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1,000,001 Things to Do with an Op Amp

June 12, 2018
Sponsored by Texas Instruments: Practically all linear signal chains today contain op amps and ADCs, and, therefore, most EEs have a strong knowledge of these devices. But it never hurts to brush up.

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If you’re designing a linear circuit of any kind today, it most likely involves an op amp and even an analog-to-digital converter (ADC). The op amp is your go-to device simply because it can be configured to perform almost any linear function. And with most applications being digital today, your design probably includes an ADC, either as an individual IC or integrated into your MCU.

If you have experience in linear design with these devices, your end product will be a success.
Such an outcome can be achieved by enhancing your knowledge with an input of fresh new knowledge about these ubiquitous devices. There are some excellent online ebooks that can provide you with all of the latest hints and tricks to optimize and bullet-proof your design.

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Op Amps Are the Answer

Op amps have been around for over 70 years. I first encountered them in an old Heathkit vacuum-tube analog computer we used in college. Then later in one of my first jobs, I was encouraged to use other vacuum-tube op amps that were on hand. These were the infamous K2Ws made by Philbrick. They worked great, but those ±300-V power supplies were just not appropriate for the truck-mounted mobile equipment I was making. I then introduced the company to the early solid-state op amps developed by Philbrick and Burr-Brown.

In the early 1970s, the first IC op amps came along. I used the popular 301 and 709. And later on, who didn’t use the 741? It’s still around today. Now, of course, we have superior CMOS op amps with mega-bandwidth. Furthermore, the specifications are so good that we often don’t have to worry so much about how to compensate for input offset voltages, bias currents, and other limitations.

Virtually all EEs learn about op amps in college. It’s one of those basic things that they do teach you in school. Most of us learned about the most popular circuits, such as the follower, inverter, the non-inverting amplifier, summer, and integrator—all still widely used.

But, as you know, hundreds or thousands more special circuits can be built with an op amp. In fact, at this point, most common basic linear circuit needs and applications have been discovered. And it’s likely that you do NOT know them all. The solution to that knowledge gap is a compendium of useful circuits that you can reference when designing your next linear project.

Signal-Chain Circuits Simplified

The op amp has been massively documented over the years. As mentioned, it’s covered in college texts and many other trade books for engineers. No doubt you have several, as do I. However, there’s probably no better source of information than that from an IC manufacturer, whose devices show up in a huge number of designs. Chip companies are the experts, thanks to their field engineers, datasheets, and application notes. Now they offer e-books.

The Analog Engineer’s Circuit Cookbook: Op Amps is an example. It compiles more than 25 popular op-amp circuit designs like current sensing, attenuating, and full-wave rectifying into a single e-book. Download the e-book for op-amp circuit ideas that you can easily adapt to your design needs.

Op Amp Quiz

Here’s a simple test of your basic op-amp knowledge. Refer to Figure 1, and answer these questions:

  1. What is the gain of this circuit?
  2. What is the input impedance of this circuit?
  3. Given the sine-wave input signal shown, what is the output?

You should probably know the answers, which can be found at the end of this article.

Welcome to the ADC

Like early op amps, ADCs were fussy devices. They suffered from low speed, noise, and linearity issues. Today these glitches have mostly been worked out, making ADCs almost as easy to use as an op amp. But you still need to know the rules and procedures to produce a clean, workable design. ADCs are also well-documented. Again, though, seeking the experience of ADC suppliers is the secret to creating a workable design the first time around.

A good starting point is The Analog Engineer’s Circuit Cookbook: ADCs, which compiles more than 15 popular ADC circuit designs like level translation, input drive circuits, and commonly used analog front-end (AFE) circuits. You can download the e-book for ADC circuit ideas that you can easily apply to your system needs.

ADC Quiz

Check your knowledge of ADCs with this simple quiz. Refer to Figure 2, and answer these questions:

  1. What is the minimum voltage that the ADC can resolve?
  2. What is the minimum sampling frequency for proper conversion?

If you passed both quizzes, congratulations. However, if you did not, then the e-books recommended here will give you a refresher.

Answers to Quiz Questions

Op Amp

1. A = Rf/Ri = 15k/4.7k = 3.19

2. ZIN = Ri = 4.7k

3. The output is a ±5-V rectangular wave. The input signal and gain are too high, and the output saturates at the limits of the supply voltages.

ADC

1. Minimum resolution = 5/1024 = 4.88 mV

2. 40 kHz; no less than twice the maximum input frequency (e.g., Nyquist)

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