Electronic Design

Op-Amp Tradeoffs

Selecting an op amp is different than selecting other components. And until someone designs the "ideal op amp," that's the way it will stay.

Supply levels have dropped and digital switching noise is on the rise. The response from high-performance analog IC manufacturers is to reduce the input-referred voltage and current noise of their amplifiers and add rail-to-rail inputs and output stages. Further performance increases can be realized by implementing these on advanced processes. By using clever topologies, designers can achieve excellent power, noise, and output drive performance even with the increased input common-mode voltage levels and output swing.

So with all of these new choices, what's the best way to navigate through the op-amp jungle?

  • A plethora of parameters. Most IC vendors have online parametric search tools, which have become a virtual necessity for op-amp selection in light of there being over 30 key parameters. But use these tools with caution because it's easy to over-prune and eliminate worthy choices. Start with the basic parameters that are most critical to your design, such as supply voltage, bandwidth, and offset. Only add further criteria if the list is overwhelming. Often, it can be helpful to manually skim a short table of parameters to see design tradeoffs made by the IC manufacturer.
  • Nonstandard standards. Frequently, parameters aren't specified using the same test conditions, even among parts from a single supplier. For example, is the distortion figure for second order, third order, or total harmonic distortion? Also, were all of the measurements taken at the same frequency, at the same supply voltage, and with the same load?
  • Unspecified parameters. Often, what's left off the data sheet will get you in trouble. What happens on the output of the amplifier if the inputs are driven beyond the supplies? What if the load becomes highly capacitive? Higher-quality IC vendors not only incorporate these features into their designs, they also test them during parts manufacturing.
  • Packages. You're attracted to that tiny leadless package, but are the specs the same as in that big SO-8 package? Do specifications change when going from the quad to the dual to the single version of the same part?
  • Needs and wants. It used to be that you could have speed but not dc precision. Or low noise, but not low power. While you may not find everything you want in a single device, it's still wise to start with the "non-negotiable" items and make sure they are guaranteed on the data sheet over the required temperature range. In a portable device, this may be supply current or package density. In instrumentation, it may be input offset voltage, bias current, or common-mode performance. If there are still multiple parts to choose from, you can fine-tune your search based on secondary criteria.
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