Chapter 6 – Instrumentation and measurement

Publisher Summary The operational amplifier (op-amp) is an amplifier with two input terminals—inverting and noninverting. A neural network is a system of interconnected processing elements called neurons or nodes. A neural network performs parallel and distributed information processing that is learned from examples, and it is used for complex bioimpedance signal processing. A variety of bridges is used for immittance measurements for a long time. The advantage of bridges is their high-resolution capabilities, a feature very important for extending dielectric measurements to low frequencies in tissue. Lock-in amplifiers are commonly used to detect minute signals buried in noise. This is accomplished if the signal of interest appears as an amplitude modulation on a reference frequency. The heart of the analogue lock-in amplifier is the synchronous rectifier that includes a phase-sensitive detector (PSD) and a low-pass filter. There are several different kinds of automated instruments for impedance measurements, which are divided into two general categories: LCR-meters and impedance analyzers. LCR-meters use the auto-balancing bridge technique to measure the impedance of materials. Impedance analyzers are also called frequency response analyzers and are in most cases combined instruments that employ two or more different techniques to cover a larger frequency range.