12EC62R20 Ramesh Synopsis

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DIGITAL ADC: VCO BASED ADC USING

DECIMATION FILTER

Synopsis Submitted in partial fulfillment of the Requirements for the degree of

Master of Technology (M. Tech)

In

Microelectronics & VLSI Design

Under the guidance of

Dr. Swapna Banerjee

By

Ramesh Prasad(Roll no: 12EC62R20)

Department of Electronics & Electrical Communication Engineering

Indian Institute of Technology, Kharagpur

Kharagpur - 721302, West Bengal, India

April 2014


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ABSTRACT

The main objective of this work is to design an ADC which is suitable for high speed

applications with medium resolution, high bandwidth and low power. Since, ADC acts as a

connecting link between the Analog domain and digital domain. In deep sub-micrometer

process technologies, it is very difficult to design efficient ADC due to low voltage headroom

available for analog circuits and it also makes the analog circuits more complex whereas

digital circuits are faster and smaller with the advances in CMOS technology. Therefore ,

VCO based ADC is one of the best alternative with respect to other ADC architecture

because it makes extensive use of digital circuits and do not need accuracy in analog design.

Due to miniaturization of CMOS transistors in the integrated circuits results for faster

switching performance and reduced value of transconductance. In the design of data

converters higher gain bandwidth product is required in order to reduce the settling error of

op-amp. Digital circuits are basically known for faster switching response, while the lower

value of transconductance deteriorates the analog design. VCO based ADC provides the

advantages of Digital circuits and consumes lesser power with respect to other ADC

architecture. In this ADC, A sigma delta modulator using VCO is designed and digital

calibration is used in order to enhance the performance of VCO. The design is implemented

in 180nm UMC technology and the ADC achieves 56dB peak SNR and 54 peak SNDR at

600MHz sampling frequency for 20MHz input bandwidth and 11.2 mW.

Index Terms - Calibration, VCO based ADC, Sigma delta modulator, Voltage Controlled

Oscillator.


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1. Introduction

As we are living in analog world. There is need of circuits in order to fill the gap between the

analog domain and digital domain. The signals are converted into digital domain using digital

systems because it is easier to process the signals for various kind of operation in digitaldomain rather than in analog domain.

As the technology is growing, there has been a continuous demand for higher bandwidth and

faster circuits. In earlier days, in order to develop faster circuit designers used the bipolar

transistor technology but it consumes a large amount of power [1]. But the use of BJT

technology was not suitable for mobile applications.

In order to achieve high speed and lower power consumption circuit designers moved to

CMOS technology [2]. Initially, with respect to speed CMOS technology always lagged

behind BJT technology but with the scaling in CMOS technology it became capable of

achieving high speed.

Figure 1(a) Performance Vs Power (Analog-Digital Converter)

Impact of Technology Scaling: In modern technology process it is very difficult to design

converters with low voltage headroom. The supply voltage is also reduced as per the

technology scaling, in order to maintain the signal to noise ratio it is also very important to

minimize the noise of converters. In most advanced processes, this limitation is handled by

utilizing thicker gate oxides but at the same time it affects the speed of operation.

Minimalistic Design:The primary motive of minimalistic design is to enhance the speed and

power efficiency by simplifying the analog circuits, because Power dissipation of ADC is


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directly dependent on the complexity of analog circuits. The simplification of analog circuits

can affect the precision of ADC and remedy of the problem is to move towards digital design.

Digitally Assisted Architectures:The digital logic can assist any converter for the purpose

of calibration and error correction.Technology scaling has significantly reduced the energy

per operation in CMOS logic circuits. For 0.5m technology the 2 input NAND gate

dissipates 1.23 pJ per operation whereas same gate dissipates 4.4fJ per logic operation in 90

nm Technology.

Table 1.1 ENANDVs EADC

As shown in table, for 50 dB signal fidelity and ENAND= 4.39fJ per logic operation, A single

conversion of ADC would consume as much as energy of approximately 38000 gates.

Generally, Converters basically depends upon three performance parameters i.e (a) Power

Consumption (B) Sampling Speed and (C) Resolution. Power consumption is one of the key

parameter to be taken into consideration for designing low power converters. The sampling

speed is defined as Samples per second. Basically it defines the measurement of number ofsamples of analog signal for a given time frame. Resolution can be defined through Signal to

Noise Distortion Ratio (SNDR). Higher the values of SNDR, higher will be the resolution.

All these three performance parameter can be combined to define a term called Figure of

Merit (FOM). This term is used to define the performance of ADC .According to following

equation,

= 2

Figure 1(b) shows the different kind of ADC architectures having different sampling speeds,

resolution and the applications demanding them. For very high bandwidth ADC the sampling

speed of the ADC must be in several of MHz.

2. Motivation

As we know analog to digital converter are very important block electronics systems in

modern time. Modern integrated technology can provide very high sampling rates but at the

cost of high power consumption. For e.g. [3,4] This ADC provides a sampling rate of

20G/sec at power dissipation of 10W at 1.2 W. In most cases, the performance and efficiency

are mainly affected by the rate at which analog signal is converted into digital.


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Figure 1(b) Different architecture of ADC

Analog to digital converter is easier to design in digital domain rather than in analog domain

because of reduced threshold voltage problem and low power supply voltage. The signal

process capability is also enhanced and improved in digital as compared to analog

counterparts - better repeatability, automatic synthesis and testing, very low sensitivity in

noise. Fig.1(c) & (d). Shows a scatter plot of results published at these venues over the past

eleven years [4].

Figure 1(c) Power efficiency versus SNDR Figure 1(d) Conversion b/w versus SNDR.

Due to several advantages, design of ADC digitally has gained so much popularity as

compared to analog ADC. There are several ways to design converters digitally but one way

is to convert the analog information into the time domain and convert the pulse widths (time

domain intervals) into digital codes. Reduction in supply voltage and shrinkage in transistor

size, ADC are designed in time domain instead of analog domain.


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3. VCO based Analog-Digital Converter

VCO based ADC is one of the best option for designing analog to digital converter in

advanced technology. A VCO based ADC is oversampling converts which uses sampling

frequency much higher than the Nyquist frequency and it uses time based architecture whichconsists of VCO and digital circuits [7]. Analog input signal controls the frequency of the

VCO frequency and VCO converts the signal from voltage domain to phase domain. The

digital part consists of Counter along with other digital circuitry and counter counts the

number of edges of multiphase oscillator outputs for a clock period and generates the

required digital output. The resolution of the ADC can be improved with the increase in the

number of phases generated from the VCO [10]. The quantization error of previous sampling

period becomes the initial phase for the next sampling period. ADC output is given by the

expression,

[] =([]+[ 1] [])

2

Where []is the VCO phase change due to analog input signal and is the number of

VCO phases.

[] =

([] +( 1)[])

2

VCO plays the role of integrator in time domain and convert the signal into time variation

from amplitude variation. But apart from several advantages it has some demerits also [7].

VCO tuning curve for control voltage is non linear in nature which is a matter of concern.

While a voltage controlled oscillator has a variety of unusual and interesting properties, it has

two major advantages that are very much attractive and relevant in the design of ADCs.

First, the VCO acts as a voltage-to-phase integrator. The instantaneous VCO output

frequency Fout is proportional to the applied input voltage Vtune(t)according to the voltage-

to-frequency gain Kv[Hz/V]. The resulting voltage controlled oscillator output phase out(t)

is proportional to the time integral of the applied input voltage.

Note that as long as the VCO oscillates, the VCO output phase will accumulate endlessly,

even for a DC input. This implies that the VCO behaves as a CT integrator with infinite DC

gain. A second property of interest is the digital nature of a ring-VCOs outputs [8]. Note that

while the VCO output phase and frequency are continuously varying, the VCO output itself

toggles between two discrete levels VDD and Gnd , much like a CMOS digital gate .Multi-

phase (or equivalently, multi-bit) quantization can be accomplished by sampling the output

phases of a ring oscillator with an array of D-flip-flops [9]. Note that since the VCO phases

are full-swing logic signals, the quantizer is robust to voltage offsets in the flip-flops. At the


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same time, only one VCO edge transitions at a given sampling instant, while the rest of the

VCO phases saturate to either VDDor Gnd. Consequently, the quantizer not only is less prone

to generate meta-stable outputs, but also has guaranteed monotonicity without requiring any

calibration.

A new methodology is proposed in which the non linearity of VCO is taken care by look up

table based calibration and the limited time resolution is taken care by the phase interpolator

which results in the improvement in SFDR, thus enhancing the efficiency of the conversion.

This architecture of VCO based is in open loop nature because the problem of non linearity

and low resolution can be resolved by employing the voltage controlled oscillator in closed

loop manner but at the same time it occupies large area. So, off chip digital calibration

technique is one of the methods which can be used to enhance the performance of ADC

The primary target of this work is to design ADC with 8 bit resolution, high bandwidth (upto

20MHz) and low power. Thus, a new design technique is defined in which digital signal edge

transition is superior to the analog voltage resolution.

Fig 1(e) Implemented analog digital converter

Fig 1(f) Implemented delay cell for VCO


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Fig 1(g) Phase interpolator

Fig 1(h) Differential ring VCO with phase interpolator

Fig 1(i) Phase Quantizer

4. Organization of Thesis

The thesis is divided into 6 chapters. The first chapter consists of the introduction, motivation

and objective of the project. The second chapter consists of literature survey on various kinds

of ADC architectures and their advantages and disadvantages. The third chapter provides the

detailed theoretical information on various kind of circuits used at block level like differentialVCO, Quantizer, filter etc. The fourth chapter provides the detailed information on the

implementation of VCO based ADC in UMC 180nm technology. The fifth chapter gives the

information regarding the schematic simulation results and post layout simulation results.

And the last chapter gives the conclusion and future work.

5. Conclusion

Digital analog to digital converters have potential applications in very low voltage, lowpower circuits. Moreover, the implementation of such converters is fully compatible with the


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standard digital CMOS technology. Actually one important feature of Digital ADC is that,

advances in process technology allow the same architecture to achieve a high sampling

frequency and better resolution as well as lower power consumption.

The design is implemented in UMC 180nm CMOS process with a supply voltage and it can

be used for lower supply voltage and sub micron technologies. The VCO ADC is designed

with approx 20 MHz bandwidth and SNR/SFDR of value 47.2 dB/ 57 dB.

The Dynamic performance of VCO based ADC can be improved in open loop configuration

with digital calibration and decimation filter techniques.

6. References

[1] R. Hagelauer, F. Oehler, G. Rohmer, J. Sauerer, and D. Seitzer, A gigasample/second 5-b ADC with on-chip track and hold based on an industrial GaAs MESFET E/D process,

IEEE Journal of Solid-State Circuits, vol. 27, no. 10, pp. 13131320,1992.

[2] M. Choi and A. Abidi, A 6 b 1.3 GSample/s A/D converter in 0.35 mu;m CMOS, in

International Solid-State Circuits Conference, pp. 126127, 438, 2001.

[3] B. Murmann, "Digitally Assisted Analog Circuits A Motivational Overview," ISSCC

Special-Topic Evening Session(SE1.1), Feb. 2007.

[4] P. Schvan, et al, A 24GS/s 6b ADC in 90nm CMOS,ISSCC Dig. Techn. Papers, pp.

544-545, Feb. 2008.

[5] K. Poulton, et al., A 20-GSample/s 8b ADC with a 1-MByte Memory in 0.18-um

CMOS, ISSCC Dig. Techn. Papers, pp. 318-319, Feb. 2003.

[6] B. Murmann, "ADC Performance Survey 1997-2008," [Online]. Available:

http://www.stanford.edu/~murmann/adcsurvey.html.

[7] Jaewook kim, Tae-Kwang jang, Young-Gyu Yoon and Seong Hwan Cho, Analysis and

Design of VCO based ADCs, IEEE transactions on circuits and systems, Vol.57, No.1, Jan

2010.

[8] Sachin Rao, Brian Young, Amr Elshazly, Wenjing Yin, Naga Sasidhar, and Pavan Kumar

Hanumolu, A 71dB SFDR Open Loop VCO-Based ADC Using 2-Level PWM Modulation,

2011, Symposium on VLSI Circuits.

[9] Engel Roza, Analog-to-Digital Conversion with duty cycle modulation, IEEE

transactions on circuits and systems, Analog and Digital Signal processing, Vol.44, No.11,

November 1997.

[10] C.H.Park and B.Kim , A Low noise , 900MHz VCO in 0.6m CMOS, IEEE J. Solid

State Circuits , vol. 34 , no. 5 ,pp. 179-194, May 1999.

12EC62R20 Ramesh Synopsis

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