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Signal Chain Basics #162: 3 considerations when selecting an op amp

One of the most common questions I see over email and on our TI E2E support forums is some form of, “What op amp should I use for a [insert application here]?” The difficulty in answering this question is twofold: there are literally thousands of op amps to choose from, and there seems to be an endless number of specifications and features to consider.

Such complexity necessitates the creation of online search and design tools, which can be complicated themselves (with literally dozens of filters, checkboxes, and slider bars). In turn, this leads to the creation of quick search tools, pre-filtered results, and other innovative tools, all designed to simplify the process of selecting an operational amplifier (op amp).

The purpose of this article is to explain a simplified process that focuses on three of the top considerations when selecting an op amp, as shown in Figure 1. The first two considerations are the primary specifications of supply voltage (Vs) and quiescent current (IQ), along with their corresponding descendent specifications. Descendent specifications are defined as those that depend directly on the primary specifications. Selecting an op amp that does not have proper primary and descendent specifications will likely not work in the application. The third and final consideration involves a couple of common op-amp features: package and price. I’d also like to clarify that the focus of this article is on general-purpose and precision voltage-feedback op amps.

diagram of op amp considerationsFigure 1 Op-amp selection considerations

Consideration No. 1: supply voltage

Supply voltage (Vs, or V- and V+) is a primary specification for two reasons. First, the supply voltage must be compatible with the system voltage; otherwise you will need to generate a new supply rail. Second, the supply voltage has two important descendent specifications: input common-mode voltage range (Vcm) and output swing/headroom (Vo, or Vol/Voh). Vcm and Vo define the range of input and output signals for linear operation. If the input signal and output signal are not within the linear operating region of the device, then you’ve selected the wrong op amp, regardless of all other specifications.

Figure 2 depicts a datasheet excerpt of how the supply voltage of an op amp defines the linear operating region and highlights a rail-to-rail input/output (RRIO) device. RRIO devices are popular because they are generally easier to use, but be aware of the trade-offs associated with different input-stage designs.

table from OPA991 datasheetFigure 2 In this excerpt from the OPA991 datasheet, Vcm and Vo depend on supply voltage rails (V- and V+).

For example, the op amp from Figure 2 with ±15V supplies would have an input common-mode range of -15.1V < Vcm < 15.1V and an output swing of approximately -14.8V < Vo < 14.8V (assuming a load impedance of 2 kΩ). If the input signal and desired output signal are not within these voltage ranges, this combination of op amp and supply voltage will not suffice for the design.

Consideration No. 2: quiescent current

Quiescent current (IQ) is a primary specification because it impacts many descendent specifications of importance such as bandwidth (BW) and slew rate (SR). An op amp with insufficient BW and SR yields undesirable effects, such as slew-induced distortion and nonlinear operation. Therefore, the system’s power supply must provide sufficient current in order for the op amp to meet performance expectations.

In general, IQ is directly related to BW and SR (a higher IQ yields more BW and faster SR). There are op amps designed for specific use cases (such as op amps with slew boost), but in general, the aforementioned relationships between IQ, BW, and SR hold true. Table 2 depicts five op amps with increasing IQ and their corresponding typical values of BW and SR. As a general rule of thumb, try to select op amps with approximately 25 to 30% more BW and SR required for the application to account for process and temperature variations. Low-distortion designs will require even more BW and SR.

Table 1 Comparison of op-amp BW and SR vs. increasing IQ

table of op amp BW and SR

Consideration No. 3: features

Now it’s time to look at op-amp features, which primarily include package and cost. Although most designers want the smallest device that costs the least amount of money, one aspect to consider is future-proofing the design from both a package and cost perspective.

There are numerous reasons why you might need to revisit a design years after going into production. These include cost-down measures, marginal designs, fabrication process shifts, and product end-of-life. To prepare for these possibilities, consider selecting op amps that come in standard packages (small-outline integrated circuit [SOIC], thin shrink small-outline package [TSSOP], very thin shrink small-outline package [VSSOP]). A number of new packages are becoming industry standards because they’re smaller and more economical to manufacture, such as small-outline no-lead (SON) and small-outline transistor (SOT) packages. So consider implementing a dual footprint, or select op amps that are part of a design family.

An op amp with the proper supply voltage, quiescent current, descendent specifications, and features is likely to be a good fit for your design. However, numerous additional specifications can impact performance: offset voltage (Vos), voltage noise density (en), power-supply rejection ratio (PSRR), and common-mode rejection ratio (CMRR). A discussion of these specifications is beyond the scope of this article, but there are resources available, some of which I’ve provided at the end of this article.

The next time you have the task of selecting an op amp, first consider the supply voltage, quiescent current, and subsequent descendent specifications to ensure that the device will operate correctly in your application. Be sure to verify linear operation of the input and output, include design margin for BW and SR, and consider op-amp families for design and package flexibility.

Peter Semig is applications manager for general-purpose amplifiers at Texas Instruments.

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1 comment on “Signal Chain Basics #162: 3 considerations when selecting an op amp

  1. meohayphongbep
    January 18, 2021

    Thanks for your post!

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