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Draw block diagram of Data Acquisition System (DAS). Write function of each block.

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Block Diagram: 

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Functions: 

A data acquisition (DAQ) system is used for the measurement and processing of plant Signal data before it is displayed on the operator desk or permanently recorded. Block diagram of a PC (computer) based data acquisition is shown in figure. It consists of individual transducers (sensors) for measurement of physical plant Parameters (such as temperature, pressure, flow, etc.). After measurement, the transducer data is fed to the signal conditioning device to bring the signal level up to a sufficient value to make it useful for conversion, processing, indicating and recording. Signal conditioner is used to amplify, modify or select certain portion of signals. The output of the signal conditioner is fed to the multiplexing (telemetry) device With the help of multiplexing all individual signal data (called lower bandwidth Communication channels) are combined and transmitted over a higher bandwidth channel. At the receiving end, de-multiplexing recovers the original lower bandwidth channels. It scans across a number of analog signals and timesharing them sequentially into a single analog output channel. The multiplexed data is converted into digital signal with the help of analog-toDigital converter. The converted digital signals are fed to the computer for further processing, Mathematical computation, storage, etc. The final and processed data is either displayed on electronic digital display panel or recorded on magnetic media And/or chart recorders. 

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Description: Fig. above shows the block diagram of generalized Data Acquisition System (DAS). It Consists of transducers, signal conditioners, multiplexer, A/D converter and output devices such as display, recorder and printer. Transducers are used for translating physical parameters into electrical signals. The Output of the transducer is given to signal conditioning circuit where the signal is modified to the required level of A/D converter. The analog multiplexer selects the inputs sequentially, one at a time. These are further converted to digital signals by the A/D converter. The processed data is further given for display/ transmission/ printing/recording/computer processing. 

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Block Description: Basic elements required for Data acquisition System are (1) Transducer. (2) Amplifiers or signal conditioners. (3) Multiplexers. (4) Sample and Hold circuit . (5) Analog to Digital converters.

(1)Transducer It is desirable that an emf obtained from transducer is proportional to quantity being measured, is used as input to the data acquisition system thus transducers such as thermocouple, strain gauges and piezoelectric devices etc. are used. An exception to this usual function of transducer ,some sensor produces frequency which can be counted with electronic counter to obtain integral of measured quantity.

(2) Amplifiers or signal conditioning EquipmentSignal conditioning Equipment includes any equipment that assists in transforming the output of transducer to the desired magnitude or form required by next stage of the DAS. It produces the required conditions in the transducers so that they work properly. Signal conditioners may include devices for amplifying, refining, or selecting certain positions of these signals. Examples of signal conditioning equipment include known constant voltage sources for strain gauge bridges, zero bridge balance devices for strain gauge circuits, temperature control devices for thermocouple junctions, voltage amplifiers and servo-systems.

(3)Multiplexers. Multiplexing is the process of sharing a single channel with more than one output. The multiplexer accepts multiple analog inputs and connects them sequentially to one measuring input. Multiplexing is a means of using the same transmission channel for transmitting more than one quantity. Multiplexing becomes necessary in measurement systems when the distance between transmitting and receiving point is large and many quantities are to be transmitted. If a separate channel is used for each quantity, the cost of installation, maintenance, and periodic replacement becomes prohibitively large and therefore a single channel is used which is shared by the various quantities. Multiplexing is commonly accomplished by either time or frequency sharing of the transmission channel between the individual quantities.

(4)A/D Converters A/D converter based on dual slope techniques are useful of low frequency data, such as from thermocouples or analog devices.

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