Tying both inputs of an opamp to the output

it'll probably rail, but could do anything, that may or may not be a problem.

tie the non inverting input to something else instead.

It'll rail, either positive or negative, depending on the offset voltage.

A better way to handle unused OpAmps is to tie Output to Vin-, then Vin+ to the rail it is capable of reaching at the output, or tie Vin+ to a midpoint. ...Jim Thompson

Thanks. I asked the question while working on a layout using an LM358. Perhaps I should have mentioned that but I really wanted to know the answer in a wider context. I usually tie Vin- to Output and Vin+ to -Vcc, sometimes to +Vcc (The LM358 is rated for inputs up to 32V without damage regardless of supply voltage). But there are times when layout convenience makes it tempting to tie both inputs to Output, especially on a home-made single-layer PCB.

As JT said, it'll rail, though if the offset voltage crosses zero at some temperature, it'll misbehave there.

It may also screw up the internal biasing for the other sections of the chip, if there's a single shared bias generator.

Cheers

Phil Hobbs

Late at night, by candle light, "Mimawl" penned this immortal opus:

When I had a 1/4 TL84 left over I made it a follower (Vout to Vin-) and tied Vin+ to the nearest neighbouring output and just left it dangling there. This ensured that it would stay within bounds. Maybe some loading of the output could have been in order but it worked fine anyway.

- YD.

One thing you have to watch out for is that some amplifiers have an output swing that's wider than their input CM range. If you're unlucky or incautious, that can cause that voltage follower to misbehave, even as far as affecting the other sections via a shared internal bias generator.

If you're using split supplies, having it follow ground is safe and convenient.

Cheers

Phil Hobbs