My Head is Spinning: Why So Many Op Amps?

Choosing the right or “best” operational amplifier (op amp) for a project can be daunting and overwhelming. Even if you restrict your search to a single vendor, there are likely tens of fairly similar devices to consider, often with new ones released as well. Vendor selection guides can help with broad categories (such as high speed, precision, high voltage) but even these divisions overlap and have ambiguities.

So, why so many op amps? A cynic might say, “It’s because they can,” but that’s really not the reason. Each op amp variation and sub-variation can be costly, requiring changes in one or more factors of design, fabrication, test, qualification, production planning, order fulfillment, packaging, and more.

In contrast, an academic would say the answer to the question is obvious: “It’s because there’s no perfect op amp.” While that is technically correct, it’s not the reason either. In fact, you might not want the ideal op amp, with infinite bandwidth, zero noise, and no other “imperfections” as it is too much of a good thing. To use it in your application, for example, you might need to add an external filter to attenuate external noise from impacting the op amp, rather than relying on the limited bandwidth of the device itself.

The reason that there are so many op amps actually is a consequence of two factors. First, there’s the sheer diversity of applications. Second, there’s the eternal engineering issue of tradeoffs. In the case of op amps – as with so many other components – these tradeoffs are not simple yes/no questions but have subtleties and nuances of degree and priority.

Some applications will tolerate less desirable values for some parameters in order to get truly superior performance in the one or two that really matter in that situation. For example, a precision instrumentation circuit may really, truly need low offset drift over a wide temperature range and be willing to accept some additional dissipation to achieve that goal. Still, the question is always, “How much are you willing to give up elsewhere to meet your primary objective?” If you can get 10% better offset drift performance but at the cost of a 50% increase in some secondary specifications, is it worth it?

Of course, then there’s cost factor: although almost all applications are cost-sensitive, the issue is to what extent is cost a critical factor. If spending a few more cents gets a device with 10% lower noise, is it worth it? The answer is not in an academic textbook, that’s for sure.

Consider two “zero-offset” op amps: the Microchip Technology MCP6V51 and the Texas Instruments OPA735. In addition to other differences, the Microchip device features initial maximum offset of ±15 microvolts (µV) and maximum offset drift of ±36 nanovolts (nV)/°C, (Figure 1). The Texas Instruments part has one-third the initial maximum offset at ±5 µV but about 50% higher maximum offset drift at ±50 nV/°C (Figure 2). So, which one is better?

Figure 1: The input offset voltage vs. ambient temperature is a critical specification in precision op amp applications. Here, it is shown for the Microchip Technology MCP6V51. (Image source: Microchip Technology)

Figure 2: The offset voltage drift is presented in a different way for the OPA735, but is clearly just a few nV/°C. (Image source: Texas Instruments)

The answer is deceptively simple, as it is in many engineering situations: “It depends.” In this case, it depends on how critical that initial offset value is when compared to its drift value, but it may be valid only for one specific application.

The decision on what and how much to give up in order to get what you want is a careful interplay of many factors and judgement, and that’s at the heart of the engineering challenge. It’s often a difficult decision since everyone at the design review may have a legitimately different point of view.

There are countless sets of different application priorities, relative weightings, and the “what you give up to get what you want” decisions that can be made. The good news is that the wide range of choice facilitates finding a very good fit (in most cases). Still, the multiplicity of choices can be overwhelming, and that leads to two possibilities: the designers may just choose the first device that comes close enough, or will just go with a vendor and device used previously and with which they are most comfortable.

Ironically, despite the many op amps already available and constant flow of new ones, many designers, rightly or wrongly, end up going with the one they’re most familiar.

关于此作者

Image of Bill Schweber

Bill Schweber 是一名电子工程师,撰写了三本关于电子通信系统的教科书,以及数百篇技术文章、意见专栏和产品特性说明。他担任过 EE Times 的多个特定主题网站的技术管理员,以及 EDN 的执行编辑和模拟技术编辑。

在 Analog Devices, Inc.(模拟和混合信号 IC 的领先供应商)工作期间,Bill 从事营销传播(公共关系),对技术公关职能的两个方面均很熟悉,即向媒体展示公司产品、业务事例并发布消息,同时接收此类信息。

担任 Analog 营销传播职位之前,Bill 在该公司颇受推崇的技术期刊担任副主编,并且还在公司的产品营销和应用工程部门工作过。在此之前,Bill 曾在 Instron Corp. 工作,从事材料测试机器控制的实际模拟和电源电路设计及系统集成。

他拥有电气工程硕士学位(马萨诸塞州立大学)和电气工程学士学位(哥伦比亚大学),是注册专业工程师,并持有高级业余无线电许可证。Bill 还规划、撰写并讲授了关于各种工程主题的在线课程,包括 MOSFET 基础知识、ADC 选择和驱动 LED。

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