14 Chapter 7 Analog Interfacing
or relative pressure to within 1% accuracy. These marvelous devices contain the diaphragm, strain gauge position sensor, compensation circuits for temperature, and output amplifier on a hybrid integrated circuit. Finally, weight is normally measured by he force that gravity generates. The weighing device, called a load cell, is essentially a piston. Objects are weighed by putting them on top of the piston, and the pressure of he fluid inside the piston is measured.
Other properties - including chemical composition and concentration, the pHof iquids, and so on - are sometimes measured by transducers. However, a discussionof hese transducers goes beyond the scope of this introductory survey.
7.2 Basic Analog Processing Components
Basic analog devices include power amplifiers, operational amplifiers, analog switches, and the timer module. These will be discussed in this section. The first subsection discusses transistors and SCRs, the next discusses OP AMPs and analog switches in general, and the last discusses practical OP AMPs and analog switches.
7.2.1 Transistors and Silicon Controlled Rectifiers
To convert a voltage or current to some other property like position or temperature, an amplifier is needed to provide enough power to run a motor or a heater. We briefly urvey the common power amplifier devices often used with microcomputers. These nclude power transistors, darlington transistors, and VFETs for control of DC devices; motors, heaters, and so on) and SCRs and triacs for control of AC devices.
The (bipolar) transistor is a device that has terminals called the collector, base, and mitter. (See Figure 7.3a.) The collector current Ic is a constant (called the beta) times he base current Ib. The power transistor can be obtained in various capacities, able to handle up to 100 A, and up to 1000 V. These are most commonly used for control of DC devices. A darlington transistor has a pair of simple transistors connected internally o it appears to be a single transistor with very high beta. (See Figure 7.3b.) Power darlington transistors require less base current Ib to drive a given load, so they are often sed with microprocessor I/O chips that have limited current output. Field-effect ransistors (FETs) can be used in place of the more conventional (bipolar) transistor. In n FET, the current flowing from drain to source is proportional to the voltage from
gate to source. Very little current flows into the gate, which is essentially |
a capacitor |
with a small leakage current. (See Figure 7.3c.) However, a vertical |
field-effect |
ransistor (VFET) is faster than a standard FET and can withstand larger voltages (about 00 V) between drain and source. The VFET is therefore a superb output amplifier that s most compatible with microcomputers. Suffice to say that for this survey, a power ransistor, a darlington, or a VFET is usually required to drive a DC device like a motor, eater, or lamp.
