7 - Analogue processing applications

Abstract

This chapter discusses the transducers that change a quantity to be measured or processed into electrical energy. A transducer is the front end or sensing stage of many measuring or instrumentation systems. It is a device for changing one form of energy into another. Transducers that change their resistance when stimulated are common. The usual way of using a transducer is to place it in a bridge circuit, which is adjusted for balance (zero output voltage) when the transducer resistor is unstimulated. Subsequent stimulation by mechanical strain or the application of heat or light causes a change in the resistance of the transducer. This unbalances the bridge circuit, producing an electrical signal proportional to the applied stimulation. Signal conditioning applications sometimes call for the accurate measurement of an unknown resistance, which may be connected to an existing circuit. This in situ measurement is possible using an operational amplifier connection. A major problem encountered when attempting to measure small capacitances is the effect of the stray capacitance from the test point to earth. The problem can be alleviated by, once again, making use of the virtual earth property of the inverting operational amplifier. Signal conditioning frequently calls for the use of filter circuits or voltage waveform shaping, which may require the use of long CR time constants. Because large capacitances are bulky, heavy, and expensive, the long-time constants are preferably achieved by using a large resistance with a small capacitance. This solution is not successful in all applications. In digital signal conditioning, it is frequently a requirement for an analog voltage waveform to be sampled and then held in store awaiting conversion to coded digital pulses.