Selecting an op amp

You could start with a similar op amp from a similar, known-good circuit or reference design and extrapolate from there. However, not everything labeled a reference design has actually been built and tested. But there’s another way to get organized and feel in control of the selection process.

A methodical approach is outlined below that guides you on what you need to know to get a grip on the design requirements and narrow down your selection. Many manufacturer’s sites have interactive selection guides with many parameters. It can get confusing when, after narrowing the selection down, you don’t know which op amp is best for your design.

There are some basic questions you should ask before you begin looking for a suitable op amp.

1. What is the input signal going to look like?
   1. Current-input or voltage-input?
   2. What is the expected operating frequency range? Maximum range?
   3. What amplitude is needed? (Typical and maximum values.)
   4. What’s the impedance of the circuit it’s going into?
2. What is an acceptable output signal going to look like?
   1. What is the expected range of frequencies the output signal might cover?
   2. What is the expected amplitude range?
   3. Will the op amp be driving another device? If so, how much power will be needed?
3. How accurate or precise does the op amp need to be?
4. The operating environment:
   1. What supply voltage(s) are available?
   2. Is there a physical size limitation? You may need to make a list of packages of an acceptable size.
   3. What is your operating temperature range? Figure out a Max, Min, and Typical. Look at how the temperature affects your most critical parameters using the graphs in the datasheet. If the information you need is missing, you can contact the company or set it aside and move on to another spec that is more thorough.
5. Buying:
   1. Are you restricted to certain manufacturers that your company deals with?
   2. Will you need to second source the op amp?
   3. What is the lifecycle of the op amp? Do not select any op amp that is Not Recommended for New Design (NRND), End of Life (EOL), or otherwise a special factory order (this might mean that it’s about to go EOL).
   4. Price might be a specification of a sort, but this should be one of the last parameters you look at when you are deciding between otherwise identical op amps.
6. Other:
   1. When selecting parameters, it’s good to allow a margin of error on the specifications. Not every op amp will be precisely the values as listed, and op amp values change with temperature, age, and stress.
   2. Make sure the finalists in your part selection are actually for sale. “Vapor-ware” is when a manufacturer announces a part to be released in the near future (e.g., “early in Q2”), but some parts have been known as “about to release” for a year or more, depending upon the manufacturer. That’s why you second source your product, and why you confirm the product’s lifecycle prior to finalizing.
   3. Finally, if you get stuck on something, you can talk to colleagues, search the web and/or post on forums to get some pointers, or call the manufacturer’s or distributor’s tech line to clarify issues.

The selection process is a decision between a succession of trade-offs. Determine which parameters are most critical to design success. However, maintain a loose grip on the specifications that can slide around a bit. If you over-specify a device before you start, you will have a narrow list of op amps to select from.

Once you have the big picture in mind, you can start looking at websites. Many manufacturers have block diagrams that give a systems-level view of several common types of end equipment or applications. For example, if you are designing a data acquisition card, you will find block diagrams for this type of device on semiconductor manufacturer’s sites listed by application. You might find that there’s a type of amplifier that you didn’t think about, such as a programmable gain amplifier. Some distributor websites have application block diagrams, as well. These are the best sites because the selection of components spans several manufacturers. Application block diagrams may have some good part suggestions but should be used as a starting point. Not all sites will have your application, either.

The next step is to start looking for op amps using a parametric search engine with your primary parameters in mind. Once you have narrowed your selection down, you will have eliminated op amps that aren’t as close to your non-critical specifications as others. Again, using a distributor’s site will provide a wider cross-selection. Manufacturers tend to specialize in a few areas, such as audio, RF, or instrumentation amplifiers. Save your final list and then head to the manufacturer’s sites to see if additional parts are available that the distributor didn’t happen to carry.