- Open Access
- Total Downloads : 5
- Authors : Leela S.Bitla
- Paper ID : IJERTCONV2IS10049
- Volume & Issue : NCETECE – 2014 (Volume 2 – Issue 10)
- Published (First Online): 30-07-2018
- ISSN (Online) : 2278-0181
- Publisher Name : IJERT
- License: This work is licensed under a Creative Commons Attribution 4.0 International License
Design and Implementation Of 4 Bit Flash ADC using LTE and NAND Gate Comparator
Priyadarshini Bhagwati College of Engineering Nagpur, Maharashtra, India. email@example.com
Abstract :-This Paper introduces 4 bit flash ADC design using Linear Tunable Transconductance Element based comparators for high speed and low power consumption using180nmtech. Thermometer to binary decoder with low power consumption, less area & short critical path is selected for the design of low power high speed. Proposed comparator provides improved PSRR (Power Supply Rejection Ratio) compared TIQ (Threshold Inverter Quantizer) comparator NAND based topology is used which improves PSRR as well as linearity; thus eliminates basic limitation of TIQ inverter.
KeywordsCMOS-LTE, CMOS-NAND gate, MUX Decoder, ADC, Gain Booster network.
Although the full-flash type A/D converter architecture is the most attractive solution for high-speed A/D converter designs, from a power dissipation and area perspective . Therefore the comparator structure is the most critical part in full-flash type architecture. The need arises for ADCs with higher resolution and faster conversion speed. The most popular type of fast converter is the flash ADC .This architecture required 2N-1 Comparator to achieved N bit resolution. For 4 bit ADC we required 15 comparators .The large numbers of comparators make it difficult to align and fabricate. The proposed improved threshold Inverter can used to preprocess the analog input signals, thus reducing the number of comparator.
II . DESIGN OF FLASH ADC
This section describes the design of 4-bit flash ADC. It consists of three blocks: (1) Comparator bank, and (2) Gain Booster and (3) Decoder.
be used, therefore effective threshold voltage are re- calculated after the transistor sizing process to handle these non-ideal effect such as narrow channel effect.
The design process can be obtained as below:
Design process involve the minimum size inverter which determine the threshold voltage value of the midpoint Quantizer Qn
Where the value of Wn = Wp we maintain the channel length is kept at minimum value.
Determine the analog input voltage range as by the following equation
Analog range = Vdd-( Vtn + |Vtp| )
Where Vtn & Vtp are the threshold voltage for NMOS and PMOS device.
Figure 2: Block diagram of design process To calculate LSB value like
Then calculate the ideal threshold points for each quantizer consider the center is Qn. the quantizer of Qn+1.Qn+p it is called PMOS side,(W/L) is kept at minimum value, The same process is applicable for NMOS but in opposite way. Replicate this entire quantizer block & complete the interconnection to get the cascaded structure, which will be
the analog part of the entire A/D code converter.
CMOS- LTE COMPA RATOR
GAIN BOOST ER CIRCUI TS
Figure 1: Block diagram of Flash ADC
THERMO METER TO BINARY CODE CONVERT
LTE Comparator Flash ADC
The reference voltage are changed when there is a noise in the power supply voltage to overcome this problem the CMOS LTE comparator are proposed. Where input voltage is compare with reference voltage to get Logic 1 or
0.when Vin > Vref then we get logic 1 at the output and When Vin< Vref the we get the logic 0 at the output of comparator circuit.
However due to non-ideal effects such as short channel and
narrow channel effects, the above design process cannot be so practical, therefore higher level MOS transistor design is to
This proposed scheme has the following advantages Use Digital Comparator for high speed & low power
consumption. To increase the voltage gain of the comparator. To reduced the noise problem & to reduced charge rejection ratio. Power supply voltage & Temperature are much smaller.
The CMOS-LTE Comparator uses Linear Tunable Transconductance Element and inverter as shown in Figure 3.The internal reference voltages are generated by systematically varying the transistor sizes of the CMOS linear tunable transconductance element. All transistor sizes of this element are identical in this design, with Vg1 and Vg4 as fixed voltages [1, 2]. The output of this component is connected to CMOS inverter to increase the voltage gain of the comparator. Using CMOS-LTE (CMOS Linear Tunable Transcoductance Element) Comparator. Voltage Transfer characteristics of an Inverter can be calculated mathematically by equation [1, 2]
Vm =(r(Vdd-Vtp)+Vtn) / (1+r) (1)
Figure 3(b): Voltage transfer curve (VTC) for LTE comparator
Structure of Proposed Comparator
where Vtp and Vtn represent the threshold voltages of the PMOS and NMOS devices respectively and
Kn = (W/L)n . n Cox Kp = (W/L)p . p Cox
The gate oxide thickness (Cox) for both transistors are same. So, Vm is shifted depending the transistor width ratio (Wp/Wn). That is, increasing Wp makes Vm larger, and increasing Wn results in Vm being smaller on the VTC
It can be shown that the Vm point on the Voltage transfer curve (VTC).The VTC is set for 15 comparator as shown in figure 3(b)
Figure 3(a): CMOS Linear Tunable Transconductance Element
Figure 4(a): CMOS NAND GATE COMPARATOR
Figure 4 (b): Voltage transfer curve (VTC) for CMOS NAND GATE COMPARATOR
The Structure of the Proposed Flash ADC using NAND GATE comparator is as shown in Figure4(a) By using CMOS-NAND gate comparator we are getting the linearity at the output as compare with CMOS-LTE Comparator. The gain boosters make shaper threshold of the comparator output voltage swing.
Each gain booster consists of two cascaded inverters. The transistor size of each gain booster is identical The gain booster is used to increase voltage gain of the output of the comparator so that it provides a full digital output voltage swing.[1,2]
The Multiplexer Based Decoder
For an N-bit flash ADC the most significant bit (MSB) of the binary output is high if more than half of the outputs in the thermometer scale are logic one. Hence MSB is same as the thermometer output at level 2N-1. To find the value at the second most significant bit (MSB-1) the original thermometer scale is divided into two partial thermometer scales, separated by the output level at 2N-1.
The partial thermometer scale to decode is chosen by a set of 2-to-1 multiplexers where the previous decoded binary output is connected to the control input of the multiplexers. MSB-1 is then found from the chosen partial thermometer scale in the same way as MSB was found from the full thermometer scale. The chosen scale is there by the scale that contains the information about MSB-1, i.e. the lower partial thermometer scale if the output at level 2N-1 is logic 0 otherwise the upper partial thermometer scale is used. This is continued recursively until only one 2-to-1 multiplexer remains. Its output is the least significant bit of the binary output.[4,3]
Figure 5: The Multiplexer Based Decoder
Figure 6: Complete Full Flash ADC Design
SIMULATION AND CHARACTERIZATION RESULTS OF 4-BIT FL ASH ADC using 0.18Âµm TECH
Figure 7: Transient response of the 4-bit flash ADC for ramp signal
TABLE I: COMPARISION OF COMPARATORS FOR Â±5% POWER SUPPLY VARIATIONS
Minimum Vm in volts
Maximum Vm in
TABLE II: COMPARATOR TRANSISTOR SIZE USED IN 0.25Âµm TECHNOLOGY
Analog input range
Min comp Max. comp
Min comp Max. comp
Min comp Max. comp
TABLE III: PROCESS VARIATIONS
Min. Vm (V)
Max. Vm (V)
TABLE IV : ADC PERFORMANCE
LTE Comparator Flash ADC and CMOS-NAND Comparator Flash ADC have been designed and simulated with 180 nm technology. The results obtained are encouraging and indicate that the CMOS-LTE Comparator approach has the advantage of better power supply noise rejection. Also the power dissipation is reduced because of the internally generated reference voltages. Future NAND based topology is used which improves PSRR as well as linearity; thus eliminates basic limitation of TIQ inverter.
Meghana Kulkarni1, V. Sridhar2 , G.H. Kulkarni3 4-Bit Flash Analog To Digital Converter Design Using CMOS-LTE Comparator 978-1- 4244-7456-1/10/$26.00 Â©2010 IEEE
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