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Design Design of a Lock-in Amplifier with Current-Mode Oscillator
Abstract
The lock-in amplifier can be used in instrumentation systems as
a part of the conditioning circuit that optimizes and supports the
signals coming from sensors, being capable of measuring the
electrical impedance (or admittance) of such sensors, by exciting
them with a local oscillator. In industrial environments and
research laboratories, it is important to ensure that the level of
current passing through the sensor, usually called Device Under
Test (DUT), does not exceed the tolerance and safety limits: 10
mA to 30 mA. In this paper, we proposed the design of a lock-in
amplifier with a current-mode oscillator to ensure that the
current level is below the tolerance limits. A current-mode Wien
oscillator was used based on Current Conveyors, a current-mode
building block. Through simulations and experimental
measurements, we could check the operation of the designed
circuit, pointing out some comparisons with the traditional lockin
amplifier with voltage-mode oscillator.
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This book describes a variety of current feedback operational amplifier (CFOA) architectures and their applications in analog signal processing/generation. Coverage includes a comprehensive survey of commercially available, off-the-shelf integrated circuit CFOAs, as well as recent advances made on the design of CFOAs, including design innovations for bipolar and CMOS CFOAs. This book serves as a single-source reference to the topic, as well as a catalog of over 200 application circuits which would be useful not only for students, educators and researchers in apprising them about the recent developments in the area but would also serve as a comprehensive repertoire of useful circuits for practicing engineers who might be interested in choosing an appropriate CFOA-based topology for use in a given application. © Springer Science+Business Media New York 2013. All rights reserved.
Similar to phase-locked loops, frequency-locked loops (FLLs) are useful in many applications involving waveform synchronization or synthesis. Simple logic circuit-based relaxation oscillators convert capacitance to frequency, which is a characteristic inverse relationship between output frequency and input capacitance. The oscillator's logic level square-wave output can be fed into an all-digital FLL that will frequency lock to the input signal and produce a digital output word N, where N is inversely proportional to the input frequency. The result is that N is linearly proportional to the unknown capacitance in the oscillator. This novel approach allows a simple FFL implementation for capacitance measurement and is demonstrated in hardware using a capacitive sensor that measures the mass of small quantities of water with an output capacitance range of 75-185 pF.
The key specification points of lock-in amplifier systems for signal recovery and signal characteristics are reviewed and it is shown how these can be improved and modified by more advanced system design. The configurations of several commercially available systems are described and the facilities available in computer-controlled lock-in systems are discussed briefly, together with some new application areas.
It is shown that the voltage-mode oscillators employing noninverting or inverting voltage controlled voltage sources can be transformed directly to current-mode oscillators by element replacement technique and using composite current conveyors. Examples of the Wien oscillator and the phase shi ft oscillator together with PSpice simulation results are included.
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