CSL F Interfacing
Transcript of CSL F Interfacing
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INTERF CINGWITHTHE
N LOGUEWORLD
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INTRODUCTION
Digital quantity 0 or 1
LOW or HIGH
True or false
Analogue Quantity Any value
Its exact value is significant.
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INTERFACINGWITHTHEANALOGUEWORLD
Most physical variables are analog in nature.
Any information that is an input to a digitalsystem must first be put into digital form.
Similarly, the outputs from a digital system arealways in digital form.
When a digital system such as a computer is to beused to monitor and/or control a physical process,
we must deal with the difference between thedigital nature of the computer and the analognature of the process variables.
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ADCANDDAC
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TRANSDUCER
The physical variable is normally a non-electrical
quantity.
A transducer is a device that converts the physical
variable to an electrical variable. thermostats, photocells, photodiodes, flow meters,
pressure transducers and tachometers.
The electrical output of the transducer is an
analog current or voltage that is proportional to
the physical variable that it is monitoring.
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ANALOG-TO-DIGITALCONVERTER(ADC)
The transducers electrical analog outputserves as the analog input to the ADC.
The ADC converts this analog input to a
digital output.This digital output consists of a number of
bits that represent the value of the analoginput.
For example, the ADC might convert thetransducers 800- to 1500-mV analog valuesto binary values ranging from 01010000(80) to 10010110 (150).
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DIGITAL-TO-ANALOG-CONVERTER(DAC)
This digital output from the computer is
connected to a DAC, which converts it to a
proportional analog voltage or current.
For example, the computer might produce a
digital output ranging from 00000000 to
11111111, which the DAC converts to a voltage
ranging from 0 to 10 V.
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ACTUATOR
The analog signal from the DAC is often connectedto some device or circuit that serves as an actuator(convert electrical signal to motion) to control the
physical variable.
The ADCs and DACs function as interfacesbetween a completely digital system, such as a
computer, and the analog world.
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DIGITAL-TO-ANALOGCONVERSION
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DIGITAL-TO-ANALOGCONVERSION
Analog output = K x digital input K is the proportionality factor and is a constant value for
a given DAC connected to a fixed reference voltage.
The analog output can be a voltage or a current.
When it is a voltage, K will be in voltage units
When the output is a current, K will be in currentunits. 10
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EXAMPLE
A five-bit DAC has a current output.For a digital input of 10100, an output current of 10 mA
is produced.
What will be for a digital input of 11110?
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SOLUTION
10100 is equal to decimal 20.
Since Iout= 10 mA
The proportionality factor must be 0.5 mA.
11110 is equal to decimal 30
Thus, we can find for any digital input such as as follows:
Iout= 0.5 mA x 30 = 15.0 mA
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EXAMPLE
What is the largest value of output voltage from an
eight-bit DAC that produces 1.0 V for a digital input of
00110010?
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SOLUTION
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ANALOGOUTPUT
The number of different possible output valuescan be increase and the difference betweensuccessive values decreased by increasing thenumber of input bits.
Produce an output that is more and more like ananalog quantity that varies continuously over arange of value.
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DIGITAL-TO-ANALOGCONVERSION
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A B C D E F Vout
0 0 0 0 0 0 0
0 0 0 0 0 1
0 0 0 0 1 0
0 0 0 0 1 1
1 1 1 1 1 1 15
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DIGITAL-TO-ANALOGCONVERSION
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A B C D E F Vout
0 0 0 0 0 0 0
0 0 0 0 0 1 0.2381
0 0 0 0 1 0 0.4762
0 0 0 0 1 1 0.7143
1 1 1 1 1 1 15
1111112
= 6310
K = 15 / 63 = 0.2381
Vin = 1
Vout = 1 * 0.2381 = 0.2381
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INPUTWEIGHTS
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EXAMPLE
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SOLUTION
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RESOLUTION(STEPSIZE)
Resolution of a D/A converter is defined as thesmallest change that can occur in the analogoutput as a result of a change in the digitalinput.
The resolution is always equal to the weight ofthe LSBand is also referred to as the step size,since it is the amount that will changes as thedigital input value is changed from one step to
the next. In general, for an N-bit DAC the number of
different levels will be 2N, and the number ofsteps will be 2N1. 21
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RESOLUTION(STEPSIZE)
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4 bit input
16 levels
15 stepsResolution = step size
= weight of LSB
= 1
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OUTPUTWAVEFORMSOFADAC
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RESOLUTION(STEPSIZE)
Afs is the analog full-scale output
nis the number of bits.
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PERCENTAGERESOLUTION
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WHATDOESRESOLUTIONMEAN?
The DACs resolution determines how manypossible voltage values can be send to the output.
6-bit DAC63 possible steps of _______ V
between 0 and 10V.8-bit DAC255 possible steps of ______V
between 0 and 10 V.
The greater the number of bits, the finer the
resolution (the smaller the step size).
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BCD INPUTCODE
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EXAMPLE
If the weight of A0 is 0.2 V, find the
step size
Full size output
Percentage of resolution
Vout if D1C1B1A1= 0101, D0C0B0A0= 1000
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SOLUTION
Step size = 0.2 V
Full size output 0.2 * 99 = 19.8V
Percentage of resolution = 0.2/19.8 *100 = 1 %
Vout if D1C1B1A1= 0101, D0C0B0A0= 1000
Vout = 58 * 0.2 = 11.6V
OR Vout = (8) + (2) +(1.6)
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EXAMPLE
A 12 bit BCD DAC has a full scale output of 9.99V.
Determine the:
Percentage resolution
Step size
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SOLUTION
Step size = 9.99 / 999 = 0.01 V
Percentage resolution = 0.01 / 9.99 *100 = 0.1%
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D/A CONVERTERCIRCUITS
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DAC SPECIFICATIONS
Resolution
Accuracy
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RESOLUTION
The percentage resolution of a DAC is dependent
solely on the number of bits.
For this reason, manufacturers usually specify a
DAC resolution as the number of bits.
A 10-bit DAC has a finer (smaller) resolution
than an 8-bit DAC
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ACCURACY
DAC manufacturers have several ways of
specifying accuracy.
The two most common are
full-scaleerror
linearity error
which are normally expressed as a percentage of the
converters full-scale output (% F.S.).
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FULLSCALEERROR
Full-scale error is the maximum deviationof the DACs output from its expected (ideal)value, expressed as a percentage of fullscale.
For example, If a DAC has an accuracy of0.01% F.S and a full-scale output of9.375V, this percentage converts to
0.01% x 9.375 V =
0.9375 mV
This means that the output of this DAC can,at any time, be off by as much as 0.9375 mVfrom its expected value. 36
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EXAMPLE
An 8 bit DAC has a full scale output of 2mAand full scale error of 0.5% F.S. What isthe range of possible outputs for an input of10000000?
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SOLUTION
An 8 bit DAC has a full scale output of 2mAand full scale error of 0.5% F.S. What isthe range of possible outputs for an input of10000000?
Step size = 2m/255 = 7.84A
100000002 = 128
The ideal output = 7.84A *128 = 1004A
The error = 0.5% * 2mA = 10 AThe range : 994 A - 1014 A
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LINEARITYERROR
Linearity error is the maximum deviation in step
size from the ideal step size.
For example, If the DAC has an expected step
size of 0.625 V, a full scale value of 9.375 V and a
linearity error of 0.01% F.S., this would meanthat the actual step size could be off by as much
as 0.9375 mV.
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OFFSETERROR
Ideally, the output of a DAC will be zero voltswhen the binary input is all 0s.
In practice, however, there will be a very smalloutput voltage for this situation; this is called
offset error. This offset error, if not corrected, will be added to
the expected DAC output for all input cases.
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OFFSETERROR(CONT)
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Input
code
Ideal
Output
mV)
Actual
Output
mV)
0000 0 2
0001 100 102
1000 800 802
1111 1500 1502
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OFFSETERROR(CONT)
Offset error can be negative as well as
positive.
Many DACs will have an external offset
adjustment that allows you to zero theoffset.
This is usually accomplished by applying
all 0s to the DAC input and monitoring
the output while an offset adjustmentpotentiometeris adjusted until the output
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SETTLINGTIME
The operating speed of a DAC is usually specifiedby giving its settling time, which is the timerequired for the DAC output to go from zero tofull scale as the binary input is changed from all0s to all 1s.
Actually, the settling time is measured as thetime for the DAC output to settle within step size (resolution) of its final value.
For example, if a DAC has a resolution of 10 mV,settling time is measured as the time it takes theoutput to settle within 5 mV of its full-scalevalue.
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ANALOG-TO-DIGITALCONVERTER(ADC)
AnADC takes an analog input voltage
and after a certain amount of time
produces a digital output code that
represents the analog input.The A/D conversion process is generally
more complex and time-consuming than
the D/A process, and many different
methods have been developed and used.
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GENERALBLOCKDIAGRAM
FORADC
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ADC DESCRIPTION
The timing for the operation is provided by the
input clock signal.
The control unit contains the logic circuitry for
generating the proper sequence of operations inresponse to the START COMMAND, which is
initiates the conversion process.
The op-amp comparator has two analog inputs
and a digital output that switches states,
depending on which analog input is greater.
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ADC OPERATION
1.The START COMMAND pulse initiates the operation.
2. At a rate determined by the clock, the control unitcontinually modified that binary number that is stored in
the register.3.The binary number in the register is converted to an
analog voltage, , by the DAC.
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ADC OPERATION(CONT)
4. The comparator compares VAXwith the analoginput VA. As long VAX< VAthe comparatoroutput stays HIGH. When VAXexceeds VA by at
least an amount equal to VT(threshold voltage),the comparator output goes LOW. At this point,VAXis close toVA. The digital number in theregister, which is the digital equivalent of VAX,
is also the approximate digital equivalent of VA,within the resolution and accuracy of the system
5. The control logic activates the end-of-conversion signal, EOC, when the conversion is
complete.
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DIGITALRAMPADC
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ADC OPERATION
Start High
Resets the counter to 0Inhibits clock pulses from
passing to the counter.
DAC output = 0
VA > VAX
End of conversion (EOC) = 1Start LowClock pulses through to counter
VAX increases one step at a time
VAX > VAEOC = 0Counter stop counting
VADigital representation of the countercontents
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4. The comparator compares VAXwith the analoginput VA. As long VAX< VAthe comparatoroutput stays HIGH. When VAXexceeds VA by at
least an amount equal to VT(threshold voltage),the comparator output goes LOW. At this point,VAXis close toVA. The digital number in theregister, which is the digital equivalent of VAX,
is also the approximate digital equivalent of VA,within the resolution and accuracy of the system
5. The control logic activates the end-of-conversion signal, EOC, when the conversion is
complete.
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ANYQUESTION?
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