Merits AND Demerits OF Digital Instruments OVER Analog ONES PDF

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Comparison of digital and Analog instrument...

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MERITS AND DEMERITS OF DIGITAL INSTRUMENTS OVER ANALOG ONES Although electronics are usually more costly than electrical instruments but are becoming more and more popular because of their various advantages over conventional ones, some of the main advantages are discussed below: 1. Detection of Low Level Signals: As analog instruments use PMMC movement for indication. This movement cannot be constructed with a full scale sensitivity of less than 50 mA. Any measurement using a PMMC movement must draw a current of 50 mA from the measured quantity for its operation for full scale deflection if conventional voltmeters are used. This would produce great loading effects especially in electronic and communication circuits. Electronic voltmeter avoids the loading errors by supplying the power required for measurement by using external circuits like amplifiers. The amplifiers not only supply power for the operation but make it possible for low level signals, which produce a current less than 50 mA for full scale deflection, to be detected which otherwise cannot be detected in the absence of amplifiers. Let us examine the loading effect. A typical meter has a resistance of 200Ω and its operating current at full scale is 50 mA. This means the power consumed is only (50 *10 –6)2 * 200 = 0.5 mW. This is extremely low power for the power circuits but not for many electronic and communication circuits. If this power is taken from the measurand, the signal gets greatly distorted in case power level the circuit is very small and to offset this power is supplied from outside through use of amplifiers. Now in case of lower ranges of voltages have to be measured, the use of a voltage amplifier becomes absolutely necessary. This is only possible through use of electronic voltmeters which allow the use of an amplifier. Therefore, it is possible to measure currents below 50 mA, voltages below 10 mV and keep drawn the power drainage below 0.5 mW by using electronic voltmeters through use of amplifiers which is otherwise not possible with conventional types of meter using PMMC movement. For the case of ac measurements, the use of an amplifier for detection of low level signals is even more necessary for sensitive measurements. 2. High Input Impedance A conventional PMMC voltmeter is a rugged and an accurate instrument, but it suffers from certain disadvantages. The principle problem is that it lacks both high sensitivity and high input resistance. It has a sensitivity of 20 kΩ/V with a 0 – 0.5 V range and has an input resistance of only 10 kΩ at its 0.5 V range with the result it has a full scale current of 50 mA which loads the measurand considerably. In electronics and communication circuits even this low value of current may not be tolerable on account of the fact that these circuits have very low operating currents. The electronic voltmeter (EVM), on the other hand, can have input resistances from 10 MΩ to 100 MΩ with the input resistance remaining constant over all ranges instead of being different at different ranges, the EVM gives for less loading effects. 3. Low Power Consumption: Electronic voltmeters utilize the amplifying properties of vacuum tubes and transistors and therefore the power required for operating the instrument can be supplied from an auxiliary source. Thus, while the circuit whose voltage is being measured controls the sensing element of the voltmeter, the power drawn from the circuit under measurement is very small or even negligible. This can be interpreted as the voltmeter circuit

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has very high input impedance. This feature of electronic voltmeter is indispensable for voltage measurement in many high impedance circuits such as encountered in communicating equipments. High Frequency Range: The most important feature of electronic voltmeters is that their response can be made practically independent of frequency within extremely wide limits. Some electronic voltmeters permit the measurement of voltage from direct current to frequency of the order of hundreds of MHz. the high frequency range may also be attributed to low input capacitance of most electronics devices. The capacitance may be of the order of a few pF. Better Resolution: Resolution (smallest reading perceivable) of analog instruments is limited by space on the scale markings and also by ability of the human operator to read such small deviations in scale markings. Whereas in a digital instrument, the measured value is displayed directly on a LED or LCD panel whose resolution is solely determined by resolution of the analog to digital converter (ADC). Use of 12 bit (or higher) ADC can make a digital instrument to read as small as 0.001 V in 0 – 5 V range. Storage Facility: Digital instruments have an additional optional advantage that their readings can be stored for future reference. Since the value displayed is obtained through an ADC, the digital data can be easily stored in a microprocessor or PC memory. Such storage facility can only be made available in analog instruments by the use of chart recorders where the pointer has a ink source that keeps on marking the values on a roll of moving paper. Accuracy: Since there are very few moving parts (or even no moving parts) in the digital instruments, in general they are usually more accurate than the analog instruments. Even the human error involved in reading these instruments is very less, which adds to the accuracy of digital instruments. However, overall accuracy of a digital instrument will largely depend on accuracies of the large number of individual electronic components used for building the instrument. In addition, digital instruments are more user friendly as they are easy to read, takes up smaller space, suitable for mass production, and also sometimes less costly.

Disadvantages of Digital Instruments 1. Effects on noise in more predominant on digital instruments than analog instruments. Analog instruments, due to inertia of its moving parts, normally remain insensitive to fast varying noise, while digital instruments continue to show erratic variations in presence of noise. 2. Analog instruments have higher overload capacity than digital instruments. The sensitive electronic components used in digital instruments are more prone to damage in case of even momentary overloads. 3. Digital instruments can sometimes loose its reliability and tend to indicate erratic values due to faulty electronic circuit components or damaged display. 4. Digital instruments and their internal electronic components are very much sensitive to external atmospheric conditions. In case of high humidity and corrosive atmosphere the internal parts may get damaged and indicate the faulty values....