In many industrial and consumer applications the measurement and control of physical conditions are very important. For example, measurements of temperature and humidity inside a diary or meat plant permit the operator to make necessary adjustments to maintain product quality. Similarly, precise temperature control of a plastic furnace is needed to produce a particular type of plastic.
Generally, a transducer is used at the measuring site to obtain the required information easily and safely. The transducer is a device that converts one form of Energy into another. For example, a strain gage when subjected to pressure or force undergoes a change in its resistance.
An Instrumentation system is used to measure the output signal produced by a transducer and often to control the physical signal producing it.
Figure shows the Block Diagram of an Instrumentation system.
The input stage is composed of a pre-amplifier and some sort of transducers, depending on the physical quantity to be measured. The output stage may use devices such as meters, oscilloscopes, charts or magnetic recorders.
The connecting lines between the blocks represents transmission lines, used especially when the transducer is at a remote test site monitoring hazardous conditions such as High temperatures or liquid levels of a flammable chemicals. These transmission lines permit signal transfer from unit to unit.
The signal source of the Instrumentation amplifier is the output of the transducer. Although some transducers produce outputs with sufficient strength to permit their use directly, many do not. To amplify the low level output signal of the transducer so that it can drive the indicator or display is the major function of an Instrumentation amplifier. In short, Instrumentation amplifier is intended for precise, low level signal amplification where low noise, low thermal and time drifts, high input resistance, and accurate closed loop gain are required. Besides, low power consumption, high common mode rejection ratio, and high slew rate are desirable for superior performance.
There are many Instrumentation operational amplifiers, such as the μA725, ICL7605 and LH0036, that make a circuit extremely stable and accurate. These ICs are, However, relatively expensive; they are very precise special purpose circuits in which most of the electrical parameters, such as offsets, drifts, and power consumption are minimised. Whereas input resistance, CMRR, and supply range are optimzed.
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