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]

On 2013-07-04, Mimawl <Mimawl@invalid.net> wrote:

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.

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[Jim Thompson]

On Thu, 4 Jul 2013 16:20:55 +0530, "Mimawl" <Mimawl@invalid.net>
wrote:

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 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
--
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I love to cook with wine. Sometimes I even put it in the food.

 

[Mimawl]

Jim Thompson wrote:

On Thu, 4 Jul 2013 16:20:55 +0530, "Mimawl"
Mimawl@invalid.net
wrote:

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 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.

 

[Phil Hobbs]

On 07/04/2013 06:50 AM, Mimawl wrote:

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.

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

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net

 

[YD]

Late at night, by candle light, "Mimawl" <Mimawl@invalid.net> penned
this immortal opus:

Jim Thompson wrote:
On Thu, 4 Jul 2013 16:20:55 +0530, "Mimawl"
Mimawl@invalid.net
wrote:

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 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.

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.
--
Remove HAT if replying by mail.

 

[Phil Hobbs]

On 07/04/2013 08:50 PM, YD wrote:

Late at night, by candle light, "Mimawl" <Mimawl@invalid.net> penned
this immortal opus:

Jim Thompson wrote:
On Thu, 4 Jul 2013 16:20:55 +0530, "Mimawl"
Mimawl@invalid.net
wrote:

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 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.


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

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net