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International Journal of Advanced Engineering Research and Technology (IJAERT) Volume 3 Issue 3, March 2015, ISSN No.: 2348 – 8190
88
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Study of Various ADCs and Compare Their
Performance and Parameters
1Mrs. Jasbir Kaur,
2Saurabh Kansal
1Assistant Professor,
2ME Scholar Electronics (VLSI Design)
Electronics and Communication Engineering Department
PEC University of Technology, Chandigarh, India
Abstract-The performance of the analog to digital
converters has become very important in signal
processing applications. Most important parameters on
which theirperformance depend are: resolution,
conversion time and the sampling frequency of input. It
became very important to understand the functioning
and performance before implementing the ADC in the
system. This paper discusses various types of ADCs and
compares the commonly used ADCs so that user can
conveniently choose between various ADCs available
for their application.
Keywords- Resolution, Conversion time, Chip area of
ADC, sampling frequency.
I. COUNTER TYPE ADC
It is the basic type of ADC which is also called as digital
ramp type ADC or stair case approximation ADC.
Fig1: Counter Type ADC
The counter generates an N bit digital output which is
applied as an input to the DAC. The analog output
corresponding to the digital input from DAC is
compared with the input analog voltage using an Op-
Amp comparator. The Op-Amp compares the two
voltages and if the generated DAC voltage is less, it
generates a high pulse to the N bit counter as a clock
pulse to increment the counter. The same process will be
repeated until the DAC output equals to or greater than
the input analog voltage.
If the DAC output voltage is equal to or greater than the
input analog voltage, then it generates low clock pulse
and it also generates a clear signal to the counter and
load signal to the storage resistor to store the
corresponding digital bits. These digital values are
closely matched with the input analog values with small
quantization error. For every sampling interval, the DAC
output follows a ramp fashion that is why, it is called as
Digital ramp type ADC, this ramp looks like stair cases
for every sampling time so, and it is also called as
staircase approximation type ADC.
Conversion time of Counter type ADC Conversion time of ADC is the time taken by the ADC
to convert the input sampled analog value to digital
value. Here the maximum conversion of high input
voltage for N bit ADC is the clock pulses required to
count its maximum count value. So, the maximum
conversion of Counter type ADC is equal to (2N-1) T
where T is the time period of clock pulse.
Limitations of Counter type of ADC is thatspeed is less
because every time the counter has to start from
ZERO.There may be clash or aliasing effect if the next
input is sampled before completion of one operation.
II. TRACKING TYPE ADC
In counter type DAC, the counter has to come to zero for
every conversion which is a disadvantageous with
respect to high conversion time.
To perform operation, instead of using normal counter,
this ADC uses up /down counter. After the first sampled
value, the up/down counter will tracks the input analog
value so that it is called tracking type of ADC [19]. The
detailed block diagram of Tracking type ADC is shown
in below figure.
Fig 2: Tracking Type ADC
International Journal of Advanced Engineering Research and Technology (IJAERT) Volume 3 Issue 3, March 2015, ISSN No.: 2348 – 8190
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The N bit up/down counter starts counting according to
the clock pulse and provides a digital input to the DAC.
The DAC converts the digital input intocorresponding
analog output which is applied to Op-Amp. The Op-
Amp compares the DAC analog output with the input
analog sampled value, if the input value greater than the
DAC output it provides a clock pulse to increment the
counter or otherwise if it is lesser than the DAC output it
provides a clock pulse to decrement the counter. But
normally for first sampled value the counter will
incremented to match to the analog value and for
subsequent samples only the counter may get decrement
pulse.
When the DAC output is equal to the sampled value, the
digital output will be taken from the up/down counter
directly. Here shift register is not needed to
capturedigital data because the counter will not go to
reset state. Then, onwards for the next sample value, the
counter will increment or decrement according to the
difference output of Op-Amp. This is also called as
Derivative Counter type ADC because the counter
output depends on difference between the previous and
next sampled value like as differentiator.
Advantages of this ADC are that Shift register is not
needed so the cost is less.Speed is high compared to
digital ramp type as the counter will not reset.
Disadvantages of tracking type ADC is that Up/Down
counter leads to complexity of the circuit. The digital
output will never be constant because of differentiator
effect. i.e., for a constant analog value also the output
will oscillating this is known as bitbobble.
III. SUCCESSIVE APPROXIMATION ADC
One method of addressing the digital ramp ADC’s
shortcomings is successive approximationADC. The
only change in this design is a very special counter
circuit known asSuccessive Approximations
RegisterInstead of counting up in binary sequence, this
register counts by trying all values of bits starting with
the most-significant bit and finishing at the least
significant bit. Throughout the count process, the
register monitor the comparator’s output to see if the
binary count is less than or greater than the analog signal
input, adjusting the bit values accordingly [19].
The advantage to this counting strategy is much faster
results: the DAC output converges on the analog signal
input in much larger steps than with the 0 to full count
sequence of a regular counter.
Important point to note on Successive Approximation
ADC is that in Counter type or digital ramp type ADC
the time taken for conversion dependson the magnitude
of the input, but in SAR the conversion time is
independent of applied input voltage.
Fig 3: Successive Approximation ADC
Advantages of this ADC arespeed is high compared to
counter type ADC.Good ratio of speed to
power.Compact design and inexpensive compared to
flash type.
Disadvantages of successive Approximation ADC Cost
is high as compare to counter type ADC because of SAR
and complexity in design.
Applications of SAR ADC is used widely data
acquisition techniques at the sampling rates higher than
10 KHz.
IV. INTEGRATING ADC
Integrating analog to digital convertersprovide high
resolution analog to digital conversions, with good noise
rejection.
Single-Slope ADC Architecture: The simplest form of
an integrating ADC uses single slope architecture
(Figures 4a and 4b). Here, an unknown input voltage is
integrated and this value is compared against a known
reference value. The time it takes for the integrator to
trip the comparator is proportional to the unknown
voltage. In this case, the known reference voltage must
be stable and accurate to guarantee the accuracy of the
measurement [5].
One drawback to this approach is that the accuracy is
also dependent on the tolerances of the integrator's R and
C values. Thus in a production environment, slight
differences in each component's value change the
conversion result and make measurement repeatability
quite difficult to attain. To overcome this sensitivity to
the component values, the dual-slope integrating
architecture is used.
International Journal of Advanced Engineering Research and Technology (IJAERT) Volume 3 Issue 3, March 2015, ISSN No.: 2348 – 8190
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Fig 4(a) Single-Slope ADC Architecture, Fig 4(b)
Output of integrator
Dual-Slope ADC Architecture: A dual-slope ADC
integrates an unknown input voltage (Vin) for a fixed
amount of time, then de-integratesusing a known
reference voltage (Vref) for a variable amount of time.
In the dual-slope converter, an integrator circuit is driven
positive and negative in alternating cycles to ramp down
and then up, rather than being reset to 0 volts at the end
of every cycle. In one direction of ramping, the
integrator is driven by the positive analog input signal
(producing a negative, variable rate of output voltage
change, or output slope) for a fixed amount of time, as
measured by a counter with a precision frequency clock.
In the other direction, with a fixed reference voltage
(producing a fixed rate of output voltage change) time is
measured by the same counter. The counter stops
counting when the integrator’s output reaches the same
voltage as it was when it started the fixed-time portion of
the cycle.
The amount of time it takes for the integrator’s capacitor
to discharge back to its original outputvoltage, as
measured by the magnitude accrued by the counter,
becomes the digital outputof the ADC circuit.
Fig 5(a): Dual-Slope ADC Architecture
Advantage of this method is that the input signal
becomes averaged as it drives the integrator during the
fixed-time portion of the cycle. Any changes in the
analog signal during that period of time have a
cumulative effect on the digital output at the end of that
cycle. Other ADC strategies merely capture the analog
signal level at a single point in time every cycle. If the
analog signal is “noisy” (contains significant levels of
spurious voltage spikes/dips), one of the other ADC
converter technologies may occasionally convert a spike
or dip because it captures the signal repeatedly at a
single point in time. A dual-slope ADC, on the other
hand, averages together all the spikes and dips within the
integration period, thus providing an output with greater
noise immunity.
These types of converters ofteninclude built-in drivers
for LCD or LED displays and are found in many
portable instrument applications, including digital panel
meters and digital multi-meters.
Fig 5(b): Integrating and De-integrating action of Dual
slope integrating ADC
Initially, switch S1 is on and S2 and S3 are off.
Theintegrator will integrate a sample of the input
voltage,Vin, for a fixed period of time T1and then, switch
S2 is closed. This disintegrates the input voltage to zero
and time taken by it to come to zero voltage is
proportional to applied input.
The disadvantage of this ADC is it is the slowest of all
ADCs but very chip and mostly used in DVMs.
V. FLASH ADC
Flash ADC is also called as the parallel A/D converter. It
is formed ofa series of comparators, each one comparing
the input signal to a unique reference voltage. The
comparator outputs connect to the inputs of a priority
encoder circuit, which then produces a binary output.
The fig 6(a) shows a 3-bit flash ADC circuit,Vref is a
stable reference voltage provided by a precision voltage
regulator as part of the converter circuit.As the analog
input voltage exceeds the reference voltage at each
comparator, the comparator outputs will sequentially
International Journal of Advanced Engineering Research and Technology (IJAERT) Volume 3 Issue 3, March 2015, ISSN No.: 2348 – 8190
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saturate to a high state. The priority encoder generates a
binary number based on the highest-order active input,
ignoring all other active inputs [2].
Fig 6(a): 3-bit Flash ADC circuit with 8 to 3 line priority
converter
For this particular application, a regular priority encoder
with all its inherent complexity isn’t necessary. Due to
the nature of the sequential comparator output states
(each comparator saturating high in sequence from
lowest to highest), the same effect (highest-order-input
selection) may be realized through a set of Exclusive-OR
gates, allowing the use of a simple encoder:
Fig 6(b): 3-bit flash ADC circuit with a set of Exclusive-
OR gates
Not only is the flash converter the simplest in terms of
operational theory, but it is the most efficient of the
ADC technologies in terms of speed, being limited only
in comparator and gate propagation delays.
And, of course, the encoder circuit itself can be made
from a matrix of diodes, demonstrating just how simply
this converter design may be constructed [19]:
Fig 6(c): 3-bit Flash ADC circuit with a matrix of diodes
Unfortunately, it is the most component-intensive for
any given number of output bits. This three-bit flash
ADC requires seven comparators. A four-bit version
would require fifteen comparators. With each additional
output bit, the number of required comparators doubles.
Considering that eight bits is generally considered the
minimum necessary for any practical ADC (255
comparators needed!), the flash methodology quickly
shows its weakness.For N bits of resolution, it requires
Ncomp. = 2N− 1comparators which require quite a lot of
area for more than 8 bits.
An additional advantage of the flash converter, often
overlooked, is the ability for it to produce a non-linear
output. With equal-value resistors in the reference
voltage divider network, each successive binary count
represents the same amount of analog signal increase,
providing a proportional response. For special
applications, however, the resistor values in the divider
network may be made non-equal. This gives the ADC a
custom, nonlinear response to the analog input signal.
No other ADC design is able to grant this signal-
conditioning behavior with just a few component value
changes [19].
Flash ADCs are suitable for applications requiring very
large bandwidths. However, these converters consume a
lot of power, have relatively low resolution, and can be
quite expensive.
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VI. TWO STEP FLASH ADC The Two Step Flash architecture evolved from the Full
Flash converter. One of the main drawbacks of the latter
is the number of necessary comparators, given by
Ncomp. = 2N− 1, which scales exponentially with the
resolution of the converter (N), making it, in some cases,
impractical to implement due to the necessary die area.
The Two-Step Flash topology alleviates the number of
necessary comparators byquantizing the input in two
steps. The effective reduction factor in the number of
comparators when compared to the Full-Flash ADC, is
exponentially proportional to the converter’s resolution,
and is approximately given by2N/2−1. In other words, the
higher the resolution the more area efficient it becomesto
use a Two-Step topology [18].
Fig 7(a): Two Step FlashADC topology employing
MDAC circuits
As shown in fig 7(a), each step (or stage) is composed of
a quantizer, with a resolution N1and N2inferior to the
resolution (N) of the entire converter, thus, requiring less
reference voltages and comparators, and consequently,
occupying less die area. Between the two steps, an
amplified residue voltage needs to be generated, which is
achieved with a DAC, a subtraction operation block and
a gain block. These three blocks constitute an MDAC
circuit. The principle of operation is as follows: the input
is sampled by the first quantizer during the sampling
phase. During the residue amplification phase, the first
quantizer decides the most significant bits (MSBs),
which are then used to reconstruct a voltage (using the
DAC), that is subtracted from the original sampled input
and then amplified (by 2N1
) to create an amplified
residue voltage. Still during this phase, the second
quantizer samples this residue. The objective of the
amplification is to restore the residue to the full voltage
range of the converter, thus facilitating the
implementation of the second quantizer fig 7(a), or
eventually, reusing the same quantizer in a cyclic way.
During the final phase, the second quantizer (of
resolution N2) quantizes the residue to obtain the least
significant bits (LSBs). The final digital output is
assembled using digital logic, by adding the MSBs
together with the LSBs.
Basically, this topology simplifies the quantization, by
trading comparators with time. The Full-Flash converter
achieves a quantization in two clock phases (one clock
cycle),while the Two-Step needs at least three phases,
i.e., one and a half clock cycles (Fig. 7 (c)).
Fig 7(b): Two stage quantization
Fig 7(c): Timing diagram of Two Step FlashADC
Although the throughput may be similar to that of the
Full-Flash converter (one digital output per clock cycle),
the Two-Step has higher latency.IfN1 is made equal to
N2, then only one quantizerneeds to be designed and,
therefore, both quantizersmay use the same reference
voltages. The latter is also made possible by using a
residue amplification gain of 2N1
.
VII. PIPELINE ADC
The Pipeline converter’s operation is basically the same
as that of the Two-Step Flash. Each stage is responsible
for quantizing Njbits, j=1, 2,… K (Nj< N) and generating
an amplified residue for further quantization (performed
by subsequent stages). However, the first stage does not
have to wait for the residue of a specific sample to reach
International Journal of Advanced Engineering Research and Technology (IJAERT) Volume 3 Issue 3, March 2015, ISSN No.: 2348 – 8190
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the end of the pipeline, to conclude its quantization. As
soon as the first stage has performed its task, it may
quantize the next input sample. This holds for all stages,
which means that at any given time, except at the
beginning when the converter starts its operation, all
stages are processing data. Thus, the throughput
ofPipeline converter may be similar to that of the Full-
Flash ADC, but its latency is high, even higher than that
of the Two-Step Flash converter. The more stages there
are in the pipeline chain, the higher the latency will be
[18].
Fig 8: Pipeline A/D converter with block diagram of a
stage
Normally, all pipeline stages are designed with the same
resolution to simplify the converter’s layout and
implementation, but, design trade-offs may determine
that each stage have different resolutions. It is usual to
find the first stage with a higher resolution. Another
important reason for all stages to be equal (in resolution)
is that the reference voltages are the same for all
quantizers and DAC functions. The pipeline ADC
further reduces the number of comparators and dies area
at the cost of increased latency.
VIII. MULTI-STEP ALGORITHMIC ADC
The Algorithmic (or Cyclic) converter, as the name
indicates, quantizes the input sample in an algorithmic or
repetitive manner [18].
The main difference with the Algorithmic converter is
that the conversion algorithm (sampling, quantization,
and residue amplification) is repeated in the same
physical space (reutilizing the same circuits) or area,
while the Pipeline ADC repeats its operation over more
area. In other words, the Algorithmic converter trades
space for time, which means it has a longer conversion
cycle as shown in Fig. 9(a). The principle of operation is
as follows: the input voltage is sampled by the first
stage. It is then quantized to generate the MSBs. These
bits are used to reconstruct a voltage (using a DAC and
reference voltages) that is subtracted from the input
voltage generating the residue voltage. This residue is
then amplified (and held) to the full scale range of the
converter and sampled by the second stage. This process
repeats itself between the first and second stages until
the LSB is generated. At this point a full digital output is
ready, while the converter is sampling the next input
sample (Fig. 9(b)).
Fig 9(a): The Algorithmic A/D converter: Block diagram
Fig 9(b): Timing diagram of Pipeline A/D converter
IX. TIME-INTERLEAVING ADCS
Time-Interleaving is a technique used to increase the
throughput or conversion rate of a converter. This
technique may be applied to all A/D converter
topologies. It consists of using an array of M parallel
converters multiplexed at the input and at the output.
Fig 10(a): Block diagram of Time-interleaving of A/D
Converters
Each converter operates at a conversion rate Fs/M (where
FSis total conversion rate), making it easier to
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implement. The analog input multiplexer, adequately
timed, is responsible for attributing an input sample to
each of the converters over time, when it reaches the last
converter in the array it starts over again. The digital
output multiplexer guarantees that the timing sequence
of digital outputs are in accordance with the sampled
inputs. Each converter added to the array inevitably
increases the die area and the power consumption of the
overall A/D conversion system. The timing diagram of
operation is shown in Fig.10 (b).
Fig 10(b) Timing diagram of Time-interleaving of A/D
converters
Besides the limitations produced by each unit
(multiplexed) ADC, the time interleaved technique
introduces its own limitations. These have mainly to do
with mismatches between the various unit ADCs that
compose the converter. This topology is very sensitive to
mismatches in the offset, gain, timing,and bandwidth of
each unit ADC. All these extra errors (inherent to time-
interleaving) cause a degradation of the converter’s
signal-to-noise ratio (SNR). No matter how large the
error of a unit ADC is, as long as all other unit ADCs
has the same errormagnitude, no mismatch will exist
[18].
X. DELTA-SIGMA ADC
One of the more advanced ADC technologies is the
Delta Sigma [15].
In this converter, the analog input voltage signal is
connected to the input of an integrator, producing a
voltage rate-of-change, or slope, at the output
corresponding to input magnitude. This ramping voltage
is then compared against ground potential (0 volts) by a
comparator. Thecomparator acts as a sort of 1-bit ADC,
producing 1 bit of output (high or low) depending on
whether the integrator output is positive or negative. The
comparator’s output is then latched through a D-type
flip-flop clocked at a high frequency, and fed back to
another input channel on the integrator, to drive the
integrator in the direction of a 0 volt output. The basic
circuit is shown in fig 12. The leftmost op-amp is the
summing integrator. The next op-amp, the integrator
feeds into is thecomparator, or 1-bit ADC. Next come
the D-type flip-flop, which latches the comparator’s
output at every clock pulse, sending either a high or low
signal to the next comparator at the top of the circuit.
This final comparator is necessary to convert the single
polarity 0V /5V logic level output voltage of the flip-
flop into a +V / -V voltage signal to be fed back to the
integrator [19]
Fig 12: Block diagram of Delta Sigma ADC
If the integrator output is positive, the first comparator
will output a high signal to the D input of the flip-flop.
At the next clock pulse, this high signal will be output
from the Q line into the non-inverting input of the last
comparator. This last comparator, seeing an input
voltage greater than the threshold voltage of 1/2 +V,
saturates in a positive direction, sending a full +V signal
to the other input of the integrator. This +V feedback
signal tends to drive the integrator output in a negative
direction. If that output voltage ever becomes negative,
the feedback loop will send a corrective signal (-V) back
around to the top input of the integrator to drive it in a
positive direction. This is the delta-sigma concept in
action: the first comparator senses a difference between
the integrator output and zero volts. The integrator sums
the comparator’s output with the analog input signal.
Advantages of this ADC is that modern Sigma-delta
converters offer high resolution, high integration, low
power consumption, and low cost, making them a good
ADC choice for applications such as process control,
precision temperature measurements, and weighing
scales.
Limitation of this ADC is that higher order (4th order or
higher) and multi-bit feedback DAC requires large area.
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Comparison of Various ADCs
FLASH
(Parallel)
SAR
DUAL SLOPE PIPELINE SIGMA DELTA
Conversi
on
Method
For N bits ADC
2N -1
comparators
required, means
increase by a
factor of 2 for
each bit [2]
Binary search
algorithm,
internal
circuitry runs at
higher speed [10]
Unknown input voltage
is integrated and
valuecompared against
known
referencevalue[19]
Small parallel
structure,each stage
works on a few bits
[16]
Comparator senses the
difference and
integrator sums the
comparator’s output
with the analog input
signal,Oversampling
ADC [15]
Selection
of this
architectu
re
Ultra-High Speed
when power
consumptionis not
primary concern?
Medium to high
resolution (8 to
16bit), low
power consumption ,
small size
Monitoring DC
signals, high
resolution, low
power consumption,
good noise
performance
High speeds,
few Msps to
100+ Msps, 8
bits to 16
bits resolution, lower
power
consumption than flash
High resolution, low to
medium speed, no
precision external
components, digital
filter reduces anti-
aliasing requirements
Encoding
Method
Thermometer
Code
Encoding [3]
Successive
Approximation
Analog
Integration
Digital
Correction
Logic
Over-Sampling
Modulator, Digital
Decimation Filter
Resolutio
n
Component
matching
typically limits
resolution to
8 bits
Componentmatchingr
equirementsdouble
withevery bitincrease
inresolution
Componentmatching
doesnot increasewith
increase inresolution
.
Componentmatchingre
quirementsdouble
withevery bitincrease
inresolution
Component matching
requirements double
with
every bit increase in
resolution
Size
Related
2N -1comparators,
Die size and
power
increases
exponentially
with
resolution
Die increaseslinearly
withincrease
inresolution
Core die size
will not
materially
change with
increase in
Resolution
Die increaseslinearly
withincrease
inresolution
Core die size will
notmaterially change
with increase in
resolution
Disadvan
tages
.
High power
consumption,
large size,
Expensive [14]
Speed limitedto
~5Msps.May
requireanti-
aliasingfilter[10]
SlowConversionrate.
Highprecisionexternalc
omponentsrequired
toachieveaccuracy
Parallelismincreasesthr
oughput atthe
expenseof power
andlatency
Higher order (4th order
or higher) and multi-bit
feedback
DAC require large area
XI. CONCLUSION
An ideal ADC has a great many bits for very fine
resolution, samples at lightning-fast speeds, and recovers
from steps instantly. It also, unfortunately, doesn’t exist
in the real world. Of course, any of these traits may be
improved through additional circuit complexity, either in
terms of increased component count and/or special
circuit designs made to run at higher clock speeds.
Different ADC technologies, though, have different
strengths [19]. Here is a summary of them ranked from
best to worst:
Resolution/complexity ratio:
Single-slope integrating, Dual-slope integrating,
Counter, Tracking, Successive approximation, Flash.
Speed:
Flash, Tracking, Successive approximation, Single-
Slope Integrating, Counter, Dual-slope integrating.
Step recovery:
Flash, Successive-approximation, Single-slope
integrating & counter, Dual-slope integrating, Tracking.
The rankings of these different ADC technologies
depend on other factors also. For instance, how an ADC
rates on step recovery depends on the nature of the step
change. A tracking ADC is equally slow to respond to
all step changes, whereas a single-slope or counter ADC
will register a high-to-low step change quicker than a
low-to-high step change. Successive-approximation
ADCs are almost equally fast at resolving any analog
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signal, but a tracking ADC will consistently beat a
successive-approximation ADC if the signal is changing
slower than one resolution step per clock pulse.
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International Journal of Advanced Engineering Research and Technology (IJAERT) Volume 3 Issue 3, March 2015, ISSN No.: 2348 – 8190
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International Journal of Advanced Engineering Research and Technology (IJAERT) Volume 3 Issue 3, March 2015, ISSN No.: 2348 – 8190
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