Operational amplifiers are used in many configurations and with many variations on specifications, depending upon the application. However, an IC package of operational amplifiers will often come with several in a package (e.g., a “quad pack” will have four op amps). If you have leftover, unused op amps on the same chip, the unused ones may be configured to share a power rail, which means that the unused op amps in the package are going to powered and operational. Powered op amps with inputs that are connected to nothing at all are called “floating” inputs, and can and will create output and actively consume power.
Connecting the unused op amps is the best answer, which minimizes power consumption. Connecting the floating pins also removes the chance that unintended activity, induced or generated by stray capacitance or inductance, will cause interaction with other areas on the same chip.
Do not tie the negative input to the positive voltage supply rail (top diagram), or the positive input pin to the negative voltage supply rail. Do not tie the input pins together (middle diagram). What may be obvious to some still needs to be mentioned: the negative supply rail and/or 0 volts is not “ground.” Find ground for your circuit, for example, the ground plane for your printed circuit board, and tie the positive input pin for the unused op amps to ground. Then connect the negative input to the op amp to the output pin in a feedback loop (bottom diagram).
For more information on what to do when connecting op amps, see the Texas Instruments “Handbook of Operational Amplifier Applications” by Bruce Carter and Thomas R. Brown or the “Op Amp Applications Handbook,” by Wal Jung, from Analog Devices. Both of these handbooks are available as PDFs for download on their respective company sites.
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