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Sci.Electronics.Basics -> Is it okay if I leave an opamp or comparator input open?
There are 9 messages in this thread.
You are currently looking at messages 1 to 9.
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Author: MRWDate: 22:18 24-07-07
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I am curious if it is okay to leave an opamp or comparator inverting
or non-inverting input open while the device is powered.
Thanks!
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Op amps have such a high gain that leaving an input open will probably
drive the output into saturation, which isn't ideal. Better to
configure unused op amps as unity-gain (tie in- to out) and use a pair
of resistors to set the input voltage to half the supply voltage.
That keeps all the pins in spec and happy.
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Author: MRWDate: 00:20 25-07-07
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On Jul 24, 10:47 pm, DJ Delorie <d...@delorie.com> wrote:
> Op amps have such a high gain that leaving an input open will probably
> drive the output into saturation, which isn't ideal. Better to
> configure unused op amps as unity-gain (tie in- to out) and use a pair
> of resistors to set the input voltage to half the supply voltage.
> That keeps all the pins in spec and happy.
Okay. I think I get it. Thanks! Hmm.. I do remember reading somewhere
that the opamp input terminal has to have a path back to ground to
make it happy.. or something like that. I don't quite remember, but I
guess that would make sense if we tie the inputs to a voltage source
or ground.
Would a circuit like this work?
GND
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V
-
| GND
| |\|
VIN2 -- ----|-\
| >-VO
VIN3-|+/
|/|
VCC
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)
VIN3 in the diagram is a fixed voltage source, while VIN2 is either
there or not (like plugged in or plugged out). Thanks!
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Author: EeyoreDate: 04:23 25-07-07
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MRW wrote:
> I am curious if it is okay to leave an opamp or comparator inverting
> or non-inverting input open while the device is powered.
It probably won't damage it but it certainly won't work in any meaningful way.
Graham
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Author: John LarkinDate: 11:03 25-07-07
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On Wed, 25 Jul 2007 04:20:33 -0000, MRW <mr.whatever@gmail.com> wrote:
>On Jul 24, 10:47 pm, DJ Delorie <d...@delorie.com> wrote:
>> Op amps have such a high gain that leaving an input open will probably
>> drive the output into saturation, which isn't ideal. Better to
>> configure unused op amps as unity-gain (tie in- to out) and use a pair
>> of resistors to set the input voltage to half the supply voltage.
>> That keeps all the pins in spec and happy.
>
>Okay. I think I get it. Thanks! Hmm.. I do remember reading somewhere
>that the opamp input terminal has to have a path back to ground to
>make it happy.. or something like that. I don't quite remember, but I
>guess that would make sense if we tie the inputs to a voltage source
>or ground.
>
>Would a circuit like this work?
>
>
>
> GND
> |
> |
> V
> -
> | GND
> | |\|
> VIN2 -- ----|-\
> | >-VO
> VIN3-|+/
> |/|
> VCC
>
>(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)
>
>
>VIN3 in the diagram is a fixed voltage source, while VIN2 is either
>there or not (like plugged in or plugged out). Thanks!
>
Some opamps will draw a lot of supply power if their outputs rail, as
they will if they are not operating closed-loop. And in some dual or
quad-section opamps, running one section open-loop will mess up other
sections. In general, keep all opamps running closed-loop, like this:
> +--------------+
> | |
> | GND |
> | |\| |
> +-----|-\ |
> | >-----+----VO
> VIN3-|+/
> |/|
> VCC
or
> vin2---+------1M------+
> | |
> | GND |
> | |\| |
> +-----|-\ |
> | >-----+----VO
> VIN3-|+/
> |/|
> VCC
or some other way to keep it happy when your input is disconnected.
John
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Author: MRWDate: 12:02 25-07-07
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Eeyore wrote:
>
>
> MRW wrote:
>
> > I am curious if it is okay to leave an opamp or comparator inverting
> > or non-inverting input open while the device is powered.
>
> It probably won't damage it but it certainly won't work in any
> meaningful way.
>
> Graham
Thanks, Graham! I'd like to make it work in a meaningful way and not
damage it. I think I understand now.
--
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Author: MRWDate: 22:34 25-07-07
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John Larkin wrote:
> On Wed, 25 Jul 2007 04:20:33 -0000, MRW <mr.whatever@gmail.com> wrote:
>
> >On Jul 24, 10:47 pm, DJ Delorie <d...@delorie.com> wrote:
> >> Op amps have such a high gain that leaving an input open will
> probably >> drive the output into saturation, which isn't ideal.
> Better to >> configure unused op amps as unity-gain (tie in- to out)
> and use a pair >> of resistors to set the input voltage to half the
> supply voltage. >> That keeps all the pins in spec and happy.
> >
> > Okay. I think I get it. Thanks! Hmm.. I do remember reading
> > somewhere that the opamp input terminal has to have a path back to
> > ground to make it happy.. or something like that. I don't quite
> > remember, but I guess that would make sense if we tie the inputs to
> > a voltage source or ground.
> >
> > Would a circuit like this work?
> >
> >
> >
> > GND
> > |
> > |
> > V
> > -
> > | GND
> > | |\|
> > VIN2 -- ----|-\
> > | >-VO
> > VIN3-|+/
> > |/|
> > VCC
> >
> > (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)
> >
> >
> > VIN3 in the diagram is a fixed voltage source, while VIN2 is either
> > there or not (like plugged in or plugged out). Thanks!
> >
>
> Some opamps will draw a lot of supply power if their outputs rail, as
> they will if they are not operating closed-loop. And in some dual or
> quad-section opamps, running one section open-loop will mess up other
> sections. In general, keep all opamps running closed-loop, like this:
>
>
> > +--------------+
> > | |
> > | GND |
> > | |\| |
> > +-----|-\ |
> > | >-----+----VO
> > VIN3-|+/
> > |/|
> > VCC
>
> or
>
> > vin2---+------1M------+
> > | |
> > | GND |
> > | |\| |
> > +-----|-\ |
> > | >-----+----VO
> > VIN3-|+/
> > |/|
> > VCC
>
> or some other way to keep it happy when your input is disconnected.
>
>
> John
Thanks, John!
--
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Author: ManojDate: 02:47 26-07-07
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On Jul 26, 7:34 am, "MRW" <mr.whate...@gmail.com> wrote:
> John Larkin wrote:
> > On Wed, 25 Jul 2007 04:20:33 -0000, MRW <mr.whate...@gmail.com> wrote:
>
> > >On Jul 24, 10:47 pm, DJ Delorie <d...@delorie.com> wrote:
> > >> Op amps have such a high gain that leaving an input open will
> > probably >> drive the output into saturation, which isn't ideal.
> > Better to >> configure unused op amps as unity-gain (tie in- to out)
> > and use a pair >> of resistors to set the input voltage to half the
> > supply voltage. >> That keeps all the pins in spec and happy.
>
> > > Okay. I think I get it. Thanks! Hmm.. I do remember reading
> > > somewhere that the opamp input terminal has to have a path back to
> > > ground to make it happy.. or something like that. I don't quite
> > > remember, but I guess that would make sense if we tie the inputs to
> > > a voltage source or ground.
>
> > > Would a circuit like this work?
>
> > > GND
> > > |
> > > |
> > > V
> > > -
> > > | GND
> > > | |\|
> > > VIN2 -- ----|-\
> > > | >-VO
> > > VIN3-|+/
> > > |/|
> > > VCC
>
> > > (created by AACircuit v1.28.6 beta 04/19/05www.tech-chat.de)
>
> > > VIN3 in the diagram is a fixed voltage source, while VIN2 is either
> > > there or not (like plugged in or plugged out). Thanks!
>
> > Some opamps will draw a lot of supply power if their outputs rail, as
> > they will if they are not operating closed-loop. And in some dual or
> > quad-section opamps, running one section open-loop will mess up other
> > sections. In general, keep all opamps running closed-loop, like this:
>
> > > +--------------+
> > > | |
> > > | GND |
> > > | |\| |
> > > +-----|-\ |
> > > | >-----+----VO
> > > VIN3-|+/
> > > |/|
> > > VCC
>
> > or
>
> > > vin2---+------1M------+
> > > | |
> > > | GND |
> > > | |\| |
> > > +-----|-\ |
> > > | >-----+----VO
> > > VIN3-|+/
> > > |/|
> > > VCC
>
> > or some other way to keep it happy when your input is disconnected.
>
> > John
>
> Thanks, John!
>
> --- Hide quoted text -
>
> - Show quoted text -
dear freinds,
there are many application notes to this effect from almost all big
manufacturers.
i think comparator pins can be left open, coz they are one way or the
other in saturation.
saturation state for op-amp is not good, one coz they take a lot of
current and second it will put in a lot of noise on the supplies.
the best idea is to keep op-amp in a known state and for that u should
tie both minus and positive pins such that u ahve a output close to
mid point of supply voltages applied. do read application notes of
national for this application.
with regards
manoj
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Author: John FieldsDate: 08:04 26-07-07
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On Wed, 25 Jul 2007 23:47:18 -0700, Manoj <manojahd@gmail.com>
wrote:
>dear freinds,
>
>there are many application notes to this effect from almost all big
>manufacturers.
>i think comparator pins can be left open, coz they are one way or the
>other in saturation.
---
From:
http://www.national.com/mpf/LM/LM393.html#Datasheet
"All input pins of any unused comparators should be tied to
the negative supply."
--
JF
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