Department of Electronics engineeringSchool of Engineering and TechnologyPondicherry University.

Seminar on

Analog to Digital Converter Presented by:-ANIL KUMAR YADAVM.TECH(1ST YEAR)REG. NO- 13304025

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Contents

• What is ADC ?• Why ADC is needed?• Types of ADC• ADC Parameter Specification• Example• Application• Conclusion• References

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What is ADC ?

An electronic integrated circuit which transforms a signal from analog (continuous) to digital (discrete) form.

The basic principle of operation is to use the comparator principle to determine whether or not to turn on a particular bit of the binary number output

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Microprocessors can only perform complex processing on digitized signals.

•When signals are in digital form they are less susceptible to the deleterious effects of additive noise.

• ADC Provides a link between the analog world of transducers and the digital world of signal processing and data handling.

Why ADC is needed

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2 steps process• Sampling and Holding (S/H)•Quantizing and Encoding (Q/E)

ADC Process

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•The behavior of S/H is analogous to that of camera. its main function is “to capture picture” of the analog signal and hold its value until the adc can process the information.

•Holding signal benefits the accuracy of the A/D Conversion

•Minimum sampling rate should be at least twice the highest data frequency of the analog signal

Sampling and Holding

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• Quantizing: Partitioning the reference signal range into a number of discrete quanta, then matching the input signal to the correct quantum.

• Encoding:Assigning a unique digital code to each quantum, then allocating the digital code to the input signal.

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Speed: Rate of conversion of a single digital input to its analog equivalent.Conversion Rate

Depends on clock speed of input signalDepends on settling time of converter

1. Ramp or stair case or Counter type A/D converter2. Tracking A/D converter3. Successive Approximation A/D Converter4. Flash A/D Converter5. Delta-Sigma A/D Converter 6. Dual Slope or integrating type A/D Converter

Types of A/D Converters

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Counter type One of the simplest types of analog to digital converter is

counter type ADC. This type of converter uses some type of counter as part of its

operation Counter type contains the following elements:

Digital to analog converter Some type of counting mechanism Comparator clock

The input signal of ADC is connected to the signal input of its internal comparator.

The ADC then systematically increases the voltage of the reference input of the comparator until the reference becomes larger than the signal.

And the comparator output goes to 0 10

Operation of counter type

Control Logic

D A C Counter

START

Vin

Comparator

Digital Output

clock

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Operation of counter type

Control Logic

D A C Counter

START

Vin

Comparator

Digital Output

clock

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Continue

Ex: consider an input signal is 4.78 volts. The initial comparator’s input would be 2.5 volts

The comparator compares the two value then the result this is less than 4.78 then the next higher voltage (5.00 volts) is applied

The comparator compares the two value and says this is greater than 4.78 and switches 0

The digital output of the ADC is the number of times the ADC increase the voltage after starting at the initial 2.5 volts

This scheme is relatively simple , but as the number of ADC increases the time it takes to scan through all possible values lower than input will grow quickly

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The conversion time on the counter type is NOT fixed but depends on the actual value of the analogue input expressed as a fraction of the full scale.This can be expressed as :-

where N is the number of bits and T is the time period of the clock pulse

Example : A counter type ADC has the following parameters, N=8, Vref=5.1V and clock=1MHz. Find the digital word for an Vin of 4.36V and the conversion time taken to reach this value?solution Step size = 5.1v / 2^N = 5.1V / 256 = 0.0199=0.02The number of steps = 4.36 / 0.02 = 218.1=219 (219)10 = 110110112Conversion time = 219 x 1/1MHz = 219 x 1uS = 219 uS

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Features of counter type

Use a clock to index the counterUse DAC to generate analog signal to compare

against inputComparator is used to compare VIN and VDAC where

VIN is the signal to be digitizedThe input to the DAC is from the counter

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Track & Hold Logic

D A C

Up/Down Counter

Vin Comparator

Digital Output

clock

Tracking ADC - similar to the counter type except it uses an up/down counter and can track a varying signal more quickly.

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Fundamental Components (For n bit Flash A/D)2^n-1 Comparators2^n ResistorsControl Logic

Flash A/D ConverterFlash adc is fastest in all adc because flash type adc is using combinational logic (not sequential logic ). Therefore ,clock is not required ,in case of flash type adc.If propagation delay time of combinational circuit is zero, then ideal conversion time of adc is zero. But practical conversion time is sum of all propagation delay of combinational circuit involve in flash type adc.

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A resistive voltage divider (see figure) can provide all the digital reference states required. There are eight reference values for the 3-bit converter.

The analog signal is compared concurrently with each reference state; therefore a separate comparator is required for each comparison.

Digital logic then combines the several comparator outputs to determine the appropriate binary code to present.

The reference voltages are set to 0.5, 1.5, 2.5, 3.5, 4.5, 5.5, 6.5, and 7.5 volts respectively. The comparator outputs are labeled correspondingly as 1, 2, 3, 4, 5, 6, and 7 respectively.

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Uses the 2^n resistors to form a ladder voltage divider, whichdivides the reference voltage into 2^n equal intervals.

Uses the 2^n-1 comparators to determine in which of these 2^nvoltage intervals the input voltage Vin lies.

The Combinational logic then translates the informationprovided by the output of the comparators

This ADC does not require a clock so the conversion time isessentially set by the settling time of the comparators and thepropagation time of the combinational logic.

How does it work

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AdvantageVery Fast (Fastest)Very simple operational theorySpeed is only limited by gate and comparator propagation delay

DisadvantageExpensive Prone to produce glitches in the outputEach additional bit of resolution requires twice the comparators

Flash A/D Converter

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Main ComponentsResistorsCapacitor Comparators Control Logic DAC

SIGMA-DELTA A/D Converter

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input is over sampled, and goes to integrator. The integration is then compared to ground.Then o/p value of comparator passes through D Latch and produces a serial bit streamOutput is a serial bit stream 1’s ,proportional to VinWith this arrangement the sigma-delta modulator automatically adjusts itsoutput to ensure that the average error at the quantize output is zero.

The integrator value is the sum of all past values of the error, so wheneverthere is a non-zero error value the integrator value just keeps building untilthe error is once again forced to zero.

How does it work

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AdvantageHigh ResolutionNo need for precision Components

DisadvantageSlow due to oversamplingOnly good for low bandwidth

Sigma-Delta A/D Converter

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Uses a n-bit DAC to compare DAC and original analog results.Uses Successive Approximation Register (SAR) supplies an approximate digital code to DAC of Vin.Comparison changes digital output to bring it closer to the input value.Uses Closed-Loop Feedback Conversion

Successive Approximation ADC Circuit

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Process1. MSB initialized as 12. Convert digital value toanalog using DAC3. Compares guess toanalog input4. Is Vin>VDAC• Set bit 1• If no, bit is 0 and testnext bit

Successive Approximation ADC

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AdvantageCapable of high speed and reliableMedium accuracy compared to other ADCGood tradeoff between speed and costCapable of outputting the binary number in serial (one bit at a time) format.

DisadvantageHigher resolutionslowerSpeed limited to ~5Msps

Successive Approximation ADC

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Example:

Given data• in 10 bit ADC,Vin= 0.6 volts (from analog device),Vref=1 volts .Find the digital value of Vin?

SolutionN=2^n (N of possible states)N=1024Vmax-Vmin/N = 1 Volt/1024 =0.0009765625V of Vref (resolution)

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Continue………

MSB (bit 9)Divided Vref by 2Compare Vref /2 with VinIf Vin >Vref /2 , turn MSB on (1)If Vin < Vref /2 , turn MSB off (0)

Vin =0.6V and V=0.5 Since Vin>V, MSB = 1 (on)

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Next Calculate MSB-1 (bit 8) Compare Vin=0.6 V to V=Vref/2 + Vref/4= 0.5+0.25 =0.75V Since 0.6<0.75, MSB is turned off.

Calculate MSB-2 (bit 7) Go back to the last voltage that caused it to be turned on (Bit 9) and add it to Vref/8, and compare with Vin.Compare Vin with (0.5+Vref/8)=0.625 Since 0.6<0.625, MSB is turned off

Continue………

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Calculate the state of MSB-3 (bit 6) Go to the last bit that caused it to be turned on (in this case MSB-1) and add it to Vref/16, and compare it to Vin.Compare Vin to V= 0.5 + Vref/16= 0.5625 Since 0.6>0.5625, MSB-3=1 (turned on)

Continue………

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This process continues for all the remaining bits.

Continue………

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Fundamental components1. integrator2. Electronically Controlled Switches3. Counter4. Clock5. Control Logic6. Comparator

Dual Slope A/D Converter

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How Does it WorkAt t<0, S1 is set to ground, S2 is closed, and counter=0.At t=0 a conversion begins and S2 is open, and S1 is set so the input to the integrator is Vin.S1 is held for Tint which is a constant predetermined time interval. When S1 is set the counter begins to count clockpulses, the counter resets to zero after TintVout of integrator at t=Tint is Vin Tint/RC is linearly proportional to Vin.At t=Tint S1 is set at -Vref to the input of theintegrator which has the voltage Vin Tint/RC storedin it.The integrator voltage then drops linearly with aslop -Vref/RC. A compartor is used to determine when the output voltage of the integrator crosses zeroWhen it is zero the digitized output value is thestate of the counter.

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Advantage•Conversion result is insensitive to errors in the component values.• Fewer adverse affects from “noise”• High Accuracy

Disadvantages•Slow• Accuracy is dependent on the use of precision external components• Cost

Dual Slope A/D Converter

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Resolution

Dual-slope integrating, CounterTrackingsuccessive approximationFlash.

Speed

FlashTrackingSuccessive ApproximationCounterDual-slope integrating.

Comparison(best to worst)

Example ADC question:

• A 10-bit digital slope integrating A/D converter has a full-scale input of 10V. If the clock period is 15 μS, how long will it take to convert an input of 4V? How long for an input of 10V?

10 bits means 210 =1024 levels.

Full scale input of 10V means each level is 10V/1024=9.77mV

4V corresponds to 4/9.7710-3=409.6 - round up to 410

A clock period of 15μs mean 4V will take 15μs410 =6.15ms

10V will take 15μs1024=15.36ms

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Example ADC question:

• A 10-bit digital slope integrating A/D converter has a full-scale input of 10V. If the clock period is 15 μS, how long will it take to convert an input of 4V? How long for an input of 10V?

10V will take 15μs1024=15.36ms

• What increase in speed can be gained by using a 12-bit successive approximation converter instead of the digital slope converter, assuming a full-scale input voltage.?

• A 12-bit SA converter will take 12 clock cycles = 180 μs, regardless of the input voltage• so for 10V full scale input, the speed increase is 15.36ms/180 μs =85.3 times. • So the SA converter is both faster and more accurate (12 bits gives 4096 levels, compared to 1024 levels for 10 bit)

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Span (or Range): difference between maximum and minimum analog values. Span= maximum value – minimum value Some common spans: range of 0 V to 5 V: span = 5 V range of –12 V to 12 V: span = 24 V range of 4 mA to 20 mA: span = 16 mA

Offset: minimum analog value

Bit Weight: analog value corresponding to a bit in the digital number

Step Size (or Resolution): smallest analog change resulting from changing one bit in the digital number, or the analog difference between two consecutive digital numbers.Let AV be Analog Value; DN be Digital Number:

AV = DN × Step Size + Offset = (DN / 2n )× Span + OffsetDN = (AV - Offset) / Step Size = (AV - Offset) × 2n / Span

ADC Parameter Specification

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How closely can we approximate the desired output signal

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Example 1

o Full scale measurement range = 0 to 10 voltso ADC resolution is 12 bits = 4096 quantization levels (codes)o ADC voltage resolution is =(10V - 0V) / 4096 codes

= 10V /4096 codes =0.00244 volts/code

= 2.44 mV/code

• Example 2

o Full scale measurement range = -10 to +10 voltso ADC resolution is 14 bits: =16384 quantization levels (codes)o ADC voltage resolution is: =(10V - (-10V)) / 16384 codes

=20V / 16384 codes = 0.00122 volts/code = 1.22 mV/code

Quantization error occur due to the finite resolution N of the A/D converter limits the signal-to-noise ratio. All inputs within ±1/2 LSB of a code center resolve to that digital code. Thus, there will be a small difference between the code center and the actual input voltage due to this quantization. Mathematically, Qe=Vin-Vstaircase, where Vstaircase=D VQ ,VQ => Quantam volatge level

If assume that this error voltage is uncorrelated and distributed uniformly, we can calculate the expected rms value of this quantization noise.“

Quantum voltage level=

expectation value of the error voltage =

The rms value of a full-scale peak-to-peak amplitude VF is:

thus the signal-to-noise ratio is =

SNR= 6.02N + 1.76 dB

Quantization Error and Quantization Noise

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Quantization error voltage for ideal analog-to-digital converter.

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Dynamic range :is the ratio of the smallest possible output (the least significant bit or quantum voltage) to the largest possible output (full-scale voltage).

`Mathematically : DR =20 log10 2^N = 6N.Signal-to-noise-and-distortion ratio ( SNDR) : is the ratio of the input signal amplitude to the rms sum of all other spectral components.

SNDR =S/N+D

Spurious-free dynamic range (SFDR): is the ratio of the input signal to the peak spurious or peak harmonic component.Spurs can be created at harmonics of the input frequency due to nonlinear- ties in the A/D converter, or at sub harmonics of the sampling frequency due to mismatch or clock coupling in the circuit. The SFDR of an A/D converter can be larger than the SNDR.

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Total Harmonic Distortion:Total harmonic distortion (THD) is the ratio of the rms sum of the first 5 harmonic components to the input signal.

where V1 is the amplitude of the fundamental, and Vn is the amplitude of the n-th harmonic.

Aperture delay :Aperture delay is the delay from when the A/D converter is triggered (perhaps the rising edge of the sampling clock) to when it actually converts the input voltage into the appropriate digital code. Aperture delay is also sometimes called aperture time.

Transient Response:Transient response is the settling time for the A/D converter to full accuracy (to within ±1/2 LSB) after a step in input voltage from zero to full scale

Overvoltage Recovery:Overvoltage recovery is the settling time for the A/D converter to full accuracy after a step in input voltage from outside the full scale voltage (for example, from 1:5VF to 0:5VF )

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Aperture Jitter:Aperture jitter is the sample-to-sample variation in the aperture delay. The rms voltage error caused by rms aperture jitter decreases the overall signal-to-noise ratio, and is a significant limiting factor in the performance of high-speed A/D converters.

If we assume that the input waveform is a sinusoid ,then , VIN = VFS sin ᾡtthen the maximum slope of the input waveform is:

which occurs at the zero crossings.If there is an rms error in the time at which we sample (aperture jitter, ta) during this maximum slope. then ,there will be an rms voltage error of

Since the aperture time variations are randomthese voltage errors will behave like a random Noise source. Thus the signal-to-jitter-noise ratio :

Effects of aperture jitter.

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AccuracyAccuracy is the total error with which the A/D converter can convert a known voltage, including the effects of quantization error, gain error, offset error, and nonlinearities.There are two ways to best improve the accuracy of A/D conversion:• increasing the resolution which improves the accuracy in measuring the amplitude of the analog signal.•increasing the sampling rate which increases the maximum frequency that can be measured.

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Offset ErrorOffset error is the deviation in the A/D converter's behavior at zero. The first transition voltage should be 1/2 LSB above analog ground. Offset error is the deviation of the actual transition voltage from the ideal 1/2 LSB.Offset error is easily trimmed by calibration. Compare the location of the first transitions in Figures 1 and 2.

Gain ErrorGain error is the deviation in the slope of the line through the A/D converter's end points at zero and full scale from the ideal slope of 2^N/VFS codes-per-volt. Like offset error, gain error is easily corrected by calibration. Compare the slope of the dashed lines in Figures 1 and 2.

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Differential NonlinearityDifferential nonlinearity (DNL) is the deviation of the code transition widths from the ideal width of 1 LSB i.e. difference b/w the actual code width of nonideal converter and the ideal case.Mathematically, DNL=actual step width-ideal step width ideal step width=Vref/8=.625V=1 LSB All code widths in the ideal A/D converter are 1 LSB wide, so the DNL would be zero everywhere.

Integral NonlinearityIntegral nonlinearity (INL) is the distance of the code centers in the A/D converter characteristic from the ideal line. If all code centers land on the ideal line, the INL is zero everywhere. See the deviations of the code centers from the ideal line in Figure .

Missing CodesMissing codes are output digital codes that are not produced for any input voltage, usually due to large DNL.In some converters, missing codes can be caused by non-monotonicity of the internal D/A. The large DNL in Figure 3 causes code 100 to be “crowded out.”

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Fig . ADC characteristic, showing nonlinearity errors and a missing code. The dashed line is the ideal characteristic, and the dotted line is the best fit.

ADC are used virtually everywhere where an analog signal has to be processed, stored, or transported in digital form.

•Some examples of ADC usage are digital volt meters, cell phone, thermocouples, and digital oscilloscope.

•Microcontrollers commonly use 8, 10, 12, or 16 bit ADCs, our micro controller uses an 8 or 10 bit ADC.

Application of ADC

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Conclusion

• Adc is main component of all the modern digital electronics devices, there are different type of adc Ic’s available in market on the behalf of their requirement like speed , converter type etc.

• There are so many application in the area of communication , automatic devices etc.

• Hence we concluded that adc is the main element of digital devices.

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References

• C-mos Circuit Design, layout and simulation- By R.Jacob baker, chapter no. 28,29.• Fundamentals of Digital Circuits By - A. Anand Kumar• LINEAR INTEGRATED CIRCUIT By: D. ROY CHOUDHARY• http://elearning.vtu.ac.in• http://web.mit.edu/klund/www/papers/• http://www.freescale.com/files/microcontrollers/doc/app_note/AN2438.pdf

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VERY MUCH……………………….……………………………………………..……………………………………………..………

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Any quarries………………?

References