 are on), transistors P1

   Therefore the total delay is given as

   and P2 turn off. In the proposed structure as soon as the com-

   =

   +

   = 2 + ×

   parator detects that for instance node f2 discharges faster, a pMOS transistor (Pc1) turns on, pulling the other node fp

   2

   ,+1,2

   back to the VDD. Therefore by the time passing, the differ-

   ( 2 ) (6)

   ence between f1 and f2 (Vf1/f2) increases in an exponential

   4

   1.21.2(,10)

   manner, leading to the reduction of latch regeneration time. The transient simulation results are shown below. The delay

   2

   1

   ,()

   of controlled dual tail comparator comprised by two param- eters .those are delay due to t0 and delay due to tlatch Each of these factors will be discussed in detail.

   Fig 4. Transient simulations of the proposed dual tail comparator for input voltage difference of Vin = 5 mV, Vcm = 0.7 V, and VDD = 0.8V

   2.3.1 Effect of enhancing V0

   The delay time t0, is define as time after which latch regen- eration starts or t0 is considered to be the time it takes (while both latch outputs are rising with different rates) until the first nMOS transistor of the back-to-back inverters turns on, so that it will pull down one of the outputs and regeneration will starts in this comparator we have

   = 2 1.2 2

   2 1

   By comparing the equations for the delay of the

   three mentioned structures, it can be observed that the pro- posed comparator takes advantage of an inner positive feed- back in dual tail operation, which strengthens the whole latch regeneration. This speed improvement is even more obvious in lower supply voltages. This is due to the fact that for larger values of VTh/VDD, the transconductance of the transistors decreases, thus the existence of an inner positive feedback in the architecture of the first stage will lead to the improved performance of the comparator .These results are shown in Simulation analysis.

   2.3.3 Reduction in Energy per Comparison

   In conventional dual tail topology, both f2 and f1 nodes discharge to the ground during the decision making phase and each time during the reset phase they should be pulled up back to the VDD. However, in our proposed com- parator, only one of the mentioned nodes (f1/f2) has to be charged during the reset phase. This is due to the fact that during the previous decision making phase, based on the sta- tus of control transistors, one of the nodes had not been dis- charged and thus less power is required. This can be seen when being compared with conventional topologies.

2. DYMANIC COMPARATORS DESIGN USING SLEEP TRANSISTOR APPROACH

   An externally switched power supply is a very basic form of power gating to achieve long term leakage power reduction. To shut off the block for small intervals of time, internal power gating is more suitable. CMOS switches that provide power to the circuitry are controlled by power gating

   = 4

   1.2 1.2 (,10) (4)

   controllers. Outputs of the power gated block discharge

   2

   1

   ,2(1)

   slowly. Hence output voltage levels spend more time in threshold voltage level. This can lead to larger short circuit current. Power gating uses low-leakage PMOS transistors as

   2.3.2 Effect of improving Effective Transconductance of latch

   In the previous comparator design the nodes f1 and f2 discharged completely to ground but in our proposed comparator only one of the first stage output nodes (f1/f2) will charge up back to the VDD at the beginning of the deci- sion making phase, will switch on one of the control transis- tors, thus the effective transconductance of the latch is in- creased. i.e the positive feedback is improved . Hence, tlatch will be

   header switches to shut off power supplies to parts of a de- sign in standby or sleep mode. NMOS footer switches can also be used as sleep transistors. Inserting the sleep transis- tors splits the chip's power network into a permanent power network connected to the power supply and a virtual power network that drives the cells and can be turned off.

   1. conventional sleep dynamic comparator

      Here we are connencting sleep transistor i.e either PMOS or NMOS is connected at Vdd or at ground resepectively, by connecting these sleep transistor we can reduce the amount

      of power dissipation and optimum delay ia achieved the schematic diagram of resepective comparator is shown be- low.

      Fig 5. Schematic diagram of conventional sleep dynamic comparator

      Fig 6. Transient simulations of conventional sleep dynamic comparator for input voltage difference of Vin = 5 mV, Vcm = 0.7 V, and VDD = 0.8V

   2. Proposed sleep controlled Dual-Tail comparator

   Fig 7. Schematic diagram of the proposed dual tail comparator

   Fig 8. Transient simulations of the proposed sleep dual tail comparator for input voltage difference of Vin = 5 mV, Vcm = 0.7 V, and VDD = 0.8V

   The operation of the proposed sleep dual tail comparator, during reset phase (CLK =in3=0, Ntail1 and Ntail2 are off, avoiding static power), P1 and P2 pulls both f1 and f2 nodes to VDD, hence transistor Pc1 and Pc2 are cut off. Intermedi- ate stage transistors, NR1 and NR2, reset both latch outputs to ground. During decision-making phase (CLK =in3= VDD, Ntail1, andNtail2 are ON) .the sleep transistor doesnt allow vdd to latch circuit. The amount of leakage power reduces negligibly small compared to proposed comparator.

3. SIMULATION RESULTS

   The comparison of proposed comparator and sleep compar- ator with existing design can be observed by simulated in 45nm CMOS technology with VDD=0.8V and offset devia- tion of 5mv at the input common mode voltage of Vcm =0.7v

   ACKNOWLEDGMENT

   Tirumalareddy anjireddby was born in An- dhra pradesh in 1991. He received his b.tech degree In Electronics and Commu- nication Engineering from jntu Kakinada,

   Kakinada A.P. in 2012, where he is pursuing his M.Tech in VLSI SysDesign. In Electronics and Communication Engi- neering dept. with research going on Energy efficient low voltage dual tail comparator

   Fig 9. Post-layout simulated (i) energy per conversion and (ii) delay as a function of supply voltage (Vin = 5 mV, Vcm = VDD 0.1)

   Table I Performance comparsion

   Comparator structure (45nm Vd=0.8v)	Sampling frequency	Delay /log(Vin) (psec/dec)	Energy per conversion (p/fs) fJ
   Conventional dy- namic compara- tor	900MHz	51.51	7.3
   Convential dual tail comparator	1.8GHz	6.14	2.22
   Controlled dual comparator	2.4GHz	3.82	1.3
   Sleep controlled dual tail compar- ator	2.4GHz	5.37	1.07

4. CONCLUSION

In this paper, we presented a comprehensive delay analysis for clocked dynamic comparators and expressions were represented. Based on theoretical analyses, a new dy- namic comparator with sleep transistor and without sleep transistor with low-voltage low-power capability was pro- posed in order to improve the performance of the compara- tor. Post-layout simulation results in 45nm CMOS technol- ogy confirmed that energy per conversion of the proposed comparator is reduced and delay was optimally reduced in comparison with the existing designs.

A.sivaiah was born in Andhra pradesh in 1986. He received his M.Tech degree In Electronics and Comm unication engi- neering from jntu Anantapur, Anantapur A.P.,he is currently doing as an assistant professior in AITS Kadapa.

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