Transcript LECTURES 15 and 16

DATA ACQUSITION SYSTEMS
1.Analog Representation. In analog representation, one quantity is
represented by another one, which is proportional to the first. For example in
an automobile speedometer, the angular position of the needle represents the
value of auto's speed, and the needle follows the variations that occur as the
auto speeds up or slows down. Another example of an analog quantity
representation is an audio-microphone, in which an output voltage is
generated in proportion to the amplitude of the sound waves impinging the
microphone. The output voltage follows the variations that occur in the input
sound.
Analog quantities such as those cited above have an important
characteristic that they can vary over a continuous range of values. The auto's
speed can have any value between zero and, say, 150 kmph. Similarly, the
microphone output might be anywhere within a range of zero to 10 mV.
• 2. Digital Representation. In digital representation the quantities
are represented not by proportional quantities but by symbols, called
the digits. For example consider a digital watch, which indicates the
time of the day in the form of decimal digits representing hours and
minutes (sometimes seconds also). The digital watch reading does
not change continuously; rather, it changes in steps of one per
minute (or per second) while the time of the day changes
continuously. In other words, this digital representation of the time of
the day changes in discrete steps, in comparison to the
representation of the time provided by an analog watch, in which the
dial reading changes continuously.
• The major difference between analog and digital quantities can be
simply stated as below
• Analog = continuous
• Digital - discrete (in steps)
• ANALOG AND DIGITAL SYSTEMS
• An analog system contains devices that manipulate the physical
quantities represented in analog form. In an analog system, the
quantities can vary continuously over a range of values.. For
example, the amplitude of output signal to the speaker in a radio
receiver can have any value between zero and its maximum limit.
Other common analog systems are the magnetic tape recording and
playback equipment, automobile odometer, and the telephone
system.
• A digital system is a combination of devices designed for
manipulating physical quantities or information represented in digital
form, i.e. they can take only discrete values. Such devices are
mostly electronic, but they can also be mechanical, magnetic, or
pneumatic. Some of the familiar digital systems are calculators,
digital watches, digital computers, traffic-signal controllers, typewriters etc.
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Merits and Limitations of Digital Techniques
Merits
Digital systems are easier to design as the circuits employed are switching
circuits, where values of voltage or current are not important, only the range
(high or low), in which they fall, is important.
Storage of information is easier as it is accomplished by special switching
circuits that can latch into information and hold it for as long as required.
Greater accuracy and precision as digital systems can handle as many digits of
precision as needed simply by adding more switching circuits. In analog
systems, precision is usually limited to three or four digits because the values
of voltage and current directly depend on the values of circuit components.
Programmable operation as the digital systems can be easily designed for
operation controllable by a set of stored instructions called a program. Analog
systems can also be programmed, but the variety and complexity of the
available operations is severely limited.
Digital circuits are less affected by noise as spurious fluctuations in voltage
(noise) are not as critical in digital systems because the exact value of voltage
is not important, as long as the noise is not large enough to prevent
distinguishing a High from a LOW.
• Limitations. There is really only one major draw-back of
using digital technique and that is due to the fact that the
real world is mainly analog.
• Most physical quantities are analog is nature and these
quantities are often the inputs and outputs that are
monitored, operated on, and controlled by a system. We
are in the habit of expressing these quantities digitally,
such as when we say that the velocity is 5.2 m/s (5.21
m/s when we want to be more accurate); but we are
really making a digital approximation to an inherently
analog quantity
• Fig (1) Block Diagram of a Pressure
Control System
• The need for conversion between analog and digital
forms of information can be considered a drawback
because of additional complexity and expense. Another
factor that is often important is the extra time required for
performing these conversions. In many applications,
these factors are outweighed by the numerous
advantages of digital techniques, and so the conversion
between analog 'and digital quantities has become quite
common in the current technology.
• DIGITAL-TO-ANALOG
AND
ANALOG-TO-DIGITAL
CONVERSION
• Because most sensors have analog output while much data
processing is accomplished with digital computers, analog-to-digital
and digital-to-analog conversion obviously play an important role.
The process of changing an analog signal to an equivalent digital
signal is accomplished with the help of an analog-to-digital converter
(ADC). For example, an ADC is used to convert an analog signal
from a transducer, (measuring some physical quantity such as
temperature, pressure, position, rotational speed or, flow rate) into
an equivalent digital signal. An analog-to-digital converter (ADC) is
often referred to as an encoding device, as it is employed for
encoding signals for entry into a digital system.
• Digital-to-analog conversion involves translation of digital
information into equivalent analog information and this is
accomplished by the use of digital-to-analog converter (DAC). DACs
are used whenever the output of a digital circuit has to provide an
analog voltage or current to drive an analog device. As an example,
the output from a digital system might be converted into an analog
control signal for adjusting the motor speed or the furnace
temperature, or for controlling almost any physical variable.
Computers can be programmed to generate the analog signals
(through a DAC) required for testing analog circuitry. A digital-toanalog converter (DAC) is sometimes considered a decoding
device.
• Digital-to-analog (D/A) conversion is a straight forward process and
is considerably easier than analog-to-digital (A/D) conversion. In
fact, DACs are used as components in some ADCs. So we will
consider D/A conversion first
• Digital-To-Analog (DIA) Conversion. Basically, D/A
conversion is the process of taking a value represented
in digital code (such as simple binary or BCD) and
converting it into a voltage or current which is
proportional to the digital value. As already mentioned,
D/A conversion is accomplished by the use of digital-toanalog converter abbreviated as DIA converter or DAC
• The basic configuration of a simple DAC is shown in fig.
21.2. It consists of a precision resistor ladder network, a
reference precision voltage supply, logic inputs,
semiconductor switches and an operational amplifier
(op-amp). The inputs A,B,C,D,....H are binary inputs
which are assumed to have values of either 0 V(LOW) or
8 V (HIGH). When the input is HIGH, the switch closes
and connects a precision reference supply to the input
resistor and when the input in LOW the switch is open.
The reference supply produces a very stable, precise
voltage required for generating an accurate analog
output. The op-amp is used as a summing amplifier,
which produces the weighted sum of the binary inputs.
• In an 8-bit code input the switch A is actuated by most
significant bit (MSB) and the switch H is actuated by
least significant bit (LSB). If the input binary number is
10,000,000 then switch A is closed and others are open.
The output voltage which depends upon feedback
resistor, is equal to the reference voltage.
• It is known that the summing amplifier multiplies each
input voltage by the ratio of feedback resistor RF to the
corresponding input resistor RIN.
•
In this circuit RF = R (say of 1 k S2) and the input
resistors range from R to 2"-' R (i.e. to 8R, 128R, 2048 R
in case of 4-bit, 8-bit and 12-bit DAC) depending upon
the number of bits of the DAC as shown in fig (2).
Fig (2) DAC Circuitry •
• The A input has RIN = R and so the op-amp posses the voltage at A
with no attenuation i.e. the output voltage VouT is equal to the
reference voltage, VREF. The B input has RIN = 2R, so it will be
attenuated by half. Similarly the input C, Input D and input H will be
attenuated by 1/4, 1/8 and , respectively. The amplifier output can
thus be expressed as
1
1
1
1


VOU T   V A  V B  Vc  V D  ... n 1 V H 
2
4
8
2


• The -ve sign is present in the above expression because
the summing amplifier is an inverting amplifier, but it will
not concern us here.
• Clearly, the summing amplifier output is an analog
voltage, which represents a weighted sum of the digital
inputs, as shown by the Table ( ) , for a 4-bit DAC. This
Table lists all the possible input conditions and the
resultant amplifier output voltage. The output is
evaluated for any input condition by setting the
appropriate inputs to either 0 V or 8 V. For example, if
the digital input 1001, then VA = VD =8V and VB = Vc =
0 V. Thus
• Binary Ladder. Fig (3) A DAC using R-2R ladder network
with four input voltages, representing4-bits of digital data
and do voltage output is Illustrated in fig.
• The output current, IOUT depends on the positions of
the four switches, and the digital inputs Do, D,, D2, D3
control the states of the switches. The current is allowed
to flow through an op-amp current-to-voltage converter
to give Vo.
• Fig (3) DAC Using R-2R
• Ladder Network With Four Input Voltages and DC
Voltage Output
• The output voltage (analog), VOUT is proportional to the
digital input and is given by the expression
VOUT 
D0 x 2 0  D1 x 21  D2 x 2 2  D3 x 2 3
2
4
V REF
• For example, if the digital input is 1010 then output
voltage VOUT will be given by the expression
0 x11 x 2  0 x 4 1 x 8
10
VOUT 
VREF  VREF
16
16
• The function of the ladder network is to convert the 16 possible
binary values (from 0000 to 1111) into one of 16 voltage levels In
steps of V REF
16
Example . Determine the (1) resolution, (II) full-scale output and weight of
each Input bit for the DAC shown In fig. -Assume VREF=10V.
Determine also the full-scale output when the feedback resistor RF Is made onefourth of R.
• Solution: The MSB passes with unity gain, so its weight
in the output is equal to VREF i.e. 10 V. Ans.
VREF 10
•
The second MSB weight
= 2  2  5V Ans.
• The third MSB weight
=
VREF
10

 2.5V Ans.
4
4
• The fourth MSB (or LSB)
weight
=
VREF
10

 1.25V Ans.
8
8
• (ii) Full scale output = 10 + 5 + 2.5 + 1.25 = 18.75 V Ans.
• (i) The resolution of the DAC is equal to the weight of the
LSB i.e. 1.25V Ans.
If RF is reduced to one-fourth, each input
will be 4 times smaller than the values
above. Thus the full-scale output will be
reduced in the same ratio and becomes
18.75
 4.6875V
4
• Analog-To-Digital (AID) Conversion. The analog-todigital (A/D) conversion is the process of converting an
analog input voltage into an equivalent digital signal. The
operation is some what more complex and timeconsuming than the D/A conversion. A number of
different methods have been developed and used for
A/D conversion. Few of these will be described here.
• Successive-Approximation AID Conversion. This is
one of the most widely used method of A/D conversion.
Though it employs more complex circuitry than that used
by ramp A/D conversion but it has much shorter
conversion time. In addition, it has a fixed value of
conversion time that does not depend upon the value of
the analog input.
• This type of ADC makes direct comparison between an
unknown input signal and a reference signal. The basic
arrangement of a successive-approximation ADC shown
in fig. (5) is similar to the digital ramp ADC.
• This type of ADC, however, does not employ a counter to provide
the input to the DAC but employs a register instead. The DAC
provides a reference variable voltage in steps. The control logic
modifies the contents of the register bit by bit until the register data
are the digital equivalent of the analog input VA within the resolution
of the converter. Usually the measurement sequence selects the
largest step of the DAC output voltage first. The number of clock
pulses represents the digital output of the DAC.
• Successive-approximation ADC can be employed at conversion
speeds of upto about 1,00,000 samples per second at resolutions of
upto 16 bits (not including sign). At lower resolutions, speeds of over
2,50,000 samples per second are practical. Factors to be
considered in the design and applications of these ADCs include
stability and regulation of the reference voltage source, overload and
recovery characteristics of the comparator, characteristics of the
analog switches and speed and response of the ladder network.
• FIGURE (5) Successive-Approximation
ADC
• Voltage-To-Frequency AID Conversion. An analog
voltage can be converted into digital form, by producing
pulses whose frequency is proportional to the analog
voltage. These pulses are counted by a counter for fixed
duration and the reading of the counter will be
proportional to the frequency of the pulses, and hence,
to the analog voltage.
• A block diagram of a voltage-to-frequency ADC is given
in fig. (6). The analog input voltage VA is applied to an
integrator, which in turn produces a ramp signal whose
slope is proportional to the input voltage.
• When the output voltage Vo attains a certain value (a preset
threshold level), a trigger pulse is produced and also a current pulse
is generated which is used to discharge the integrator capacitor C.
Now a new ramp is initiated. The time between successive threshold
level crossings is inversely proportional to the slope of the ramp.
Since the ramp slope is proportional to the input analog voltage VA,
the frequency of the output pulses from the comparator is, therefore,
directly proportional to the input analog voltage. This output
frequency may be measured with the help of a digital frequency
counter.
• The above method provides measurement of the true average of the
input signal over the ramp duration, and so provides high
discrimination against noise present at the input. However, the
digitizing rates are slow because of long integration durations. The
accuracy of this method is comparable with the ramp type ADC, and
is limited by the stability of the integrator time constant, and the
stability and accuracy of the comparator.
• Fig (6) Voltage- To- Frequency AID
Conversion