June 15, 2019
June 15, 2019
June 19, 2019
Experimentation and Laboratory-Oriented Studies
Instrumentation amplifiers are a standard industrial building block found in a diverse range of applications ranging from strain gauge measurement systems for weight or pressure to electrocardiogram (ECG) or electroencephalogram (EEG) measurements. They are employed wherever a differential measurement of low-level signals is required, especially in the presence of large amounts of noise that is common to both signals. The underlying principles of this circuit are straightforward to understand and may be used to reinforce understanding of fundamental circuit laws as well as operational amplifiers. Instrumentation amplifier experiments are typically found in 2nd or 3rd-year laboratory experiments in electrical engineering as well as instrumentation courses in other disciplines, such as might be found in mechanical or biomedical engineering. By way of example, in “Fundamentals 3” at the UXX, the final project is an ECG instrumentation system. A key element of the project is the use of an instrumentation amplifier as the first stage of amplification and serves to amplify the millivolt level cardiac signals available at the wrists while rejecting much of the 60 HZ common mode interference present on the human body. In a typical laboratory sequence, the classic 3 op-amp configuration is analyzed and experiments conducted around constructing and testing the basic circuit. However, the primary function of the instrumentation amplifier, rejecting common mode signals while amplifying the difference between two voltages, is difficult to reproduce with conventional laboratory equipment. The function generators found in virtually all undergraduate educational laboratory stations provide a single output signal that is referenced to ground. This makes it challenging to measure two signals that are offset about a central reference voltage without requiring a separate function generator in which the outputs are isolated from the signal ground – an expensive proposition. This may lead to simulation-only scenarios for exploring the complete range of instrumentation amplifier applications. In this BYOE paper, we describe several inexpensive and straightforward techniques for designing compelling instrumentation amplifier experiments. They employ readily available parts and common student laboratory equipment allowing instructors to design instrumentation amplifier experiments in which small differential signals may be generated and have offsets added as a D.C. component that is several orders of magnitude larger than the A.C. signal to be amplified. We also describe how this experiment may be combined with other typical experiments using multivibrators and filters to produce a high-frequency common mode signal that may be added to the low-level differential signals to provide an extremely compelling experiment in common mode interference reduction. These additional experiments also provide instructors to add homework problems and analysis related to these circuits both from a time-domain perspective as well as frequency domain analysis of filters with non-sinusoidal inputs. We also present a design for a printed circuit board that snaps directly into a solderless breadboard and incorporates all of the above experimental techniques. This board is simple and inexpensive and provides a quick solution in cases where the instructor may not have laboratory time available for students to construct the complete suite of experiments, and wishes to concentrate only on the instrumentation amplifier. Plans and assembly instructions are available from the author.
Powell, H. C. (2019, June), BYOE: Simple Techniques for Visualizing Instrumentation Amplifier Operation Paper presented at 2019 ASEE Annual Conference & Exposition , Tampa, Florida. https://peer.asee.org/32492
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