8.2 Signal Conditioning Circuits

Chapter 8.2 Signal Conditioning Circuits

Physics Lecture Notes – Phys 395 Electronics Book
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Physics Lecture Notes – Phys 395 Electronics Book

  • CHAPTER 8. DATA ACQUISITION AND PROCESS CONTROL158are: a radio antenna, a photo-diode, a phototube, a piezoelectric crystal, a thermocouple, aHall effect device, a mechanical switch, a strain gauge, an ionization chamber, etc..The output transducer transfers signals out of the electrical domain and into the do-main that can be perceived by one of the five human senses.A substantial amount ofpower is usually required to transfer information out of the electrical domain. Examplesof output transducers are the motor, cathode-ray tube, loudspeaker, light-emitting diode,radio-frequency transmitter, etc.8.2Signal Conditioning CircuitsSignal conditioning occurs in the interface between the transducers and the electrical circuit.A low-level signal amplifier and a low-pass filter are common signal conditioners after theinput transducer. The output signal is usually conditioned by a low-pass filter and sometype of power amplifier.8.2.1De-bouncing the Mechanical SwitchThe pushbutton or toggle switch is a simple form of data entry into a digital system. However,a problem occurs since the normal human reaction time is about 10−1 s and digital electronicsresponds to times of the order of 10−8 s. Thus any unnoticed mechanical contact bounce ofa few milliseconds will be seen as several distinct switch closures by a digital system. Wemay de-bounce the mechanical switch by using an RC circuit and Schmitt trigger logic ora flip-flop latch. The latter design requires a break-before-make action, which means thatduring the throw there is a time when the common is connected to neither terminal.8.2.2Op Amps for Gain, Offset and Function ModificationAn operational amplifier can be used to provide the following signal conditioning:1. increase the amplitude of the signal,2. filter the signal,3. decrease the signal output impedance or4. provide a variable gain and offset control.The latter is most useful for calibrating a transducer’s output signal.The dynamic range of a signal from an input transducer may be too large to processthrough the DAQ system (eg. the ADC is often the limiting factor). One can use a linearamplifier and choose to overflow or reduce the overall gain. The latter approach will causea loss in precision. Another approach is to use a nonlinear amplifier, such as one with alogarithmic gain, Vout =log(Vin).