Examine every chart and think of what your fellow engineer who measured the data is trying to tell you. Often, your fellow engineer at a semiconductor company includes a chart that highlights a less flattering specification of an amplifier. If a chart shows that an amplifier has 90% overshoot at 10 pF of output capacitance, the part is subject to instability.
The general description and application section follows the chart section of a typical data sheet. In this section, you can learn of appropriate applications and read about any peculiarities or special features of the amplifier. The application section may warn you that the part will burn up if you overdrive the outputs. In some older parts, the application section may warn that the part exhibits phase reversal—that is, when you bring the input pin past its common-mode range, the output of the amplifier suddenly inverts, even though the inputs never cross zero.
The part number, or suffix, section of the data sheet may be toward the end, but some manufacturers, such as TI, put this information on the front page. Every package and voltage rating of the part gets its own part number. The manufacturer may also include numbers for lead-free ROHS (restriction-of-hazardous-substances) parts. Part numbers also differ for parts in a rail or in a 4000-part reel. It is exasperating to lay out a board with a different package from the one you intended because you used an incomplete part number. Errors such as these can cost weeks or months in the development cycle.
One of the last sections of the data sheet is often the packages section, which includes drawings and suggested PCB (printed-circuit-board) patterns. If your PCB has a low profile, the overall height of the package may be the critical performance specification you must meet.
Online tools can help
Never hesitate to call the local field-application engineer or the factory-applications group. Analog Devices and Texas Instruments sell almost every type of op amp, so they have no reason to steer you to a specific part. One exception to this rule is that manufacturers often want to promote their newest parts in the hope of recovering the costs of designing them. For this reason, National Semiconductor’s Grohe likes to use selector guides. “A parametric search will return all the parts that meet your required specifications, whether the part was designed yesterday or 20 years ago,” he says. Grohe developed the downloadable selector guide you can get from the company’s amplifier Web page. TI, Analog Devices, STMicroelectronics, and others also provide online selector guides.
Linear Technology developed another helpful, free, fully functional, downl
oadable tool, LTSPICE, which Mike Engelhardt designed. He assures that the program converges, even with magnetic elements. Texas Instruments also offers the downloadable, node-limited, fully functional Tiny TI SPICE program, which provides accurate results when you use it with accurate models. Analog Devices’ Web site also has a downloadable simulator and the ADIsim op-amp-evaluation tool. The program does a simple evaluation using National Instruments’ LabView engine. Once you select a part, the tool switches to using National Instruments’ MultiSIM full-SPICE engine if a part model is available. In addition to the SPICE tools, Analog Devices, National Semiconductor, and TI also offer Web tools to help design instrumentation amps or to properly bias a single-ended amplifier, as well as for scores of other applications.
For designing filter chains, TI offers its FilterPro software. This downloadable software performs the math calculations to show you the response of multipole filters. National Semiconductor offers its Webench online environment for designing filters. It runs SPICE simulations online to show you the response of the part.
Selecting op amps can be daunting. In addition to conventional voltage-feedback amplifiers, many specialty amplifiers exist (see sidebar “Specialty op amps”). You may need to read relevant trade magazines and books before you understand all the subtleties of amplifier selection. Application engineers can be a great help in getting you to understand the right specifications and amplifier types at which you should be looking. Once you know those facts, you can use the variety of downloadable selector guides and online guides. You can then simulate your circuit online or through the downloadable tools, as well as use the vendor-supplied SPICE models to simulate you circuit in Orcad, Altium, PADS, or Electronics Workbench.
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