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Tying both inputs of an opamp to the output

Started by Mimawl July 4, 2013
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. 


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. -- &#9858;&#9859; 100% natural --- news://freenews.netfront.net/ - complaints: news@netfront.net ---
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 -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
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.
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
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.
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