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CHAPTER 1
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INTRODUCTION
The low power applications were becoming more and more
essential due to the increasing portable electronics market during the last
decades. The emerging rush to the mobile communications, mobile computers
and their immobile counterparts are preceding the constant improvement of
low power and high performance ICs. Due to the energy density limitations of
commonly used batteries, the weight reduction of portable devices became a
bottleneck. Therefore, it was necessary to reduce the total power consumption
for saving weight of such portable devices. But, it was divergent to the rapidly
growing number of devices on a single chip, due to increasing system
complexity. So, it was important to solve these power problems on the
transistor and circuit levels. The power consideration, which grew so great,
acquired as an important design parameter along with other two inevitable
parameters; speed and chip area, each one was being dependent on the others
(N. A. Shah and F. A. Khanday, 2009).
VLSI where millions of transistors can be integrated on a single
chip was perfectly suitable for portable electronics requirements specifically
CMOS technology due to its low power consumption and scalability. For
many years, bipolar transistor was dominant in the design of silicon ICs. The
emergent of Metal Oxide Semiconductor Field Effect Transistor (MOSFET)
which demonstrated area and power reduction changed the scenario and
became the vehicle for growth of VLSI designs. The CMOS technology,
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which was designed with complementary and symmetric pairs of n-channel
Metal Oxide
Semiconductor
(NMOS)
and
p-channel
Metal Oxide
Semiconductor (PMOS) transistors, dominated both digital and analog IC
designs. In CMOS technology, the power was dissipated only when the circuit
actually switches. The CMOS circuits had almost zero static power dissipation
unlike bipolar circuits.
The advancement of CMOS technology had been the stronghold
in the implementation of entire electronic SoC which consisted of digital,
analog and mixed signal functions. Signal processing was significant in many
of the SoC applications. The real world signals are inherently analog signals.
Processing of the analog signals was very complex. So, there was a necessity
to convert it to the digital form with the intention that the processing of the
digital domain was easier. ADC is a mixed signal device which act as interface
between the real world and the digital circuits, facilitate the effortless
processing of the sensors or transducers output. Hence, there was a huge
demand of high speed and low power ADCs (Chen et al. 2012).
1.1
ADC AND ITS TYPES
The ADC is a broadly used mixed signal electronic device that
converts an input analog electrical signal (commonly a voltage) to a digital
(binary) word as shown in Figure 1.1. The ADC had an analog reference
voltage or current to which the input analog signal was compared. The digital
output words expressed the fraction of reference current or voltage relevant to
that of the input signal.
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Figure 1.1
Basic Block Diagram of an ADC
The ADC performed the conversion periodically that is by taking
samples of the input instead of performing continuous conversion. Therefore,
there was a requisition of defining the rate at which new digital values were
sampled from the input analog signal. The rate of new digital values was
called as sampling frequency or sampling rate of the converter. For the faithful
reproduction of original signal, the sampling rate should be higher than twice
the amount of highest frequency of the input signal which was stated as
Shannon – Nyquist sampling theorem. The conversion also included the
quantization of input to obtain the digital signal.
The ADC had wide range of applications such as sensors, data
acquisition systems, digital signal processors, radar and communication
interfaces. The ADC architecture was chosen on the basis of application
requirements. There were many ways had been developed for conversion
process, each with its own strengths and weaknesses. If there was a need for
speed, a fast ADC can be used else if there was a need of precision, an
accurate ADC can be used else if there was constrain in space, a compact
ADC can be used appropriately. Hence, there were wide variety of ADC
architectures were available with respect to the application requirements. The
types of ADC were:
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 Successive Approximation Register ADC
 Integrating or Dual Slope ADC
 Flash ADC
 Averaging ADC
 Folding and Interpolating ADC
 Sub Ranging ADC
 Pipelined ADC
 Algorithmic ADC
 Time Interleaved ADC
 Delta Encoded ADC
 Sigma Delta ADC
 Wilkinson ADC
1.1.1
Successive Approximation Register ADC
SAR ADC evaluated each bit at a time from most significant bit
to least significant bit through all possible quantization levels. The comparator
was used in this ADC to compare the input signal with the reference signal
obtained from the DAC. SAR logic was used to obtain the digit bits as output
of the ADC which was also input of the DAC.
Advantages
 Compact implementation
 Low power consumption
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Disadvantages
 The size of DAC grew with number of bits.
 They took as many clock cycles for conversion as number of bits.
1.1.2
Integrating or Dual Slope ADC
The dual slope ADC used counters to generate the output digital
bits. As per the name, the ADC had two phases where first phase ramped up
with a slope proportional to the input voltage for a fixed time period and
second ramped down with a different slope proportional to the reference
voltage for a varied time period.
Advantages
 Very precise
 The sources of errors were only comparison with zero and clock period.
Disadvantages
 Low speed
 Needed time to ramp up and down the output voltage which doubled
with addition of each bit added to the representation.
1.1.3
Flash ADC
Flash ADC had a resistor ladder which divided the reference
voltage into equal parts. For every part, a comparator was used to compare the
input signal and the voltage obtained from the part of resistor ladder. The
outputs of all the comparators were called as thermometer code which was
converted to binary code using encoder.
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Advantages
 Very fast
 Instant conversion
Disadvantages
 The size of the ADC doubled for each bit added to the representation.
 Very high power consumption
1.1.4
Averaging ADC
In averaging ADC, the input analog signal was applied to more
than one number of ADCs with same clock frequency and phase. The output
digital words were generated by averaging them at each clock cycle.
Advantages
 Signal to noise ratio was improved.
 Uncorrelated noise sources between the ADCs were reduced.
Disadvantages
 Very high power consumption
 Large area
1.1.5
Folding and Interpolating ADC
The architecture consisted of folding blocks and interpolation
blocks. The folding architecture was used to reduce the number of
comparators and interpolation architecture was used to reduce the number of
differential pairs attached to the input signal. The architecture also consisted of
coarse quantizer and encoder.
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Advantages
 No sub stage DAC
 No need of linear processing.
Disadvantages
 Jitter errors at high frequencies.
 Increase in folding factor proportion to ADC resolution
1.1.6
Sub Ranging ADC
The architecture of sub ranging ADC was built in two important
steps. The input was tracked and held (T/H) in first step. Then it was digitized
by a sub stage ADC to produce Most Significant bits (MSBs) in second step.
The digital bits were then applied to sub stage DAC and the result was
subtracted from T/H output to determine the Least Significant bits (LSBs). A
summer was used to yield the final digital word.
Advantages
 No residue amplifier required compared to pipelined ADC.
 Reduced complexity compared to flash ADC
Disadvantages
 Three clock phases required for each conversion.
 Low speed
1.1.7
Pipelined ADC
Pipelined
ADC
required
number
of
conversion
stages
proportional to number of bits. At each conversion stage, the input signal was
compared to half the reference value. If it was higher, the reference value was
subtracted to input and bit corresponding to that stage was binary 1 else it was
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binary 0. In both cases, the remaining value was doubled and conceded to next
stage.
Advantages
 Higher speed
 Higher Bandwidth
Disadvantages
 High latency
 Any error introduced in a stage propagated to the following stages.
1.1.8
Algorithmic ADC
Algorithmic ADC, which was also called cyclic ADC, was
designed on the basis of binary division principle. The architecture was similar
to that of pipelined ADC but single stage was used for all partial conversions.
But, it was unable to process more than one sample at a time. Hence, this ADC
needed N+1 clock cycles to quantize N bit representation.
Advantages
 Less area
 Low power consumption
Disadvantages
 Low speed
 Less throughput
1.1.9
Time Interleaved ADC
The time interleaved ADC used M identical parallel ADCs where
each ADC sampled the data at every Mth cycle. Thus, the sampling rate was
increased by M times in comparison. The architecture could be designed in
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two ways. One with M identical parallel channels consisted of sample and
hold (S/H) circuit and ADC in every channel. Another way was designed with
single input S/H circuit and M ADCs.
Advantages
 High speed
 Increased bandwidth
Disadvantages
 High power consumption
 Large area
1.1.10
Delta Encoded ADC
Delta encoded ADC, which was also known as counter-ramp
ADC, had an up-down counter which fed a DAC. The comparator was used to
compare the input signal and the DAC and control the counter. The ADC used
negative feedback from the comparator to regulate the counter until DAC
output was nearer to input signal.
Advantages
 High resolution
 High bandwidth
Disadvantages
 Low speed
 High power consumption
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1.1.11
Sigma Delta ADC
Sigma delta ADC was a unique ADC which sampled the signal in
much higher frequency than the Nyquist frequency. Hence it was also called as
oversampling ADC. The error between reference signal and input signal was
integrated to obtain the outputs. Then, the output of the integrator was
compared with zero. The process was repeated again and again to obtain the
streams of binary zeros and ones. A decimation filter was used to convert the
bit stream into desired binary code.
Advantages
 Quantization noise spectral density was reduced.
 Very simple design
Disadvantages
 Decimation filter was required in the end.
 Limited bandwidth
1.1.12
Wilkinson ADC
The Wilkinson ADC was based on the comparison of input signal
with the voltage produced by a charging capacitor. The capacitor was allowed
to charge until its voltage equivalent to the amplitude of input signal and
discharge linearly. When the capacitor started to discharge, a gate pulse was
initiated and remained on until the capacitor discharged completely. The
duration of the gate pulse was directly proportional to amplitude of the input
signal.
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Advantages
 Compact design
 Less area
Disadvantages
 Low speed
 Limited bandwidth
1.2
PERFORMANCE METRICS OF ADCS
The metrics that characterize the performance of ADCs are
divided into two groups namely static and dynamic. The static performance
metrics of ADCs are Figure of Merit (FOM), Differential Non-Linearity
(DNL) and Integral Non-Linearity (INL). The dynamic performance metrics
of ADCs are Effective Number of Bits (ENOB), Total Harmonic Distortion
(THD), Signal to Noise Ratio (SNR), Signal to Noise and Distortion Ratio
(SNDR) and Spurious Free Dynamic Range (SFDR).
1.2.1
Differential Non-Linearity
The difference between an actual step width and ideal value of
one LSB is termed as DNL error. The calculated DNL value must be less than
1 LSB.
1 LSB = VFSR / 2N
(1.1)
Where VFSR is full scale voltage range
N is bit resolution of the ADC
DNL ( LSB ) 
V p 1  VP
Videal  1
for 0  p  2 N  2
(1.2)
where Vp is the physical value corresponding to the digital output code
Videal is ideal spacing of two adjacent digital codes
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1.2.2
Integral Non-Linearity
The deviation in LSB or the percent of full scale range of an
actual transfer function is INL error. The INL is classified into two types
namely the best straight line INL and end point INL.
1.2.2.1
Best Straight Line INL
The best straight line INL provides information about gain
(slope) error, offset (intercept) error and also position of the transfer function.
It also determines the closest approximation to the ADC’s actual transfer
function in the form of a straight line. This approach provides the best
repeatability and also gives out true representation of linearity.
1.2.2.2
End Point INL
The end point INL passes straight line through the end points of
ADC’s transfer function and thereby defines precise position for the straight
line. This straight line for a N-bit ADC is defined by its full scale range or its
zero as outputs.
INL ( LSB ) 
V p  Vzero
Videal  p
for 0  p  2 N  1
(1.3)
where Vzero is minimum analog input voltage corresponding to all zero output
code
1.2.3
Figure of Merit
The FOM is a quantity used to represent the measure of
efficiency and effectiveness of ADC. The FOM is given as
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FOM ( J / step) 
Psignal
min ( f s , 2 fin )  2 ENOB
(1.4)
where Psignal is the average power of the signal
fs is sampling frequency
fin is frequency of the input signal
1.2.4
Effective Number of Bits
ENOB can be obtained by
ENOB (bits) 
SNDR  1.76
6.02
(1.5)
where 1.76 is the term comes from quantization error in an ideal ADC
6.02 is the term which is used to convert decibels into bits
1.2.5
Signal to Noise and Distortion Ratio
SNDR is defined as the ratio of total received power to noise plus
distortion power.
SNDR (dB ) 10 log
Ps  Pn  Pd
Pn  Pd
(1.6)
where Ps is the average power of the signal
Pn is the average power of the noise components
Pd is the average power of the distortion components
1.2.6
Signal to Noise Ratio
SNR is defined as the ratio of signal power to noise power which
is expressed as
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SNR (dB)  6.02  N  1.76
(1.7)
where N is the resolution of ADC in bits
1.2.7
Total Harmonic Distortion
THD is defined as the ratio of sum of powers of all harmonic
components to power of fundamental signal which is used to measure the
harmonic distortion in the signal.
THD (dB) 
V22  V32  V42  
V1
(1.8)
where Vn is the root mean square voltage of nth harmonic and n = 1 is the
fundamental signal voltage
1.2.8
Spurious Free Dynamic Range
SFDR is defined as the strength ratio of the fundamental signal to
the highest noise or harmonic distortion component in the FFT spectrum. This
is an important specification which is used to determine the minimum signal
level that can be differentiated from noise or distortion components.
SFDR (dB )  20 log
1.3
fundamental signal
highest spurious
(1.9)
CHALLENGES IN DESIGNING ADC
In mixed signal designs, combining analog circuits on a digital IC
substrate was very difficult. Hence, there must be performance trade off and
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sometimes there might be error in the designs. There were key challenges to
be aware of while designing ADCs. The unavoidable challenges were:
 Sampling rates
 Bit resolution
 SNR
 ENOB
 Power
 Supply voltage
1.3.1
Sampling Rates
To satisfy Nyquist sampling theorem, the sampling rate should be
higher than twice the higher frequency of the input signal. However, faster
sampling rate demanded higher bandwidth which in turn consumed more
power and required better synchronization among the bits.
1.3.2
Bit Resolution
Bit resolution was a reflection of how closely the analog signal
represented as a digital word. The resolution was used to convey how many
discrete values can be produced over the range of analog values. So, it was
necessary to determine the bit resolution of the ADC.
1.3.3
SNR
In ADC, any noise that presented in the signal reduced the
accuracy of the digital output. So, care should be taken while considering the
placement of neighbouring components and circuitry on the chip.
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1.3.4
ENOB
ENOB was a broad way to measure the impact of distortion,
noise and temperature impact on the performance of ADC. So, it was essential
to have ENOB as high as number of ADC bits.
1.3.5
Power
In all SOC designs, power became an increasingly critical
concern. There was an increase in power consumption whenever any of the
performance metrics was improved. Always power became a trade off in the
design of ADC.
1.3.6
Supply Voltage
As the need for portable devices were drastically increased, there
was a necessity to design the circuits with reduced supply voltage. The design
of analog circuits with reduced supply voltage was critical when compared to
that of digital circuits. So, more concentration should be taken when it came to
supply voltage for the ADC architectural design.
1.4
MOTIVATION AND SCOPE OF THE THESIS
The push for portable systems and longer battery life time in the
electronic applications demands less area and low power consuming ADCs.
The advancement in CMOS technologies had supported the design of analog
circuits. In order to design efficient ADCs to achieve requirements using
CMOS technology, the limitations and scope of the circuit designs should be
understood.
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1.4.1
Power Consumption
Power consumption can be calculated by using various factors
such as supply voltage, capacitance, leakage current, short circuit current,
frequency and data activity. The commonly used expression for power
consumption was given by
Pc  Pd  Psc  Pl
(1.10)
where Pc – Power Consumption
Pd – Dynamic Power due to switching activities
Psc – Short Circuit Power
Pl – Leakage Power
The short circuit power resulted from finite rise and fall time of
the input signals that led to pull up and pull down network to ON for a short
time period. The expression for short circuit power was given by
Psc  I sc  Vdd
(1.11)
where Isc - Short Circuit Current
Vdd – Supply voltage
The reduction in supply voltage led to reduction in threshold
voltage to maintain the performance. This resulted in exponential growth of
the sub-threshold leakage current.
Pl  I l  Vdd
(1.12)
where Il – Leakage Current
1.4.2
CMOS as Active Resistor
The goal of the thesis is to optimize the building blocks of ADCs
by overcoming their limitations to achieve high speed low power and area
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efficient ADC architectures. Among the numerous ADC architectures, there
are three important architectures namely flash ADC, SAR ADC and pipelined
ADC are widely used (Razavi 2015). Hence, all three ADC architectures are
considered for the research to attain their optimized designs and also record
their static and dynamic performance metrics.
The comparator is the essential element in the design of ADCs
but also limit the performance of these ADCs (Katyal et al. 2006). So, there is
a need of designing low power, low offset and high speed comparator. To
arrive at the above specifications, the dynamic comparators are often used
which compare the inputs once in every clock period (Jeon & Kim 2010).
Hence, a dynamic comparator was proposed to meet the above mentioned
criterions.
The research includes proposed multiplexer based encoder for the
design of flash ADC and proposed two SAR logics where one with FSM and
other with shift registers. The research also includes the proposed DAC with
zero average switching energy.
The circuit designs are implemented in 130nm CMOS process
using Tanner tool 13.0 version at 1.2 V supply voltage. The characteristics of
the ADCs are manipulated with the help of MATLAB R2010a.
1.5
OBJECTIVES AND CONTRIBUTIONS OF THE THESIS
The main objectives and contributions of the thesis are,
 To analyze various elements of ADC such as dynamic comparators,
encoders, DACs, operational amplifiers by doing widespread survey
on existing designs and study their merits and demerits.
 To propose a design of low power, area efficient and high speed
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dynamic comparator.
 To design a four bit flash ADC architecture based on proposed
dynamic comparator and proposed multiplexer based thermometer
code to binary code encoder.
 To design a four bit SAR ADC architecture based on proposed
dynamic comparator, proposed SAR logic designs using FSM and also
using shift register and proposed DAC with zero average switching
energy.
 To design a four bit pipelined ADC architecture based on proposed
flash ADC as sub stage ADC, proposed DAC with zero average
switching energy and operational amplifier.
 To implement the design of proposed ADCs in an automotive
application.
 To carry out a comparative analysis between conventional and
proposed designs by considering the parameters such as transistor
count, delay, average power consumption, power delay product, static
power dissipation and dynamic power dissipation.
 To observe and record the ADC static and dynamic characteristics
such as: SFDR, SINAD, ENOB, INL, DNL and FOM.
1.6
ORGANIZATION OF THE THESIS
The Chapters are organized as follows.
Chapter 2
presents a detailed survey on various dynamic comparators and
various components of ADCs.
Chapter 3
proposes a design of low power, area efficient and high speed
dynamic comparator.
Chapter 4
proposes a design of four bit flash ADC architecture based on
dynamic comparator and thermometer code to binary code
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encoder.
Chapter 5
proposes a design of four bit SAR ADC architecture based on
dynamic comparator, two SAR logic designs where one using
FSM and other using shift register, DAC with zero average
switching energy.
Chapter 6
proposes a design of four bit pipelined ADC architecture based
on flash ADC as sub stage ADC, DAC with zero average
switching energy and operational amplifier.
Chapter 7
explains the implementation of the proposed ADC in the
automotive applications using LabVIEW tool.
Chapter 8
analyzes the performance of the proposed dynamic comparator.
A comparative analysis is performed between the conventional
and proposed designs to establish the effectiveness of the
proposed designs.
Chapter 9
concludes and recapitulates the overall effectiveness of the
proposed designs and possible scope for the future work.