Int. Journal of Electrical & Electronics Engg.
Vol. 2, Spl. Issue 1 (2015)
e-ISSN: 1694-2310 | p-ISSN: 1694-2426
Layout Design Analysis of CMOS Comparator using 180nm Technology 1
Jyoti , 2Rajesh Mehra
1
ME Scholar , 2Associate Professor, Department of Electronics & Communication Engineering, National Institute of Technical Teachers Training and Research, Chandigarh, India
1, 2
Jyoti13july@gmail.com
ABSTRACT- Comparator is a very useful and basic arithmetic component of digital system. In the world of technology the demand of portable devices are increasing day by day. This paper presents CMOS design of 1-bit comparator on 180nm technology. The layout of 1-bit comparator has been developed using Automatic and semi-custom techniques. Both the layouts are compared and analyzed in terms of their Power and Area consumption. Automatic layout is generated from its equivalent schematic whereas semi-custom layout is developed manually. The result shows that semi-custom consumes less power as compared to Automatic. Key Words: CMOS technology, Power dissipation, Layout, Performance analysis, combinational circuit
INTRODUCTION Comparator is one of the fundamental building blocks in most analog-to-digital converters (ADCs) [1]. Comparator are the most important design element for various application such as in embedded processor, general purpose processor, DSP core, image/signal processing and built in self test circuits [2]. Minimizing the power dissipation for the digital circuits requires optimization at all level of the design. So, this optimization depends on circuit style, topologies and in fact includes the technology which is used to implement the digital circuits [3]. In VLSI design Comparator is a basic component which compares two binary number and then determine whether the number is greater than, less than or equal to the other input. The n-bit Comparator is shown in figure 1 A
B
A>B
n- Bit Magnitude Comparator
A=B A>B
1-BIT MAGNITUDE COMPARATOR Digital Comparator also called “Magnitude Comparator” is a combinational circuit that compares two numbers in which A and B are two inputs and three outputs A> , = , < and only one of the three outputs would be high accordingly if A is greater than or equal to or less than B. The truth table of 1-bit comparator is shown in Table 1 Table 1. Truth table of 1-bit comparator
Input A 0 0 1 1
B 0 1 0 1
Output A> 0 0 1 0
A=B 1 0 0 1
A< 0 1 0 0
Karnaugh -Map is used to minimize Boolean function obtained from truth table and shown in figure 2
Fig.1 Block diagram of n-bit Comparator
The outcome of comparison is specified by three binary variables that show whether A>B, A=B, or A<B. In the truth table, the circuit for comparing two n-bit numbers, has 2n inputs & 22n entries. So, 4 inputs & 16-rows in the truth table for 2-bit numbers and similarly, for 3-bit numbers 6-inputs & 64-rows in the truth table [4]. The logic style used in logic gates basically influences the size, speed, power dissipation and the wiring complexity of a circuit [5]. Circuit size depends on the number of transistors and their sizes and on
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the wiring complexity [6]. The wiring complexity is determined by the number of connections and their lengths. All mentioned characteristics may vary considerably from one logic style to another and thus proper choice of logic style is very important for circuit performance [7, 8]. The speed, power consumption and chip area are the important factors while designing comparators. The continually-growing application of portable devices makes the power consumption a very critical constraint for circuit designers [9].The CMOS technology produce degraded output in the circuit. As NMOS transistor pass strong logic 0 and weak 1 pass but PMOS transistor is strong 1 pass and weak 1 pass. It is possible to combine NMOS and PMOS transistor into a single switch that is capable of driving its output terminal either to a low or high voltage equally well [10]. Here we use Microwind3.1 to draw the layout of the CMOS circuit. In order to differentiate designs, simulations are carried out for Power and Area. Simulations are performed at 180nm technology. CMOS can be designed by using PMOS and NMOS transistor and CMOS consumes no steady state power.
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Equation for (A > B) =
′
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Int. Journal of Electrical & Electronics Engg.
Vol. 2, Spl. Issue 1 (2015)
e-ISSN: 1694-2310 | p-ISSN: 1694-2426
Equation for (A < B)= ′
Equation for ( A = B) =
+ =AʘB ′
′
Fig.4 Switch Level Comparator
Fig.2 Equations from Karnaugh -Map
SCHEMATIC DESIGN SIMULATIONS A Comparator has two inputs and three outputs which are A > B , A< B and A=B. The logic circuit of this comparator can be implemented with the help of XNOR gate, AND gates and NOT gates. The logic for A=B requires XNOR gate while the logic for A >B and A< B requires AND and NOT gates. The basic logic diagram for Comparator with basic gates can be represented as shown below figure 3
Fig.5 Timing diagram of Switch Level Comparator
LAYOUT DISCUSSIONS The schematic of 1-bit Comparator is designed. Using Microwind software, the auto generated layout of 1-bit Comparator is created with 180nm foundary. The figure 6 represents the autogenerated layout. Layout is the general concept that describes the geometrical representation of the circuits by the means of layers. Different logical layers is used by designers to generate the layout.
Fig.3 Logic Level comparator
The Switch level Comparator consist of NMOS and PMOS transistors as shown in figure 4 and showing 1-bit Comparator. Also the timing diagram is shown in fig. 5
Fig.6 Auto generated layout of the comparator
The Output waveform for Automatic generated layout is shown in figure 7
Fig.7 Output waveform for automatic generated layout
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Int. Journal of Electrical & Electronics Engg.
Vol. 2, Spl. Issue 1 (2015)
The representation of Semi-Custom Layout of the 1-bit Comparator using nMOS, pMOS and its output waveform is shown in figure 8 and figure 9 respectively.
e-ISSN: 1694-2310 | p-ISSN: 1694-2426
1600 1400 1200 1000 800
Automatic Generated
600
Semi-Custom
400 200 0 Area Fig.10 Area shown between Automatic Generated and Semi-Custom layout
Power(mw)
Fig.8 Semi-custom layout of the comparator
SemiCustom 48%
Fig11 Power shown between Automatic Generated and Semi-Custom layout
Fig.9 Output waveform for semi-custom layout
The performance of proposed 1-bit Comparator layout is compared with semicustom approach. The performance in terms of Area and Power is compared. Comparative analysis is shown in Table 2 Table 2: Comparative Analysis For Area And Power
Approach Automatic Generated SemiCustom
Automatic Generated 52%
Area (μm2) 1446.5
Power (mWatt) 0.176
Width(μm)
Height(μm)
73.8
19.6
855.4
0.163
31.8
26.9
In terms of area and power the Semi-Custom layout has better performance among two design approaches. Area and Power graph representation of these design approach is shown below in figure 10 and Figure 11 respectively
From the above graph we observed that there is a reduction in power and Area for semi custom approach as compared to Automatic generated layout. CONCLUSION Above analysis conclude that this technique is very useful to reduce the effective area on a chip and power dissipation. In this paper, we presented the analysis for layout of 1-bit comparator using auto generated and semi-custom technique and also expressions were derived using karnaugh -map. Finally from the comparative analysis it is clear that the Semi-Custom layout is more efficient than auto generated in terms of power and area. So this design approach can be implemented where area and power reduction is the main consideration. REFERENCES [1] Samaneh Babayan-Mashhadi and Reza Lotfi, “ Analysis & Design of a Low Voltage Low-Power Double-Tail Comparator” IEEE Transactions On Very Large Scale Integration (VLSI) Systems, Vol.22, No. 2, pp. 343-352,Feb. 2014. [2] Saleh Abdel- Hafeez, “Scalable digital CMOS comparator using a parallel prefix tree,” IEEE Transactions On Very Large Scale Integration (VLSI) Systems, Vol.21, No. 11, pp. 1989 - 1997, Nov. 2013. [3] Geetanjali Sharma, Uma Nirmal, Yogesh Misra, “ A low power8- bit magnitude comparator with small transistor count hybrid PTL/CMOS logic,” International Journal of Computational Engineering & Management, Vol. 12,pp. 110-115, April 2011.
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Int. Journal of Electrical & Electronics Engg.
Vol. 2, Spl. Issue 1 (2015)
e-ISSN: 1694-2310 | p-ISSN: 1694-2426
[4]M. Morris Mano, “Digital Design” Pearson Education, 3rd Ed, 2002. [5] R. Zimmermann and W. Fichtner, “Low Power Logic Styles: CMOS Versus Pass Transistor Logic” IEEE Journal of Solid State Circuits, vol.32, No.7, pp.1079-1090, July 1997. [6] S. Kang and Y. Leblebici “CMOS Digital Integrated Circuit, Analysis and Design” Tata McGraw-Hill, 3rd Ed, 2003. [7] A. Bellaouar and Mohamed I. Elmasry “Low Power Digital VLSI Design: Circuits and Systems” Kluwer Academic Publishers, 2nd Ed, 1995. [8] Anantha P. Chandrakasan and Robert W. Brodersen, “Minimizing Power Consumption in digital CMOS circuits”. Department of EECS, University of California at Barkeley, vol. 83, No. 4, pp. 498-523,1995. [9] R. S. Gamad, S. Kale, "Design of a CMOS Comparator for Low Power and High Speed," International Journal of Electronic Engineering Research, vol. 2, No. 1, pp. 29-34, 2010. [10] Madhusudhan Dangeti, S.N.Singh, “ Minimization of Transistors Count and Power in an Embedded System using GDI Technique : A realization with digital circuits”, International Journal of Electronics and Electrical Engineering, vol. 2, No. 9 ,pp. 21-30, 2012
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