I selected a Rail-to-Rail Input/Output amplifier but the output does not go all the way to the negative rail or all the way to the positive rail. What am I doing wrong?
The term “Rail-to-Rail” is misleading. To be completely accurate, it should be “almost Rail-to-Rail” or “very nearly Rail-to-Rail.” The output for most R-R amps is from 20 to 200mv from either supply rail, almost never all the way to the rail. And as more load current is required, the output will “pull” further away from the supply rail voltage. Most amplifiers will provide their maximum output voltage swing with a load of 100k Ω or greater. The Electrical Characteristics table and the Characteristic Curves in the product datasheet will specify the output voltage swings that can be expected. In addition, when creating a design using Amplifiers WEBENCH, the user can request a preference for rail-to-rail output amplifiers. The op amps presented in the selection table will meet the output swing requirements of the design, with the rail-to-rail output devices showing up higher in the table if that preference is selected.
