Tying both inputs of an opamp to the output

[Mimawl]

Is there a possibility of something undesireable happening if
*both* the inverting and non-inverting inputs of an opamp are
tied to the output? A possible scenario is when one unit of a
dual or quad type is unused and tying both inputs to the output
is convenient in the PCB layout.

 

[Jasen Betts]

Is there a possibility of something undesireable happening if
*both* the inverting and non-inverting inputs of an opamp are
tied to the output? A possible scenario is when one unit of a
dual or quad type is unused and tying both inputs to the output
is convenient in the PCB layout.

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.

 

[Mimawl]

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.

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.

 

[YD]

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.

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.