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Signal conditioning


Figure 1. An op amp, an in-amp, and a comparator.


It is just a triangle, or what does a symbol really mean?


Symbols are important, but what if the symbol can mean several things? In this article, Analog Devices’ Harry Holt, staff applications engineer, and Mike Skroch, applications engineer, explains how choosing the right op amp will result in a trouble-free, production-worthy design for years to come.


mean several things? This can lead to problems, as we shall see. In the


D


analogue world, a triangle can represent an op amp, a comparator, or an instrumentation amplifier. You could force one of them to do the function of one of the others, but system performance would not be optimum. Let us look at their differences and what to be cautious of so we can design around them if possible. As we shall see, there are some cases when you do not even want to try to design with the wrong type of part. Looking at Figure 1, which triangle is the op


amp? Which triangle is the comparator? And which triangle is the instrumentation amplifier? The answer is: They all are! So, what is the difference and why do we care? Looking at Table 1, we can see that there


oes a symbol help or hinder our thinking about a design? Symbols are important, but what if the symbol can


are some big differences in several characteristics, but what do they mean at the circuit and system level? Let us see how you can get into trouble…


FeeDbAck An op amp has a huge gain. We were told in engineering school to start the analysis with the difference between the two inputs equal to zero. But in real life, this cannott be true. If the open- loop gain is one million, then to get 5 V on the output, you would have to have 5 μV on the input. For a usable circuit, we need to apply feedback, so when the output tries to go too high, a control signal is fed back to the input, counteracting the original stimulus—for example, negative feedback. When used as a comparator, with no feedback, the output will slam against one rail or the other; with positive feedback, it will be driven farther in the same


direction. So, op amps need negative feedback. In fact, when some op amps are used as comparators with no feedback, the supply current can be five to 10 times higher than the max on the data sheet.1 For a comparator, however, positive feedback


is exactly what we need. With no feedback, if one input to a comparator slowly crosses the level of the other input, the output will slowly start to change. If there is noise in the system, such as ground bounce, the output may reverse, which is certainly undesirable in a control system. But then it starts changing back, resulting in oscillatory behavior, sometimes called chatter. For an instrumentation amplifier, the feedback


is already internal, so adding feedback just produces an inaccurate gain. A typical way to build an instrumentation amplifier with op amps is shown in Figure 2. Note: there is feedback around each individual


op amp. Let us begin by using the standard negative feedback diagram (see Figure 3) with the in-amp being G, with a desired gain of 10, implying a feedback factor of 0.1. Next, choose an in-amp fixed gain of 100. Using Equation 1, the actual closed-loop gain will be 9.09, almost a 10 per cent error. So, using an in-amp triangle as an op amp and putting feedback around it does not make sense. For an op amp, we really need negative


feedback; for a comparator, we really need positive feedback; and for an in-amp, we do not need any feedback.


Table 1. Comparisons of Op Amps, Comparators, and Instrumentation Amplifiers. 54 March 2021 Instrumentation Monthly


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