Analog Instruments

Analog instruments are characterized by their continuous signals. A purely analog instrument measures, transmits, displays, and stores data in analog form. The signal processing is realized by analog components that are integrating together as functional blocks, as illustrated in Figure 

Some examples of functional blocks are bridges, amplifiers, filters, oscillators, modulators, offset circuits, level converters, and buffers. Three basic components are common in all types of analog signal processing:- resistors, capacitors, and inductors. The main function of a resistor is to limit the current. The current will flow through a capacitor only if the voltage changes across it. In the case of inductors, voltage is established only as a result of a change in the current across it. Other analog components, including semiconductor devices such as diodes, transistors, operational amplifiers, and rectifiers, are based on these three basic elements. Two basic types of semiconductor devices exist:- bipolar and metal oxide semiconductors (MOS). Many modern analog components and circuits are manufactured in integrated circuit (ICs). ICs contain all basic circuit components such as resistors, transistors, and diodes densely packed on silicon chips. The operational amplifier (op amp) is an example of an IC. Operational amplifiers are fundamental building blocks of analog signal processing circuits. They are made either as integrated (monolithic) circuits or hybrid circuits (combination of monolithic and discrete parts). An op amp is made from hundreds of transistors, resistors, and capacitors in a single chip. It has two inputs (inverting and noninverting) and a single output. A good operational amplifier has the following properties

• High input resistance, hundreds of megaohms or few gigaohms
• Low output resistance, less than few ohms or fraction of one ohm
• Low input offset voltage, a few millivolts or micro ohms
• Low input bias current, a few picoamps
• Very high open loop gain, 10^4 to 10^6
• Low intrinsic noise
• High common mode rejection ratio (CMRR)
• Low sensitivity to changes in power voltage
• A broad operating frequency range
• High environmental stability

Operational amplifiers can be configured as inverting or noninverting amplifiers. In addition, they can perform many other functions, for example as multipliers, adders, limiters, and filters. Another version of op amps is the instrumentation amplifiers, which are essentially high performance differential amplifiers that consist of several closed loops within the chip. Instrumentation amplifiers are used extensively in applications where sensor signals are extremely weak and noisy. An instrumentation amplifier has improved CMRR (up to 160 dB), high input impedances (e.g., 500 MΩ), low output impedance, low offset currents and voltages, and better temperature characteristics than common op amps do.