1. DATA ACQUSITION SYSTEMS

2. 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.

3. 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)

4. 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, type- writers etc.

5. 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.

6. 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

7. Fig (1) Block Diagram of a Pressure Control System

8. 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.

9. 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.

10. 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-to- analog 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

11. 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-to- analog converter abbreviated as DIA converter or DAC

12. 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.

13. 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).

14. Fig (2) DAC Circuitry

15. 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

16. 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

17. 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.

18. Fig (3) DAC Using R-2R Ladder Network With Four Input Voltages and DC Voltage Output

19. The output voltage (analog), VOUT is proportional to the digital input and is given by the expression For example, if the digital input is 1010 then output voltage VOUT will be given by the expression

20. 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 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 one- fourth of R.

21. Solution: The MSB passes with unity gain, so its weight in the output is equal to VREF i.e. 10 V. Ans. The second MSBweight = The third MSB weight = The fourth MSB (or LSB)weight = (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.

22. 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

23. Analog-To-Digital (AID) Conversion. The analog-to- digital (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 time- consuming 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.

24. 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.

25. 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.

26. FIGURE (5) Successive-Approximation ADC

27. 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.

28. 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.

29. Fig (6) Voltage- To- Frequency AID Conversion